File: System\Windows\Data\CollectionView.cs
Web Access
Project: src\src\Microsoft.DotNet.Wpf\src\PresentationFramework\PresentationFramework.csproj (PresentationFramework)
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
 
//
// Description: Base implementation of ICollectionView that enforces
// affinity to the UI thread dispatcher.
//
// See spec at CollectionView.mht
//
 
 
using System;
using System.ComponentModel;
using System.Globalization;
using System.Reflection;
 
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Diagnostics;
using System.Windows;
using System.Threading;
using System.Windows.Threading;
using MS.Internal.Data;
using MS.Internal;              // Invariant.Assert
using MS.Internal.Hashing.PresentationFramework;    // HashHelper
 
namespace System.Windows.Data
{
    /// <summary>
    /// ICollectionView with checks for affinity to the UI thread dispatcher
    /// </summary>
    public class CollectionView : DispatcherObject, ICollectionView, INotifyPropertyChanged
    {
        //------------------------------------------------------
        //
        //  Constructors
        //
        //------------------------------------------------------
 
        #region Constructors
 
        /// <summary>
        /// Create a view to given collection.
        /// </summary>
        /// <remarks>
        /// Note that this instance of CollectionView is bound to the
        /// UI thread dispatcher of the caller of this constructor.
        /// </remarks>
        /// <param name="collection">underlying collection</param>
        public CollectionView(IEnumerable collection)
            : this(collection, 0)
        {
        }
 
        internal CollectionView(IEnumerable collection, int moveToFirst)
        {
            ArgumentNullException.ThrowIfNull(collection);
 
            // Constructing a CollectionView itself (as opposed to a derived class)
            // is deprecated in NetFx3.5.  This used to use IndexedEnumerable to
            // support a view over a plain IEnumerable, but this scenario is now
            // supported by the EnumerableCollectionView derived class.  Internal
            // code does not create a CollectionView, but it is still
            // possible for an app to call "new CollectionView(myCollection)"
            // directly;  this is public API that we cannot remove.  Such an app
            // will continue to get the old behavior with IndexedEnumerable, bugs
            // and all.  As a courtesy, we detect this here and warn the user about
            // it through the tracing channel.
            if (this.GetType() == typeof(CollectionView))
            {
                if (TraceData.IsEnabled)
                {
                    TraceData.TraceAndNotify(TraceEventType.Warning,
                        TraceData.CollectionViewIsUnsupported);
                }
            }
 
            _engine = DataBindEngine.CurrentDataBindEngine;
 
            if (!_engine.IsShutDown)
            {
                SynchronizationInfo syncInfo = _engine.ViewManager.GetSynchronizationInfo(collection);
                SetFlag(CollectionViewFlags.AllowsCrossThreadChanges, syncInfo.IsSynchronized);
            }
            else
            {
                // WPF doesn't really support doing anything on a thread whose dispatcher
                // has been shut down.  But for app-compat we should limp along
                // as well as we did in 4.0.  This means avoiding anything that
                // touches the ViewManager.
                moveToFirst = -1;
            }
 
            _sourceCollection = collection;
 
            // forward collection change events from underlying collection to our listeners.
            INotifyCollectionChanged incc = collection as INotifyCollectionChanged;
            if (incc != null)
            {
                // BindingListCollectionView already listens to IBindingList.ListChanged;
                // Don't double-subscribe (bug 452474, 607512)
                IBindingList ibl;
                if (!(this is BindingListCollectionView) ||
                    ((ibl = collection as IBindingList) != null && !ibl.SupportsChangeNotification))
                {
                    incc.CollectionChanged += new NotifyCollectionChangedEventHandler(OnCollectionChanged);
                }
                SetFlag(CollectionViewFlags.IsDynamic, true);
            }
 
            // set currency to the first item if available
            object currentItem = null;
            int currentPosition = -1;
            if (moveToFirst >= 0)
            {
                BindingOperations.AccessCollection(collection,
                    () =>
                    {
                        IEnumerator e = collection.GetEnumerator();
                        if (e.MoveNext())
                        {
                            currentItem = e.Current;
                            currentPosition = 0;
                        }
 
                        IDisposable d = e as IDisposable;
                        if (d != null)
                        {
                            d.Dispose();
                        }
                    },
                    false);
            }
 
            _currentItem = currentItem;
            _currentPosition = currentPosition;
            SetFlag(CollectionViewFlags.IsCurrentBeforeFirst,  _currentPosition < 0);
            SetFlag(CollectionViewFlags.IsCurrentAfterLast,  _currentPosition < 0);
            SetFlag(CollectionViewFlags.CachedIsEmpty, _currentPosition < 0);
        }
 
        internal CollectionView(IEnumerable collection, bool shouldProcessCollectionChanged)
            : this(collection)
        {
            SetFlag(CollectionViewFlags.ShouldProcessCollectionChanged, shouldProcessCollectionChanged);
        }
 
        #endregion Constructors
 
        //------------------------------------------------------
        //
        //  Public Interfaces
        //
        //------------------------------------------------------
 
        #region Public Interfaces
 
        #region ICollectionView
 
        /// <summary>
        /// Culture to use during sorting.
        /// </summary>
        [TypeConverter(typeof(System.Windows.CultureInfoIetfLanguageTagConverter))]
        public virtual CultureInfo Culture
        {
            get { return _culture; }
            set
            {
                ArgumentNullException.ThrowIfNull(value);
 
                if (_culture != value)
                {
                    _culture = value;
                    OnPropertyChanged(CulturePropertyName);
                }
            }
        }
 
        /// <summary>
        /// Return true if the item belongs to this view.  No assumptions are
        /// made about the item. This method will behave similarly to IList.Contains().
        /// </summary>
        /// <remarks>
        /// <p>If the caller knows that the item belongs to the
        /// underlying collection, it is more efficient to call PassesFilter.
        /// If the underlying collection is only of type IEnumerable, this method
        /// is a O(N) operation</p>
        /// </remarks>
        public virtual bool Contains(object item)
        {
            VerifyRefreshNotDeferred();
 
            return (IndexOf(item) >= 0);
        }
 
        /// <summary>
        /// Returns the underlying collection.
        /// </summary>
        public virtual IEnumerable SourceCollection
        {
            get { return _sourceCollection; }
        }
 
        /// <summary>
        /// Filter is a callback set by the consumer of the ICollectionView
        /// and used by the implementation of the ICollectionView to determine if an
        /// item is suitable for inclusion in the view.
        /// </summary>
        /// <exception cref="NotSupportedException">
        /// Simpler implementations do not support filtering and will throw a NotSupportedException.
        /// Use <seealso cref="CanFilter"/> property to test if filtering is supported before
        /// assigning a non-null value.
        /// </exception>
        public virtual Predicate<object> Filter
        {
            get
            {
                return _filter;
            }
            set
            {
                if (!CanFilter)
                    throw new NotSupportedException();
 
                _filter = value;
 
                RefreshOrDefer();
            }
        }
 
        /// <summary>
        /// Indicates whether or not this ICollectionView can do any filtering.
        /// When false, set <seealso cref="Filter"/> will throw an exception.
        /// </summary>
        public virtual bool CanFilter
        {
            get
            {
                return true;
            }
        }
 
        /// <summary>
        /// Collection of Sort criteria to sort items in this view over the SourceCollection.
        /// </summary>
        /// <remarks>
        /// <p>
        /// Simpler implementations do not support sorting and will return an empty
        /// and immutable / read-only SortDescription collection.
        /// Attempting to modify such a collection will cause NotSupportedException.
        /// Use <seealso cref="CanSort"/> property on CollectionView to test if sorting is supported
        /// before modifying the returned collection.
        /// </p>
        /// <p>
        /// One or more sort criteria in form of <seealso cref="SortDescription"/>
        /// can be added, each specifying a property and direction to sort by.
        /// </p>
        /// </remarks>
        public virtual SortDescriptionCollection SortDescriptions
        {
            get { return SortDescriptionCollection.Empty; }
        }
 
        /// <summary>
        /// Test if this ICollectionView supports sorting before adding
        /// to <seealso cref="SortDescriptions"/>.
        /// </summary>
        public virtual bool CanSort
        {
            get { return false; }
        }
 
