// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
//
// Description: allows the author of a IValueConverter
//              to specify what source and target property types the 
//              ValueConverter is capable of converting
//
// Specs:       Data Binding.mht
//
 
namespace System.Windows.Data
{
    /// <summary>
    ///     This attribute allows the author of a <seealso cref="System.Windows.Data.IValueConverter"/>
    ///     to specify what source and target property types the ValueConverter is capable of converting.
    ///     This meta data is useful for designer tools to help categorize and match ValueConverters.
    /// </summary>
    /// <remarks>
    /// Add this custom attribute to your IValueConverter class definition.
    /// <code>
    ///     [ValueConversion(typeof(Employee), typeof(Brush))]
    ///     class MyConverter : IValueConverter
    ///     {
    ///         public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    ///         {
    ///             if (value is Dev)       return Brushes.Beige;
    ///             if (value is Employee)  return Brushes.Salmon;
    ///             return Brushes.Yellow;
    ///         }
    ///     }
    /// </code>
    /// </remarks>
    [AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
    public sealed class ValueConversionAttribute : Attribute
    {
        //------------------------------------------------------
        //
        //  Constructors
        //
        //------------------------------------------------------
 
        /// <summary>
        ///     Creates a new ValueConversionAttribute to indicate between 
        ///     what types of a data binding source and target this ValueConverter can convert.
        /// </summary>
        /// <param name="sourceType">the expected source type this ValueConverter can handle</param>
        /// <param name="targetType">the target type to which this ValueConverter can convert to</param>
        public ValueConversionAttribute(Type sourceType, Type targetType)
        {
            ArgumentNullException.ThrowIfNull(sourceType);
            ArgumentNullException.ThrowIfNull(targetType);
            _sourceType = sourceType;
            _targetType = targetType;
        }
 
        /// <summary>
        ///     The expected source type this ValueConverter can handle.
        /// </summary>
        public Type SourceType
        {
            get { return _sourceType; }
        }
 
        /// <summary>
        ///     The target type to which this ValueConverter can convert to.
        /// </summary>
        public Type TargetType
        {
            get { return _targetType; }
        }
 
        /// <summary>
        ///     The type of the optional ValueConverter Parameter object.
        /// </summary>
        public Type ParameterType
        {
            get { return _parameterType; }
            set { _parameterType = value; }
        }
        
        ///<summary>
        ///     Returns the unique identifier for this Attribute.
        ///</summary>
        // Type ID is used to remove redundant attributes by 
        // putting all attributes in a dictionary of [TypeId, Attribute].
        // If you want AllowMultiple attributes to work with designers,
        // you must override TypeId.  The default implementation returns
        // this.GetType(), which is appropriate for AllowMultiple = false, but
        // not for AllowMultiple = true;
        public override object TypeId
        {
             // the attribute itself will be used as a key to the dictionary
            get { return this; }
        }
 
        ///<summary>
        ///     Returns the hash code for this instance.
        ///</summary>
        public override int GetHashCode()
        {
            // the default implementation does some funky enumeration over its fields
            // we can do better and use the 2 mandatory fields source/targetType's hash codes
            return _sourceType.GetHashCode() + _targetType.GetHashCode();
        }
 
 
        //--------------------------------------------
        // Private members
        //--------------------------------------------
        private Type _sourceType;
        private Type _targetType;
        private Type _parameterType;
    }
}