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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Collections.Generic;
using System.Diagnostics;
namespace System.Linq
{
public static partial class Enumerable
{
public static TSource First<TSource>(this IEnumerable<TSource> source)
{
TSource? first = source.TryGetFirst(out bool found);
if (!found)
{
ThrowHelper.ThrowNoElementsException();
}
return first!;
}
public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
TSource? first = source.TryGetFirst(predicate, out bool found);
if (!found)
{
ThrowHelper.ThrowNoMatchException();
}
return first!;
}
public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source) =>
source.TryGetFirst(out _);
/// <summary>Returns the first element of a sequence, or a default value if the sequence contains no elements.</summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <param name="source">The <see cref="IEnumerable{T}" /> to return the first element of.</param>
/// <param name="defaultValue">The default value to return if the sequence is empty.</param>
/// <returns><paramref name="defaultValue" /> if <paramref name="source" /> is empty; otherwise, the first element in <paramref name="source" />.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, TSource defaultValue)
{
TSource? first = source.TryGetFirst(out bool found);
return found ? first! : defaultValue;
}
public static TSource? FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) =>
source.TryGetFirst(predicate, out _);
/// <summary>Returns the first element of the sequence that satisfies a condition or a default value if no such element is found.</summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <param name="source">An <see cref="IEnumerable{T}" /> to return an element from.</param>
/// <param name="predicate">A function to test each element for a condition.</param>
/// <param name="defaultValue">The default value to return if the sequence is empty.</param>
/// <returns><paramref name="defaultValue" /> if <paramref name="source" /> is empty or if no element passes the test specified by <paramref name="predicate" />; otherwise, the first element in <paramref name="source" /> that passes the test specified by <paramref name="predicate" />.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> or <paramref name="predicate" /> is <see langword="null" />.</exception>
public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, TSource defaultValue)
{
TSource? first = source.TryGetFirst(predicate, out bool found);
return found ? first! : defaultValue;
}
private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, out bool found)
{
if (source is null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
}
return
#if !OPTIMIZE_FOR_SIZE
source is Iterator<TSource> iterator ? iterator.TryGetFirst(out found) :
#endif
TryGetFirstNonIterator(source, out found);
}
private static TSource? TryGetFirstNonIterator<TSource>(IEnumerable<TSource> source, out bool found)
{
if (source is IList<TSource> list)
{
if (list.Count > 0)
{
found = true;
return list[0];
}
}
else
{
using (IEnumerator<TSource> e = source.GetEnumerator())
{
if (e.MoveNext())
{
found = true;
return e.Current;
}
}
}
found = false;
return default;
}
private static TSource? TryGetFirst<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate, out bool found)
{
if (source is null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
}
if (predicate is null)
{
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
}
if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
{
foreach (TSource element in span)
{
if (predicate(element))
{
found = true;
return element;
}
}
}
else
{
foreach (TSource element in source)
{
if (predicate(element))
{
found = true;
return element;
}
}
}
found = false;
return default;
}
}
}
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