        /// <summary>
        /// Returns true if this view really supports grouping.
        /// When this returns false, the rest of the interface is ignored.
        /// </summary>
        public virtual bool CanGroup
        {
            get { return false; }
        }
 
        /// <summary>
        /// The description of grouping, indexed by level.
        /// </summary>
        public virtual ObservableCollection<GroupDescription> GroupDescriptions
        {
            get { return null; }
        }
 
        /// <summary>
        /// The top-level groups, constructed according to the descriptions
        /// given in GroupDescriptions.
        /// </summary>
        public virtual ReadOnlyObservableCollection<object> Groups
        {
            get { return null; }
        }
 
        /// <summary>
        /// Re-create the view, using any <seealso cref="SortDescriptions"/> and/or <seealso cref="Filter"/>.
        /// </summary>
        public virtual void Refresh()
        {
            IEditableCollectionView ecv = this as IEditableCollectionView;
            if (ecv != null && (ecv.IsAddingNew || ecv.IsEditingItem))
                throw new InvalidOperationException(SR.Format(SR.MemberNotAllowedDuringAddOrEdit, "Refresh"));
 
            RefreshInternal();
        }
 
        internal void RefreshInternal()
        {
            if (AllowsCrossThreadChanges)
                VerifyAccess();
 
            RefreshOverride();
 
            SetFlag(CollectionViewFlags.NeedsRefresh, false);
        }
 
        /// <summary>
        /// Enter a Defer Cycle.
        /// Defer cycles are used to coalesce changes to the ICollectionView.
        /// </summary>
        public virtual IDisposable DeferRefresh()
        {
            if (AllowsCrossThreadChanges)
                VerifyAccess();
 
            IEditableCollectionView ecv = this as IEditableCollectionView;
            if (ecv != null && (ecv.IsAddingNew || ecv.IsEditingItem))
                throw new InvalidOperationException(SR.Format(SR.MemberNotAllowedDuringAddOrEdit, "DeferRefresh"));
 
            ++ _deferLevel;
            return new DeferHelper(this);
        }
 
        /// <summary>
        /// Return the "current item" for this view
        /// </summary>
        /// <remarks>
        /// Only wrapper classes (those that pass currency handling calls to another internal
        /// CollectionView) should override CurrentItem; all other derived classes
        /// should use SetCurrent() to update the current values stored in the base class.
        /// </remarks>
        public virtual object CurrentItem
        {
            get
            {
                VerifyRefreshNotDeferred();
 
                return _currentItem;
            }
        }
 
        /// <summary>
        /// The ordinal position of the <seealso cref="CurrentItem"/> within the (optionally
        /// sorted and filtered) view.
        /// </summary>
        /// <returns>
        /// -1 if the CurrentPosition is unknown, because the collection does not have an
        /// effective notion of indices, or because CurrentPosition is being forcibly changed
        /// due to a CollectionChange.
        /// </returns>
        /// <remarks>
        /// Only wrapper classes (those that pass currency handling calls to another internal
        /// CollectionView) should override CurrenPosition; all other derived classes
        /// should use SetCurrent() to update the current values stored in the base class.
        /// </remarks>
        public virtual int CurrentPosition
        {
            get
            {
                VerifyRefreshNotDeferred();
 
                return _currentPosition;
            }
        }
 
        /// <summary>
        /// Return true if <seealso cref="CurrentItem"/> is beyond the end (End-Of-File).
        /// </summary>
        public virtual bool IsCurrentAfterLast
        {
            get
            {
                VerifyRefreshNotDeferred();
 
                return CheckFlag(CollectionViewFlags.IsCurrentAfterLast);
            }
        }
 
 
        /// <summary>
        /// Return true if <seealso cref="CurrentItem"/> is before the beginning (Beginning-Of-File).
        /// </summary>
        public virtual bool IsCurrentBeforeFirst
        {
            get
            {
                VerifyRefreshNotDeferred();
 
                return CheckFlag(CollectionViewFlags.IsCurrentBeforeFirst);
            }
        }
 
        /// <summary>
        /// Move <seealso cref="CurrentItem"/> to the first item.
        /// </summary>
        /// <returns>true if <seealso cref="CurrentItem"/> points to an item within the view.</returns>
        public virtual bool MoveCurrentToFirst()
        {
            VerifyRefreshNotDeferred();
 
            int index = 0;
            IEditableCollectionView ecv = this as IEditableCollectionView;
            if (ecv != null && ecv.NewItemPlaceholderPosition == NewItemPlaceholderPosition.AtBeginning)
            {
                index = 1;
            }
 
            return MoveCurrentToPosition(index);
        }
 
        /// <summary>
        /// Move <seealso cref="CurrentItem"/> to the last item.
        /// </summary>
        /// <returns>true if <seealso cref="CurrentItem"/> points to an item within the view.</returns>
        public virtual bool MoveCurrentToLast()
        {
            VerifyRefreshNotDeferred();
 
            int index = Count - 1;
            IEditableCollectionView ecv = this as IEditableCollectionView;
            if (ecv != null && ecv.NewItemPlaceholderPosition == NewItemPlaceholderPosition.AtEnd)
            {
                index -= 1;
            }
 
            return MoveCurrentToPosition(index);
        }
 
        /// <summary>
        /// Move <seealso cref="CurrentItem"/> to the next item.
        /// </summary>
        /// <returns>true if <seealso cref="CurrentItem"/> points to an item within the view.</returns>
        public virtual bool MoveCurrentToNext()
        {
            VerifyRefreshNotDeferred();
 
            int index = CurrentPosition + 1;
            int count = Count;
            IEditableCollectionView ecv = this as IEditableCollectionView;
 
            if (ecv != null && index == 0 && ecv.NewItemPlaceholderPosition == NewItemPlaceholderPosition.AtBeginning)
            {
                index = 1;
            }
            if (ecv != null && index == count-1 && ecv.NewItemPlaceholderPosition == NewItemPlaceholderPosition.AtEnd)
            {
                index = count;
            }
 
            if (index <= count)
            {
                return MoveCurrentToPosition(index);
            }
            else
            {
                return false;
            }
        }
 
        /// <summary>
        /// Move <seealso cref="CurrentItem"/> to the previous item.
        /// </summary>
        /// <returns>true if <seealso cref="CurrentItem"/> points to an item within the view.</returns>
        public virtual bool MoveCurrentToPrevious()
        {
            VerifyRefreshNotDeferred();
 
            int index = CurrentPosition - 1;
            int count = Count;
            IEditableCollectionView ecv = this as IEditableCollectionView;
 
            if (ecv != null && index == count-1 && ecv.NewItemPlaceholderPosition == NewItemPlaceholderPosition.AtEnd)
            {
                index = count-2;
            }
            if (ecv != null && index == 0 && ecv.NewItemPlaceholderPosition == NewItemPlaceholderPosition.AtBeginning)
            {
                index = -1;
            }
 
            if (index >= -1)
            {
                return MoveCurrentToPosition(index);
            }
            else
            {
                return false;
            }
        }
 
        /// <summary>
        /// Move <seealso cref="CurrentItem"/> to the given item.
        /// If the item is not found, move to BeforeFirst.
        /// </summary>
        /// <param name="item">Move CurrentItem to this item.</param>
        /// <returns>true if <seealso cref="CurrentItem"/> points to an item within the view.</returns>
        public virtual bool MoveCurrentTo(object item)
        {
            VerifyRefreshNotDeferred();
 
            // if already on item, or item is the placeholder, don't do anything
            if (System.Windows.Controls.ItemsControl.EqualsEx(CurrentItem, item) || System.Windows.Controls.ItemsControl.EqualsEx(NewItemPlaceholder, item))
            {
                // also check that we're not fooled by a false null _currentItem
                if (item != null || IsCurrentInView)
                    return IsCurrentInView;
            }
 
            int index = -1;
            IEditableCollectionView ecv = this as IEditableCollectionView;
            bool isNewItem = (ecv != null && ecv.IsAddingNew && System.Windows.Controls.ItemsControl.EqualsEx(item, ecv.CurrentAddItem));
            if (isNewItem || PassesFilter(item))
            {
                // if the item is not found IndexOf() will return -1, and
                // the MoveCurrentToPosition() below will move current to BeforeFirst
                index = IndexOf(item);
            }
 
            return MoveCurrentToPosition(index);
        }
 
        /// <summary>
        /// Move <seealso cref="CurrentItem"/> to the item at the given index.
        /// </summary>
        /// <param name="position">Move CurrentItem to this index</param>
        /// <returns>true if <seealso cref="CurrentItem"/> points to an item within the view.</returns>
        public virtual bool MoveCurrentToPosition(int position)
        {
            VerifyRefreshNotDeferred();
 
            ArgumentOutOfRangeException.ThrowIfLessThan(position, -1);
            ArgumentOutOfRangeException.ThrowIfGreaterThan(position, Count);
 
            // ignore request to move onto the placeholder
            IEditableCollectionView ecv = this as IEditableCollectionView;
            if (ecv != null &&
                    ((position == 0 && ecv.NewItemPlaceholderPosition == NewItemPlaceholderPosition.AtBeginning) ||
                     (position == Count-1 && ecv.NewItemPlaceholderPosition == NewItemPlaceholderPosition.AtEnd)))
            {
                return IsCurrentInView;
            }
 
            if ((position != CurrentPosition || !IsCurrentInSync)
                && OKToChangeCurrent())
            {
                bool oldIsCurrentAfterLast = IsCurrentAfterLast;
                bool oldIsCurrentBeforeFirst = IsCurrentBeforeFirst;
 
                _MoveCurrentToPosition(position);
                OnCurrentChanged();
 
                if (IsCurrentAfterLast != oldIsCurrentAfterLast)
                    OnPropertyChanged(IsCurrentAfterLastPropertyName);
 
                if (IsCurrentBeforeFirst != oldIsCurrentBeforeFirst)
                    OnPropertyChanged(IsCurrentBeforeFirstPropertyName);
 
                OnPropertyChanged(CurrentPositionPropertyName);
                OnPropertyChanged(CurrentItemPropertyName);
            }
 
            return IsCurrentInView;
        }
 
        ///<summary>
        /// Raise this event before changing currency.
        ///</summary>
        public virtual event CurrentChangingEventHandler CurrentChanging;
 
        ///<summary>
        ///Raise this event after changing currency.
        ///</summary>
        public virtual event EventHandler  CurrentChanged;
 
        #endregion ICollectionView
 
        #region IEnumerable
 
        /// <summary>
        /// Returns an object that enumerates the items in this view.
        /// </summary>
        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
 
        #endregion IEnumerable
 
        #endregion Public Interfaces
 
        //------------------------------------------------------
        //
        //  Public Methods
        //
        //------------------------------------------------------
 
        #region Public Methods
 
        /// <summary>
        /// Return true if the item belongs to this view.  The item is assumed to belong to the
        /// underlying DataCollection;  this method merely takes filters into account.
        /// It is commonly used during collection-changed notifications to determine if the added/removed
        /// item requires processing.
        /// Returns true if no filter is set on collection view.
        /// </summary>
        public virtual bool PassesFilter(object item)
        {
            if (CanFilter && Filter != null)
                return Filter(item);
 
            return true;
        }
 
        /// <summary> Return the index where the given item belongs, or -1 if this index is unknown.
        /// </summary>
        /// <remarks>
        /// If this method returns an index other than -1, it must always be true that
        /// view[index-1] &lt; item &lt;= view[index], where the comparisons are done via
        /// the view's IComparer.Compare method (if any).
        /// (This method is used by a listener's (e.g. System.Windows.Controls.ItemsControl)
        /// CollectionChanged event handler to speed up its reaction to insertion and deletion of items.
        /// If IndexOf is  not implemented, a listener does a binary search using IComparer.Compare.)
        /// </remarks>
        /// <param name="item">data item</param>
        public virtual int IndexOf(object item)
        {
            VerifyRefreshNotDeferred();
 
            return EnumerableWrapper.IndexOf(item);
        }
 
        /// <summary>
        /// Retrieve item at the given zero-based index in this CollectionView.
        /// </summary>
        /// <remarks>
        /// <p>The index is evaluated with any SortDescriptions or Filter being set on this CollectionView.
        /// If the underlying collection is only of type IEnumerable, this method
        /// is a O(N) operation.</p>
        /// <p>When deriving from CollectionView, override this method to provide
        /// a more efficient implementation.</p>
        /// </remarks>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown if index is out of range
        /// </exception>
        public virtual object GetItemAt(int index)
        {
            // only check lower bound because Count could be expensive
            ArgumentOutOfRangeException.ThrowIfNegative(index);
 
            return EnumerableWrapper[index];
        }
 
        /// <summary>
        /// Detach from the source collection.  (I.e. stop listening to the collection's
        /// events, or anything else that makes the CollectionView ineligible for
        /// garbage collection.)
        /// </summary>
        public virtual void DetachFromSourceCollection()
        {
            INotifyCollectionChanged incc = _sourceCollection as INotifyCollectionChanged;
            if (incc != null)
            {
                IBindingList ibl;
                if (!(this is BindingListCollectionView) ||
                    ((ibl = _sourceCollection as IBindingList) != null && !ibl.SupportsChangeNotification))
                {
                    incc.CollectionChanged -= new NotifyCollectionChangedEventHandler(OnCollectionChanged);
                }
            }
 
            _sourceCollection = null;
        }
 
        #endregion Public Methods
 
        //------------------------------------------------------
        //
        //  Public Properties
        //
        //------------------------------------------------------
 
        #region Public Properties
 
        /// <summary>
        /// Return the number of items (or -1, meaning "don't know");
        /// if a Filter is set, this counts only items that pass the filter.
        /// </summary>
        /// <remarks>
        /// <p>If the underlying collection is only of type IEnumerable, this count
        /// is a O(N) operation; this Count value will be cached until the
        /// collection changes again.</p>
        /// <p>When deriving from CollectionView, override this property to provide
        /// a more efficient implementation.</p>
        /// </remarks>
        public virtual int Count
        {
            get
            {
                VerifyRefreshNotDeferred();
 
                return EnumerableWrapper.Count;
            }
        }
 
        /// <summary>
        /// Returns true if the resulting (filtered) view is emtpy.
        /// </summary>
        public virtual bool IsEmpty
        {
            get { return EnumerableWrapper.IsEmpty; }
        }
 
        /// <summary>
        /// Return an object that compares items in this view.
        /// </summary>
        public virtual IComparer Comparer
        {
            get { return this as IComparer; }
        }
 
        /// <summary>
        ///     Returns true if this view needs to be refreshed.
        /// </summary>
        public virtual bool NeedsRefresh
        {
            get { return CheckFlag(CollectionViewFlags.NeedsRefresh); }
        }
 
        /// <summary>
        ///     Returns true if this view is in use (i.e. if anyone
        ///     is listening to its events).
        /// </summary>
        public virtual bool IsInUse
        {
            get { return CollectionChanged != null || PropertyChanged != null ||
                        CurrentChanged != null || CurrentChanging != null; }
        }
 
        public static object NewItemPlaceholder
        {
            get { return _newItemPlaceholder; }
        }
 
        #endregion Public Properties
 
 
        //------------------------------------------------------
        //
        //  Public Events
        //
        //------------------------------------------------------
 
        #region Public Events
 
        /// <summary>
        /// Raise this event when the (filtered) view changes
        /// </summary>
        protected virtual event NotifyCollectionChangedEventHandler CollectionChanged;
 
        /// <summary>
        /// CollectionChanged event (per <see cref="INotifyCollectionChanged" />).
        /// </summary>
        event NotifyCollectionChangedEventHandler INotifyCollectionChanged.CollectionChanged
        {
            add
            {
                CollectionChanged += value;
            }
            remove
            {
                CollectionChanged -= value;
            }
        }
 
 
        //------------------------------------------------------
        #region IPropertyChange implementation
 
        /// <summary>
        /// PropertyChanged event (per <see cref="INotifyPropertyChanged"/>).
        /// </summary>
        event PropertyChangedEventHandler INotifyPropertyChanged.PropertyChanged
        {
            add
            {
                PropertyChanged += value;
            }
            remove
            {
                PropertyChanged -= value;
            }
        }
 
        /// <summary>
        /// Raises a PropertyChanged event (per <see cref="INotifyPropertyChanged"/>).
        /// </summary>
        protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, e);
            }
        }
 
        /// <summary>
        /// PropertyChanged event (per <see cref="INotifyPropertyChanged"/>).
        /// </summary>
        protected virtual event PropertyChangedEventHandler PropertyChanged;
        #endregion IPropertyChange implementation
 
        #endregion Public Events
 
 
        //------------------------------------------------------
        //
        //  Protected Methods
        //
        //------------------------------------------------------
 
        #region Protected Methods
 
        /// <summary>
        /// Re-create the view, using any <seealso cref="SortDescriptions"/> and/or <seealso cref="Filter"/>.
        /// </summary>
        protected virtual void RefreshOverride()
        {
            if (SortDescriptions.Count > 0)
                throw new InvalidOperationException(SR.Format(SR.ImplementOtherMembersWithSort, "Refresh()"));
 
            object oldCurrentItem = _currentItem;
            bool oldIsCurrentAfterLast = CheckFlag(CollectionViewFlags.IsCurrentAfterLast);
            bool oldIsCurrentBeforeFirst = CheckFlag(CollectionViewFlags.IsCurrentBeforeFirst);
            int oldCurrentPosition = _currentPosition;
 
            // force currency off the collection (gives user a chance to save dirty information)
            OnCurrentChanging();
 
            InvalidateEnumerableWrapper();
 
            if (IsEmpty || oldIsCurrentBeforeFirst)
            {
                _MoveCurrentToPosition(-1);
            }
            else if (oldIsCurrentAfterLast)
            {
                _MoveCurrentToPosition(Count);
            }
            else if (oldCurrentItem != null) // set currency back to old current item, or first if not found
            {
                int index = EnumerableWrapper.IndexOf(oldCurrentItem);
                if (index  < 0)
                {
                    index  = 0;
                }
                _MoveCurrentToPosition(index);
            }
 
 
            OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
            OnCurrentChanged();
 
            if (IsCurrentAfterLast != oldIsCurrentAfterLast)
                OnPropertyChanged(IsCurrentAfterLastPropertyName);
 
            if (IsCurrentBeforeFirst != oldIsCurrentBeforeFirst)
                OnPropertyChanged(IsCurrentBeforeFirstPropertyName);
 
            if (oldCurrentPosition != CurrentPosition)
                OnPropertyChanged(CurrentPositionPropertyName);
 
            if (oldCurrentItem != CurrentItem)
                OnPropertyChanged(CurrentItemPropertyName);
        }
 
        /// <summary>
        /// Returns an object that enumerates the items in this view.
        /// </summary>
        protected virtual IEnumerator GetEnumerator()
        {
            VerifyRefreshNotDeferred();
 
            if (SortDescriptions.Count > 0)
                throw new InvalidOperationException(SR.Format(SR.ImplementOtherMembersWithSort, "GetEnumerator()"));
 
            return EnumerableWrapper.GetEnumerator();
        }
 
 
        /// <summary>
        ///     Notify listeners that this View has changed
        /// </summary>
        /// <remarks>
        ///     CollectionViews (and sub-classes) should take their filter/sort/grouping
        ///     into account before calling this method to forward CollectionChanged events.
        /// </remarks>
        /// <param name="args">
        ///     The NotifyCollectionChangedEventArgs to be passed to the EventHandler
        /// </param>
        protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs args)
        {
            ArgumentNullException.ThrowIfNull(args);
 
            unchecked { ++ _timestamp; }    // invalidate enumerators because of a change
 
            if (CollectionChanged != null)
                CollectionChanged(this, args);
 
            // Collection changes change the count unless an item is being
            // replaced or moved within the collection.
            if (args.Action != NotifyCollectionChangedAction.Replace &&
                args.Action != NotifyCollectionChangedAction.Move)
            {
                OnPropertyChanged(CountPropertyName);
            }
 
            bool isEmpty = IsEmpty;
            if (isEmpty != CheckFlag(CollectionViewFlags.CachedIsEmpty))
            {
                SetFlag(CollectionViewFlags.CachedIsEmpty, isEmpty);
                OnPropertyChanged(IsEmptyPropertyName);
            }
        }
 
        /// <summary>
        /// set CurrentItem and CurrentPosition, no questions asked!
        /// </summary>
        /// <remarks>
        /// CollectionViews (and sub-classes) should use this method to update
        /// the Current__ values.
        /// </remarks>
        protected void SetCurrent(object newItem, int newPosition)
        {
            int count = (newItem != null) ? 0 : IsEmpty ? 0 : Count;
            SetCurrent(newItem, newPosition, count);
        }
 
        /// <summary>
        /// set CurrentItem and CurrentPosition, no questions asked!
        /// </summary>
        /// <remarks>
        /// This method can be called from a constructor - it does not call
        /// any virtuals.  The 'count' parameter is substitute for the real Count,
        /// used only when newItem is null.
        /// In that case, this method sets IsCurrentAfterLast to true if and only
        /// if newPosition >= count.  This distinguishes between a null belonging
        /// to the view and the dummy null when CurrentPosition is past the end.
        /// </remarks>
        protected void SetCurrent(object newItem, int newPosition, int count)
        {
            if (newItem != null)
            {
                // non-null item implies position is within range.
                // We ignore count - it's just a placeholder
                SetFlag(CollectionViewFlags.IsCurrentBeforeFirst, false);
                SetFlag(CollectionViewFlags.IsCurrentAfterLast, false);
            }
            else if (count == 0)
            {
                // empty collection - by convention both flags are true and position is -1
                SetFlag(CollectionViewFlags.IsCurrentBeforeFirst, true);
                SetFlag(CollectionViewFlags.IsCurrentAfterLast, true);
                newPosition = -1;
            }
            else
            {
                // null item, possibly within range.
                SetFlag(CollectionViewFlags.IsCurrentBeforeFirst, newPosition < 0);
                SetFlag(CollectionViewFlags.IsCurrentAfterLast, newPosition >= count);
            }
 
            _currentItem = newItem;
            _currentPosition = newPosition;
        }
 
        /// <summary>
        /// ask listeners (via <seealso cref="ICollectionView.CurrentChanging"/> event) if it's OK to change currency
        /// </summary>
        /// <returns>false if a listener cancels the change, true otherwise</returns>
        protected bool OKToChangeCurrent()
        {
            CurrentChangingEventArgs args = new CurrentChangingEventArgs();
            OnCurrentChanging(args);
            return (!args.Cancel);
        }
 
        /// <summary>
        /// Raise a CurrentChanging event that is not cancelable.
        /// Internally, CurrentPosition is set to -1.
        /// This is called by CollectionChanges (Remove and Refresh) that affect the CurrentItem.
        /// </summary>
        /// <exception cref="InvalidOperationException">
        /// This CurrentChanging event cannot be canceled.
        /// </exception>
        protected void OnCurrentChanging()
        {
            _currentPosition = -1;
            OnCurrentChanging(uncancelableCurrentChangingEventArgs);
        }
 
        /// <summary>
        /// Raises the CurrentChanging event
        /// </summary>
        /// <param name="args">
        ///     CancelEventArgs used by the consumer of the event.  args.Cancel will
        ///     be true after this call if the CurrentItem should not be changed for
        ///     any reason.
        /// </param>
        /// <exception cref="InvalidOperationException">
        ///     This CurrentChanging event cannot be canceled.
        /// </exception>
        protected virtual void OnCurrentChanging(CurrentChangingEventArgs args)
        {
            ArgumentNullException.ThrowIfNull(args);
 
            if (_currentChangedMonitor.Busy)
            {
                if (args.IsCancelable)
                    args.Cancel = true;
                return;
            }
 
            if (CurrentChanging != null)
            {
                CurrentChanging(this, args);
            }
        }
 
        /// <summary>
        /// Raises the CurrentChanged event
        /// </summary>
        protected virtual void OnCurrentChanged()
        {
            if (CurrentChanged != null && _currentChangedMonitor.Enter())
            {
                using (_currentChangedMonitor)
                {
                    CurrentChanged(this, EventArgs.Empty);
                }
            }
        }
 
        /// <summary>
        ///     Must be implemented by the derived classes to process a single change on the
        ///     UI thread.  The UI thread will have already been entered by now.
        /// </summary>
        /// <param name="args">
        ///     The NotifyCollectionChangedEventArgs to be processed.
        /// </param>
        protected virtual void ProcessCollectionChanged(NotifyCollectionChangedEventArgs args)
        {
            //
            // Steps for ProcessCollectionChanged:
            //
            // 1) Validate that the values in the args are acceptable.
            // 2) Translate the indices if necessary.
            // 3) Raise CollectionChanged.
            // 4) Adjust Currency.
            // 5) Raise any PropertyChanged events that apply.
            //
 
            ValidateCollectionChangedEventArgs(args);
 
            object oldCurrentItem = _currentItem;
            bool oldIsCurrentAfterLast = CheckFlag(CollectionViewFlags.IsCurrentAfterLast);
            bool oldIsCurrentBeforeFirst = CheckFlag(CollectionViewFlags.IsCurrentBeforeFirst);
            int oldCurrentPosition = _currentPosition;
            bool raiseChanged = false;
 
            switch (args.Action)
            {
                case NotifyCollectionChangedAction.Add:
                    if (PassesFilter(args.NewItems[0]))
                    {
                        raiseChanged = true;
                        AdjustCurrencyForAdd(args.NewStartingIndex);
                    }
                    break;
 
                case NotifyCollectionChangedAction.Remove:
                    if (PassesFilter(args.OldItems[0]))
                    {
                        raiseChanged = true;
                        AdjustCurrencyForRemove(args.OldStartingIndex);
                    }
                    break;
 
                case NotifyCollectionChangedAction.Replace:
                    if (PassesFilter(args.OldItems[0]) || PassesFilter(args.NewItems[0]))
                    {
                        raiseChanged = true;
                        AdjustCurrencyForReplace(args.OldStartingIndex);
                    }
                    break;
 
                case NotifyCollectionChangedAction.Move:
                    if (PassesFilter(args.NewItems[0]))
                    {
                        raiseChanged = true;
                        AdjustCurrencyForMove(args.OldStartingIndex, args.NewStartingIndex);
                    }
                    break;
 
                case NotifyCollectionChangedAction.Reset:
                    // collection has completely changed
                    RefreshOrDefer();
                    return;     // Refresh already raises the event
            }
 
            // we've already returned if (args.Action == NotifyCollectionChangedAction.Reset) above
            if (raiseChanged)
                OnCollectionChanged(args);
 
            // currency has to change after firing the deletion event,
            // so event handlers have the right picture
            if (_currentElementWasRemovedOrReplaced)
            {
                MoveCurrencyOffDeletedElement();
                _currentElementWasRemovedOrReplaced = false;
            }
 
 
            // notify that the properties have changed.
            if (IsCurrentAfterLast != oldIsCurrentAfterLast)
                OnPropertyChanged(IsCurrentAfterLastPropertyName);
 
            if (IsCurrentBeforeFirst != oldIsCurrentBeforeFirst)
                OnPropertyChanged(IsCurrentBeforeFirstPropertyName);
 
            if (_currentPosition != oldCurrentPosition)
                OnPropertyChanged(CurrentPositionPropertyName);
 
            if (_currentItem != oldCurrentItem)
                OnPropertyChanged(CurrentItemPropertyName);
        }
 
        ///<summary>
        ///     Handle CollectionChanged events.
        ///
        ///     Calls ProcessCollectionChanged() or
        ///     posts the change to the Dispatcher to process on the correct thread.
        ///</summary>
        /// <remarks>
        ///     User should override <see cref="ProcessCollectionChanged"/>
        /// </remarks>
        /// <param name="sender">
        /// </param>
        /// <param name="args">
        /// </param>
        protected void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs args)
        {
            if (CheckFlag(CollectionViewFlags.ShouldProcessCollectionChanged))
            {
                if (!AllowsCrossThreadChanges)
                {
                    if (!CheckAccess())
                        throw new NotSupportedException(SR.MultiThreadedCollectionChangeNotSupported);
                    ProcessCollectionChanged(args);
                }
                else
                {
                    PostChange(args);
                }
            }
        }
 
        /// <summary>
        ///     This method is called when the value of AllowsCrossThreadChanges
        ///     is changed.   It gives a derived class an opportunity to
        ///     initialize its support for cross-thread changes (or to retire
        ///     that support).
        /// </summary>
        /// <notes>
        ///     This method will only be called if the application chooses to
        ///     change the synchronization information for a collection after
        ///     creating one or more collection views for that collection.
        ///     This is an unusual situation - most applications will register
        ///     the synchronization information for a collection before creating
        ///     any collection views, and never change it.   If so,
        ///     this method will not be called.
        /// </notes>
        protected virtual void OnAllowsCrossThreadChangesChanged()
        {
        }
 
        /// <summary>
        ///     Clear any pending changes.
        /// </summary>
        /// <notes>
        ///     When AllowsCrossThreadChanges is true, CollectionChanged events
        ///     received from the source collection are held in a buffer until
        ///     the Dispatcher's thread can process them.  A when a derived class
        ///     resets itself to the current content of the source collection
        ///     (e.g. during RefreshOverride), it should discard these pending
        ///     changes by calling this method.
        /// </notes>
        protected void ClearPendingChanges()
        {
            lock(_changeLog.SyncRoot)
            {
                _changeLog.Clear();
                _tempChangeLog.Clear();
            }
        }
 
        /// <summary>
        ///     Process all the pending changes
        /// </summary>
        /// <notes>
        ///     A derived class would call this method when it needs to bring
        ///     the collection view up-to-date with respect to the underlying
        ///     collection.  This is often required before modifying
        ///     the underlying collection (e.g. via AddNew, Remove, RemoveAt),
        ///     so that the derived class can supply the correct index.
        /// </notes>
        protected void ProcessPendingChanges()
        {
            lock(_changeLog.SyncRoot)
            {
                ProcessChangeLog(_changeLog, true);
                _changeLog.Clear();
            }
        }
 
        /// <summary>
        ///     Obsolete.   Retained for compatibility.
        ///     Use OnAllowsCrossThreadChangesChanged instead.
        /// </summary.
        /// <param name="args">
        ///     The NotifyCollectionChangedEventArgs that is added to the change log
        /// </param>
        [Obsolete("Replaced by OnAllowsCrossThreadChangesChanged")]
        protected virtual void OnBeginChangeLogging(NotifyCollectionChangedEventArgs args)
        {
        }
 
        /// <summary>
        ///     Obsolete.   Retained for compatibility.
        ///     Use ClearPendingChanges instead.
        /// </summary>
        [Obsolete("Replaced by ClearPendingChanges")]
        protected void ClearChangeLog()
        {
            ClearPendingChanges();
        }
 
        /// <summary>
        ///     Refresh, or mark that refresh is needed when defer cycle completes.
        /// </summary>
        protected void RefreshOrDefer()
        {
            if (IsRefreshDeferred)
            {
                SetFlag(CollectionViewFlags.NeedsRefresh, true);
            }
            else
            {
                RefreshInternal();
            }
        }
 
        #endregion Protected Methods
 
        //------------------------------------------------------
        //
        //  Protected Properties
        //
        //------------------------------------------------------
 
        #region Protected Properties
 
        /// <summary>
        ///     returns true if the underlying collection provides change notifications
        ///     the collection view is listening to the change events.
        /// </summary>
        protected bool IsDynamic
        {
            get
            {
                return CheckFlag(CollectionViewFlags.IsDynamic);
            }
        }
 
        /// <summary>
        ///     Returns true if this view supports CollectionChanged events raised
        ///     by the source collection on a foreign thread (a thread different
        ///     from the Dispatcher's thread).
        /// </summary>
        /// <notes>
        ///     The value of this property depends on the synchronization information
        ///     registered for the source collection via
        ///     BindingOperations.EnableCollectionSynchronization.
        ///     The value is set when the view is created.
        /// </notes>
        protected bool AllowsCrossThreadChanges
        {
            get { return CheckFlag(CollectionViewFlags.AllowsCrossThreadChanges); }
        }
 
        internal void SetAllowsCrossThreadChanges(bool value)
        {
            bool oldValue = CheckFlag(CollectionViewFlags.AllowsCrossThreadChanges);
            if (oldValue == value)
                return;
 
            SetFlag(CollectionViewFlags.AllowsCrossThreadChanges, value);
            OnAllowsCrossThreadChangesChanged();
        }
 
        /// <summary>
        ///     Obsolete.   Retained for compatibility.
        ///     Use AllowsCrossThreadChanges instead.
        /// </summary>
        protected bool UpdatedOutsideDispatcher
        {
            get { return AllowsCrossThreadChanges; }
        }
 
        /// <summary>
        /// IsRefreshDeferred returns true if there
        /// is still an outstanding DeferRefresh in
        /// use.  If at all possible, derived classes
        /// should not call Refresh if IsRefreshDeferred
        /// is true.
        /// </summary>
        protected bool IsRefreshDeferred
        {
            get
            {
                return _deferLevel > 0;
            }
        }
 
        /// <summary>
        /// IsCurrentInSync returns true if CurrentItem and CurrentPosition are
        /// up-to-date with the state and content of the collection.
        /// </summary>
        protected bool IsCurrentInSync
        {
            get
            {
                if (IsCurrentInView)
                    return GetItemAt(CurrentPosition) == CurrentItem;
                else
                    return CurrentItem == null;
            }
        }
 
        #endregion Protected Properties
 
        //------------------------------------------------------
        //
        //  Internal Methods
        //
        //------------------------------------------------------
 
        #region Internal Methods
 
        /// <summary>
        /// This method is for use by an agent that manages a set of
        /// one or more views.  Normal applications should not use it directly.
        /// </summary>
        /// <remarks>
        /// It is used to control the lifetime of the view, so that it gets
        /// garbage-collected at the right time.
        /// </remarks>
        internal void SetViewManagerData(object value)
        {
            object[] array;
 
            if (_vmData == null)
            {
                // 90% case - store a single value directly
                _vmData = value;
            }
            else if ((array = _vmData as object[]) == null)
            {
                // BindingListCollectionView appears in the table for both
                // DataTable and DataView - keep both references (bug 1745899)
                _vmData = new object[]{_vmData, value};
            }
            else
            {
                // in case a view is held by more than two tables, keep all
                // references.  This doesn't happen in current code, but there's
                // nothing preventing it, either.
                object[] newArray = new object[array.Length + 1];
                array.CopyTo(newArray, 0);
                newArray[array.Length] = value;
                _vmData = newArray;
            }
        }
 
 
        // determine whether the items have reliable hash codes
        internal virtual bool HasReliableHashCodes()
        {
            // default implementation - sample the first item
            return (IsEmpty || HashHelper.HasReliableHashCode(GetItemAt(0)));
        }
 
        // helper to validate that we are not in the middle of a DeferRefresh
        // and throw if that is the case.
        internal void VerifyRefreshNotDeferred()
        {
            #pragma warning disable 1634, 1691 // about to use PreSharp message numbers - unknown to C#
            #pragma warning disable 6503
 
            if (AllowsCrossThreadChanges)
                VerifyAccess();
 
            // If the Refresh is being deferred to change filtering or sorting of the
            // data by this CollectionView, then CollectionView will not reflect the correct
            // state of the underlying data.
 
            if (IsRefreshDeferred)
                throw new InvalidOperationException(SR.NoCheckOrChangeWhenDeferred);
 
            #pragma warning restore 6503
            #pragma warning restore 1634, 1691
        }
 
        internal void InvalidateEnumerableWrapper()
        {
            IndexedEnumerable wrapper = (IndexedEnumerable) Interlocked.Exchange(ref _enumerableWrapper, null);
            if (wrapper != null)
            {
                wrapper.Invalidate();
            }
        }
 
        internal ReadOnlyCollection<ItemPropertyInfo> GetItemProperties()
        {
            IEnumerable collection = SourceCollection;
            if (collection == null)
                return null;
 
            IEnumerable properties = null;
 
            ITypedList itl = collection as ITypedList;
            Type itemType;
            object item;
 
            if (itl != null)
            {
                // ITypedList has the information
                properties = itl.GetItemProperties(null);
            }
            else if ((itemType = GetItemType(false)) != null)
            {
                // If we know the item type, use its properties.
                properties = TypeDescriptor.GetProperties(itemType);
            }
            else if ((item = GetRepresentativeItem()) != null)
            {
                // If we have a representative item, use its properties.
                // It's cheaper to use the item type, but we cannot do that
                // when all we know is a representative item.  If the item
                // has synthetic properties (via ICustomTypeDescriptor or
                // TypeDescriptorProvider), they don't show up on the type -
                // only on the item.
                ICustomTypeProvider ictp = item as ICustomTypeProvider;
                if (ictp == null)
                {
                    properties = TypeDescriptor.GetProperties(item);
                }
                else
                {
                    properties = ictp.GetCustomType().GetProperties();
                }
            }
 
            if (properties == null)
                return null;
 
            // convert the properties to ItemPropertyInfo
            List<ItemPropertyInfo> list = new List<ItemPropertyInfo>();
            foreach (object property in properties)
            {
                PropertyDescriptor pd;
                PropertyInfo pi;
 
                if ((pd = property as PropertyDescriptor) != null)
                {
                    list.Add(new ItemPropertyInfo(pd.Name, pd.PropertyType, pd));
                }
                else if ((pi = property as PropertyInfo) != null)
                {
                    list.Add(new ItemPropertyInfo(pi.Name, pi.PropertyType, pi));
                }
            }
 
            // return the result as a read-only collection
            return new ReadOnlyCollection<ItemPropertyInfo>(list);
        }
 
        internal Type GetItemType(bool useRepresentativeItem)
        {
            Type collectionType = SourceCollection.GetType();
            Type[] interfaces = collectionType.GetInterfaces();
 
            // Look for IEnumerable<T>.  All generic collections should implement
            // this.  We loop through the interface list, rather than call
            // GetInterface(IEnumerableT), so that we handle an ambiguous match
            // (by using the first match) without an exception.
            for (int i=0; i<interfaces.Length; ++i)
            {
                Type interfaceType = interfaces[i];
 
                if (interfaceType.Name == IEnumerableT)
                {
                    // found IEnumerable<>, extract T
                    Type[] typeParameters = interfaceType.GetGenericArguments();
                    if (typeParameters.Length == 1)
                    {
                        Type type = typeParameters[0];
 
                        if (typeof(ICustomTypeProvider).IsAssignableFrom(type))
                        {
                            // if the item type can point to a custom type
                            // for reflection, we need the custom type.
                            // We can only get it from a representative item.
                            break;
                        }
 
                        if (type == typeof(Object))
                        {
                            // IEnumerable<Object> is useless;  we need a representative
                            // item.   But keep going - perhaps IEnumerable<T> shows up later.
                            continue;
                        }
 
                        return type;
                    }
                }
            }
 
            // No generic information found.  Use a representative item instead.
            if (useRepresentativeItem)
            {
                // get type of a representative item
                object item = GetRepresentativeItem();
                return System.Windows.Markup.ReflectionHelper.GetReflectionType(item);
            }
 
            return null;
        }
 
        internal object GetRepresentativeItem()
        {
            if (IsEmpty)
                return null;
 
            object result = null;
            IEnumerator ie = this.GetEnumerator();
            while (ie.MoveNext())
            {
                object item = ie.Current;
                if (item != null && item != NewItemPlaceholder)
                {
                    result = item;
                    break;
                }
            }
 
            IDisposable d = ie as IDisposable;
            if (d != null)
            {
                d.Dispose();
            }
 
            return result;
        }
 
        internal virtual void GetCollectionChangedSources(int level, Action<int, object, bool?, List<string>> format, List<string> sources)
        {
            format(level, this, null, sources);
            if (_sourceCollection != null)
            {
                format(level+1, _sourceCollection, null, sources);
            }
        }
 
        #endregion Internal Methods
 
 
        //------------------------------------------------------
        //
        //  Internal Properties
        //
        //------------------------------------------------------
 
        #region Internal Properties
 
        internal object SyncRoot
        {
            get { return _syncObject; }
        }
 
        // Timestamp is used by the PlaceholderAwareEnumerator to determine if a
        // collection change has occurred since the enumerator began.  (If so,
        // MoveNext should throw.)
        internal int Timestamp
        {
            get { return _timestamp; }
        }
 
        #endregion Internal Properties
 
        //------------------------------------------------------
        //
        //  Internal Types
        //
        //------------------------------------------------------
 
        #region Internal Types
 
        internal class PlaceholderAwareEnumerator : IEnumerator
        {
            enum Position { BeforePlaceholder, OnPlaceholder, OnNewItem, AfterPlaceholder}
 
            public PlaceholderAwareEnumerator(CollectionView collectionView, IEnumerator baseEnumerator, NewItemPlaceholderPosition placeholderPosition, object newItem)
            {
                _collectionView = collectionView;
                _timestamp = collectionView.Timestamp;
                _baseEnumerator = baseEnumerator;
                _placeholderPosition = placeholderPosition;
                _newItem = newItem;
            }
 
            public bool MoveNext()
            {
                if (_timestamp != _collectionView.Timestamp)
                    throw new InvalidOperationException(SR.EnumeratorVersionChanged);
 
                switch (_position)
                {
                    case Position.BeforePlaceholder:
                        // AtBeginning - move to the placeholder
                        if (_placeholderPosition == NewItemPlaceholderPosition.AtBeginning)
                        {
                            _position = Position.OnPlaceholder;
                        }
                        // None or AtEnd - advance base, skipping the new item
                        else if (_baseEnumerator.MoveNext() &&
                                    (_newItem == NoNewItem || _baseEnumerator.Current != _newItem
                                            || _baseEnumerator.MoveNext()))
                        {
                        }
                        // if base has reached the end, move to new item or placeholder
                        else if (_newItem != NoNewItem)
                        {
                            _position = Position.OnNewItem;
                        }
                        else if (_placeholderPosition == NewItemPlaceholderPosition.None)
                        {
                            return false;
                        }
                        else
                        {
                            _position = Position.OnPlaceholder;
                        }
                        return true;
 
                    case Position.OnPlaceholder:
                        // AtBeginning - move from placeholder to new item (if present)
                        if (_newItem != NoNewItem && _placeholderPosition == NewItemPlaceholderPosition.AtBeginning)
                        {
                            _position = Position.OnNewItem;
                            return true;
                        }
                        break;
 
                    case Position.OnNewItem:
                        // AtEnd - move from new item to placeholder
                        if (_placeholderPosition == NewItemPlaceholderPosition.AtEnd)
                        {
                            _position = Position.OnPlaceholder;
                            return true;
                        }
                        break;
                }
 
                // in all other cases, simply advance base, skipping the new item
                _position = Position.AfterPlaceholder;
                return (_baseEnumerator.MoveNext() &&
                            (_newItem == NoNewItem || _baseEnumerator.Current != _newItem
                                            || _baseEnumerator.MoveNext()));
            }
 
            public object Current
            {
                get
                {
                    return  (_position == Position.OnPlaceholder) ? CollectionView.NewItemPlaceholder
                        :   (_position == Position.OnNewItem)   ?   _newItem
                        :                                           _baseEnumerator.Current;
                }
            }
 
            public void Reset()
            {
                _position = Position.BeforePlaceholder;
                _baseEnumerator.Reset();
            }
 
            CollectionView _collectionView;
            IEnumerator _baseEnumerator;
            NewItemPlaceholderPosition _placeholderPosition;
            Position _position;
            object _newItem;
            int _timestamp;
        }
 
        #endregion Internal Types
 
        //------------------------------------------------------
        //
        //  Private Properties
        //
        //------------------------------------------------------
 
        #region Private Properties
 
        private bool IsCurrentInView
        {
            get
            {
                VerifyRefreshNotDeferred();
                return (0 <= CurrentPosition && CurrentPosition < Count);
            }
        }
 
        private IndexedEnumerable EnumerableWrapper
        {
            get
            {
                if (_enumerableWrapper == null)
                {
                    IndexedEnumerable newWrapper = new IndexedEnumerable(SourceCollection, new Predicate<object>(this.PassesFilter));
                    Interlocked.CompareExchange(ref _enumerableWrapper, newWrapper, null);
                }
 
                return _enumerableWrapper;
            }
        }
 
        #endregion Private Properties
 
        //------------------------------------------------------
        //
        //  Private Methods
        //
        //------------------------------------------------------
 
        #region Private Methods
 
        // Just move it.  No argument check, no events, just move current to position.
        private void _MoveCurrentToPosition(int position)
        {
            if (position < 0)
            {
                SetFlag(CollectionViewFlags.IsCurrentBeforeFirst, true);
                SetCurrent(null, -1);
            }
            else if (position >= Count)
            {
                SetFlag(CollectionViewFlags.IsCurrentAfterLast, true);
                SetCurrent(null, Count);
            }
            else
            {
                SetFlag(CollectionViewFlags.IsCurrentBeforeFirst | CollectionViewFlags.IsCurrentAfterLast, false);
                SetCurrent(EnumerableWrapper[position], position);
            }
        }
 
        private void MoveCurrencyOffDeletedElement()
        {
            int lastPosition = Count - 1;
            // if position falls beyond last position, move back to last position
            int newPosition = (_currentPosition < lastPosition) ? _currentPosition : lastPosition;
 
            // ignore cancel, there's no choice in this currency change
            OnCurrentChanging();
            _MoveCurrentToPosition(newPosition);
            OnCurrentChanged();
        }
 
        private void EndDefer()
        {
            -- _deferLevel;
 
            if (_deferLevel == 0 && CheckFlag(CollectionViewFlags.NeedsRefresh))
            {
                Refresh();
            }
        }
 
        /// <summary>
        ///     DeferProcessing is to be called from OnCollectionChanged by derived classes  that
        ///     wish to process the remainder of a changeLog after allowing other events to be
        ///     processed.
        /// </summary>
        /// <param name="changeLog">
        ///     ArrayList of NotifyCollectionChangedEventArgs that could not be precessed.
        /// </param>
        private void DeferProcessing(ICollection changeLog)
        {
            Debug.Assert(changeLog != null && changeLog.Count > 0, "don't defer when there's no work");
 
            lock(SyncRoot)
            {
                lock(_changeLog.SyncRoot)
                {
                    if (_changeLog == null)
                    {
                        _changeLog = new ArrayList(changeLog);
                    }
                    else
                    {
                        _changeLog.InsertRange(0, changeLog);
                    }
 
                    if (_databindOperation != null)
                    {
                        _engine.ChangeCost(_databindOperation, changeLog.Count);
                    }
                    else
                    {
                        _databindOperation = _engine.Marshal(new DispatcherOperationCallback(ProcessInvoke), null, changeLog.Count);
                    }
                }
            }
        }
 
        /// <summary>
        ///     Must be implemented by the derived classes to process changes on the
        ///     UI thread.  Called by ProcessInvoke wich is called by the Dispatcher, so
        ///     the UI thread will have allready been entered by now.
        /// </summary>
        /// <param name="changeLog">
        ///     List of NotifyCollectionChangedEventArgs that is to be processed.
        /// </param>
        private ICollection ProcessChangeLog(ArrayList changeLog, bool processAll=false)
        {
            int currentIndex = 0;
            bool mustDeferProcessing = false;
            long beginTime = DateTime.Now.Ticks;
            int startCount = changeLog.Count;
 
            for ( ; currentIndex < changeLog.Count && !(mustDeferProcessing); currentIndex++)
            {
                NotifyCollectionChangedEventArgs args = changeLog[currentIndex] as NotifyCollectionChangedEventArgs;
 
                if (args != null)
                {
                    ProcessCollectionChanged(args);
                }
 
                if (!processAll)
                {
                    mustDeferProcessing = DateTime.Now.Ticks - beginTime > DataBindEngine.CrossThreadThreshold;
                }
            }
 
            if (mustDeferProcessing && currentIndex < changeLog.Count)
            {
                // create an unprocessed subset of changeLog
                changeLog.RemoveRange(0,currentIndex);
                return changeLog;
            }
 
            return null;
        }
 
        // returns true if ANY flag in flags is set.
        private bool CheckFlag(CollectionViewFlags flags)
        {
            return (_flags & flags) != 0;
        }
 
        private void SetFlag(CollectionViewFlags flags, bool value)
        {
            if (value)
            {
                _flags = _flags | flags;
            }
            else
            {
                _flags = _flags & ~flags;
            }
        }
 
        // Post a change on the UI thread Dispatcher and updated the _changeLog.
        private void PostChange(NotifyCollectionChangedEventArgs args)
        {
            lock(SyncRoot)
            {
                lock(_changeLog.SyncRoot)
                {
                    // we can ignore everything before a Reset
                    if (args.Action == NotifyCollectionChangedAction.Reset)
                    {
                        _changeLog.Clear();
                    }
 
                    if (_changeLog.Count == 0 && CheckAccess())
                    {
                        // when a change arrives on the UI thread and there are
                        // no pending cross-thread changes, process the event
                        // synchronously.   This is important for editing operations
                        // (AddNew, Remove), which expect to get notified about
                        // the changes they make directly.
                        ProcessCollectionChanged(args);
                    }
                    else
                    {
                        // the change (or another pending change) arrived on the
                        // wrong thread.  Marshal it to the UI thread.
                        _changeLog.Add(args);
 
                        if(_databindOperation == null)
                        {
                            _databindOperation = _engine.Marshal(
                                new DispatcherOperationCallback(ProcessInvoke),
                                null, _changeLog.Count);
                        }
                    }
                }
            }
        }
 
        // Callback that is passed to Dispatcher.BeginInvoke in PostChange
        private object ProcessInvoke(object arg)
        {
            // work on a private copy of the change log, so that other threads
            // can add to the main change log
            lock(SyncRoot)
            {
                lock(_changeLog.SyncRoot)
                {
                    _databindOperation = null;
                    _tempChangeLog = _changeLog;
                    _changeLog = new ArrayList();
                }
            }
 
            // process the changes
            ICollection unprocessedChanges = ProcessChangeLog(_tempChangeLog);
 
            // if changes remain (because we ran out of time), reschedule them
            if (unprocessedChanges != null && unprocessedChanges.Count > 0)
            {
                DeferProcessing(unprocessedChanges);
            }
 
            _tempChangeLog = EmptyArrayList;
 
            return null;
        }
 
        private void ValidateCollectionChangedEventArgs(NotifyCollectionChangedEventArgs e)
        {
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add:
                    if (e.NewItems.Count != 1)
                        throw new NotSupportedException(SR.RangeActionsNotSupported);
                    break;
 
                case NotifyCollectionChangedAction.Remove:
                    if (e.OldItems.Count != 1)
                        throw new NotSupportedException(SR.RangeActionsNotSupported);
                    if (e.OldStartingIndex < 0)
                        throw new InvalidOperationException(SR.RemovedItemNotFound);
                    break;
 
                case NotifyCollectionChangedAction.Replace:
                    if (e.NewItems.Count != 1 || e.OldItems.Count != 1)
                        throw new NotSupportedException(SR.RangeActionsNotSupported);
                    break;
 
                case NotifyCollectionChangedAction.Move:
                    if (e.NewItems.Count != 1)
                        throw new NotSupportedException(SR.RangeActionsNotSupported);
                    if (e.NewStartingIndex < 0)
                        throw new InvalidOperationException(SR.CannotMoveToUnknownPosition);
                    break;
 
                case NotifyCollectionChangedAction.Reset:
                    break;
 
                default:
                    throw new NotSupportedException(SR.Format(SR.UnexpectedCollectionChangeAction, e.Action));
            }
        }
 
        // fix up CurrentPosition and CurrentItem after a collection change
        private void AdjustCurrencyForAdd(int index)
        {
            // adjust current index if insertion is earlier
            if (Count == 1)
                _currentPosition = -1;
            else if (index <= _currentPosition)
            {
                ++_currentPosition;
 
                if (_currentPosition < Count)
                {
                    _currentItem = EnumerableWrapper[_currentPosition];
                }
            }
        }
 
        // Fix up CurrentPosition and CurrentItem after an item was removed.
        private void AdjustCurrencyForRemove(int index)
        {
            // adjust current index if deletion is earlier
            if (index < _currentPosition)
                --_currentPosition;
 
            // move currency off the deleted element
            else if (index == _currentPosition)
            {
                _currentElementWasRemovedOrReplaced = true;
            }
        }
 
        // Fix up CurrentPosition and CurrentItem after an item was moved.
        private void AdjustCurrencyForMove(int oldIndex, int newIndex)
        {
            // if entire move was before or after current item, then there
            // is nothing that needs to be done.
            if ((oldIndex < CurrentPosition && newIndex < CurrentPosition)
                || (oldIndex > CurrentPosition && newIndex > CurrentPosition))
                return;
 
            if (oldIndex <= CurrentPosition)
                AdjustCurrencyForRemove(oldIndex);
            else if (newIndex <= CurrentPosition)
                AdjustCurrencyForAdd(newIndex);
        }
 
 
        // fix up CurrentPosition and CurrentItem after a collection change
        private void AdjustCurrencyForReplace(int index)
        {
            // CurrentItem was replaced
            if (index == _currentPosition)
            {
                _currentElementWasRemovedOrReplaced = true;
            }
        }
 
        /// <summary>
        /// Helper to raise a PropertyChanged event  />).
        /// </summary>
        private void OnPropertyChanged(string propertyName)
        {
            OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
        }
 
        #endregion Private Methods
 
 
        //------------------------------------------------------
        //
        //  Private Types
        //
        //------------------------------------------------------
 
        #region Private Types
 
 
        private class DeferHelper : IDisposable
        {
            public DeferHelper(CollectionView collectionView)
            {
                _collectionView = collectionView;
            }
 
            public void Dispose()
            {
                if (_collectionView != null)
                {
                    _collectionView.EndDefer();
                    _collectionView = null;
                }
 
                GC.SuppressFinalize(this);
            }
 
            private CollectionView _collectionView;
        }
 
 
        // this class helps prevent reentrant calls
        private class SimpleMonitor : IDisposable
        {
            public bool Enter()
            {
                if (_entered)
                    return false;
 
                _entered = true;
                return true;
            }
 
            public void Dispose()
            {
                _entered = false;
                GC.SuppressFinalize(this);
            }
 
            public bool Busy { get { return _entered; } }
 
            bool _entered;
        }
 
        [Flags]
        private enum CollectionViewFlags
        {
            UpdatedOutsideDispatcher        =   0x2,
            ShouldProcessCollectionChanged  =   0x4,
            IsCurrentBeforeFirst            =   0x8,
            IsCurrentAfterLast              =   0x10,
            IsDynamic                       =   0x20,
            IsDataInGroupOrder              =   0x40,
            NeedsRefresh                    =   0x80,
            AllowsCrossThreadChanges        =   0x100,
            CachedIsEmpty                   =   0x200,
        }
 
 
        #endregion Private Types
 
        //------------------------------------------------------
        //
        //  Private Fields
        //
        //------------------------------------------------------
        #region Private Fields
 
        ArrayList               _changeLog = new ArrayList();
        ArrayList               _tempChangeLog = EmptyArrayList;
        DataBindOperation       _databindOperation;
        object                  _vmData;            // view manager's private data
        IEnumerable             _sourceCollection;  // the underlying collection
        CultureInfo             _culture;           // culture to use when sorting
        SimpleMonitor           _currentChangedMonitor = new SimpleMonitor();
        int                     _deferLevel;
        IndexedEnumerable       _enumerableWrapper;
        Predicate<object>       _filter;
        object                  _currentItem;
        int                     _currentPosition;
        CollectionViewFlags     _flags = CollectionViewFlags.ShouldProcessCollectionChanged |
                                        CollectionViewFlags.NeedsRefresh;
        bool                    _currentElementWasRemovedOrReplaced;
        static object           _newItemPlaceholder = new NamedObject("NewItemPlaceholder");
        object                  _syncObject = new object();
        DataBindEngine          _engine;
        int                     _timestamp;
        static readonly ArrayList EmptyArrayList = new ArrayList();
        static readonly string IEnumerableT = typeof(IEnumerable<>).Name;
        internal static readonly object NoNewItem = new NamedObject("NoNewItem");
 
        // since there's nothing in the uncancelable event args that is mutable,
        // just create one instance to be used universally.
        static readonly CurrentChangingEventArgs uncancelableCurrentChangingEventArgs = new CurrentChangingEventArgs(false);
 
        internal const string CountPropertyName = "Count";
        internal const string IsEmptyPropertyName = "IsEmpty";
        internal const string CulturePropertyName = "Culture";
        internal const string CurrentPositionPropertyName = "CurrentPosition";
        internal const string CurrentItemPropertyName = "CurrentItem";
        internal const string IsCurrentBeforeFirstPropertyName = "IsCurrentBeforeFirst";
        internal const string IsCurrentAfterLastPropertyName = "IsCurrentAfterLast";
 
        #endregion Private Fields
    }
}