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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.Threading;
using System.Threading.Tasks;
namespace System.Linq
{
public static partial class AsyncEnumerable
{
/// <summary>
/// Returns the only element of a sequence, and throws an exception if there is not
/// exactly one element in the sequence.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}"/> to return the single element of.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>The single element of the input sequence.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source"/> is <see langword="null"/>.</exception>
/// <exception cref="InvalidOperationException">The <paramref name="source"/> sequence is empty (via the returned task).</exception>
/// <exception cref="InvalidOperationException">The <paramref name="source"/> sequence contains more than one element. (via the returned task).</exception>
public static ValueTask<TSource> SingleAsync<TSource>(
this IAsyncEnumerable<TSource> source,
CancellationToken cancellationToken = default)
{
ArgumentNullException.ThrowIfNull(source);
return Impl(source, cancellationToken);
static async ValueTask<TSource> Impl(
IAsyncEnumerable<TSource> source, CancellationToken cancellationToken)
{
await using IAsyncEnumerator<TSource> e = source.GetAsyncEnumerator(cancellationToken);
if (!await e.MoveNextAsync())
{
ThrowHelper.ThrowNoElementsException();
}
TSource result = e.Current;
if (await e.MoveNextAsync())
{
ThrowHelper.ThrowMoreThanOneElementException();
}
return result;
}
}
/// <summary>
/// Returns the only element of a sequence that satisfies a specified condition,
/// and throws an exception if more than one such element exists.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}"/> to return the single element of.</param>
/// <param name="predicate">A function to test an element for a condition.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>The single element of the input sequence that satisfies a condition.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source"/> is <see langword="null"/>.</exception>
/// <exception cref="ArgumentNullException"><paramref name="predicate" /> is <see langword="null" />.</exception>
/// <exception cref="InvalidOperationException">The <paramref name="source"/> sequence is empty (via the returned task).</exception>
/// <exception cref="InvalidOperationException">No element satisfies the condition in <paramref name="predicate"/> (via the returned task).</exception>
/// <exception cref="InvalidOperationException">More than one element satisfies the condition in <paramref name="predicate"/> (via the returned task).</exception>
public static ValueTask<TSource> SingleAsync<TSource>(
this IAsyncEnumerable<TSource> source,
Func<TSource, bool> predicate,
CancellationToken cancellationToken = default)
{
ArgumentNullException.ThrowIfNull(source);
ArgumentNullException.ThrowIfNull(predicate);
return Impl(source, predicate, cancellationToken);
static async ValueTask<TSource> Impl(
IAsyncEnumerable<TSource> source,
Func<TSource, bool> predicate,
CancellationToken cancellationToken)
{
await using IAsyncEnumerator<TSource> e = source.GetAsyncEnumerator(cancellationToken);
while (await e.MoveNextAsync())
{
TSource result = e.Current;
if (predicate(result))
{
while (await e.MoveNextAsync())
{
if (predicate(e.Current))
{
ThrowHelper.ThrowMoreThanOneMatchException();
}
}
return result;
}
}
ThrowHelper.ThrowNoElementsException();
return default!; // Unreachable
}
}
/// <summary>
/// Returns the only element of a sequence that satisfies a specified condition,
/// and throws an exception if more than one such element exists.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}"/> to return the single element of.</param>
/// <param name="predicate">A function to test an element for a condition.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>The single element of the input sequence that satisfies a condition.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source"/> is <see langword="null"/>.</exception>
/// <exception cref="ArgumentNullException"><paramref name="predicate" /> is <see langword="null" />.</exception>
/// <exception cref="InvalidOperationException">The <paramref name="source"/> sequence is empty (via the returned task).</exception>
/// <exception cref="InvalidOperationException">No element satisfies the condition in <paramref name="predicate"/> (via the returned task).</exception>
/// <exception cref="InvalidOperationException">More than one element satisfies the condition in <paramref name="predicate"/> (via the returned task).</exception>
public static ValueTask<TSource> SingleAsync<TSource>(
this IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<bool>> predicate,
CancellationToken cancellationToken = default)
{
ArgumentNullException.ThrowIfNull(source);
ArgumentNullException.ThrowIfNull(predicate);
return Impl(source, predicate, cancellationToken);
static async ValueTask<TSource> Impl(
IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<bool>> predicate,
CancellationToken cancellationToken)
{
await using IAsyncEnumerator<TSource> e = source.GetAsyncEnumerator(cancellationToken);
while (await e.MoveNextAsync())
{
TSource result = e.Current;
if (await predicate(result, cancellationToken))
{
while (await e.MoveNextAsync())
{
if (await predicate(e.Current, cancellationToken))
{
ThrowHelper.ThrowMoreThanOneMatchException();
}
}
return result;
}
}
ThrowHelper.ThrowNoElementsException();
return default!; // Unreachable
}
}
/// <summary>
/// Returns the only element of a sequence, or a default value if the sequence is
/// empty; this method throws an exception if there is more than one element in the sequence.
/// </summary>
/// <typeparam name="TSource">The type of the elements of source.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}"/> to return the single element of.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>
/// The single element of the input sequence, or the default value of <typeparamref name="TSource"/>
/// if the sequence contains no elements.
/// </returns>
/// <exception cref="ArgumentNullException"><paramref name="source"/> is <see langword="null"/>.</exception>
/// <exception cref="InvalidOperationException">The <paramref name="source"/> sequence contains more than one element. (via the returned task).</exception>
public static ValueTask<TSource?> SingleOrDefaultAsync<TSource>(
this IAsyncEnumerable<TSource> source,
CancellationToken cancellationToken = default) =>
SingleOrDefaultAsync(source, default(TSource), cancellationToken);
/// <summary>Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.</summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}" /> to return the single element of.</param>
/// <param name="defaultValue">The default value to return if the sequence is empty.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>The single element of the input sequence, or <paramref name="defaultValue" /> if the sequence contains no elements.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <exception cref="InvalidOperationException">The input sequence contains more than one element.</exception>
public static ValueTask<TSource> SingleOrDefaultAsync<TSource>(
this IAsyncEnumerable<TSource> source,
TSource defaultValue,
CancellationToken cancellationToken = default)
{
ArgumentNullException.ThrowIfNull(source);
return Impl(source, defaultValue, cancellationToken);
static async ValueTask<TSource> Impl(
IAsyncEnumerable<TSource> source,
TSource defaultValue,
CancellationToken cancellationToken)
{
await using IAsyncEnumerator<TSource> e = source.GetAsyncEnumerator(cancellationToken);
if (!await e.MoveNextAsync())
{
return defaultValue;
}
TSource result = e.Current;
if (await e.MoveNextAsync())
{
ThrowHelper.ThrowMoreThanOneElementException();
}
return result;
}
}
/// <summary>
/// Returns the only element of a sequence that satisfies a specified condition or
/// a default value if no such element exists; this method throws an exception if
/// more than one element satisfies the condition.
/// </summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}" /> to return the single element of.</param>
/// <param name="predicate">A function to test an element for a condition.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>
/// The single element of the input sequence that satisfies the condition, or the default value of
/// <typeparamref name="TSource"/> if no such element is found.
/// </returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <exception cref="ArgumentNullException"><paramref name="predicate" /> is <see langword="null" />.</exception>
/// <exception cref="InvalidOperationException">The input sequence contains more than one element.</exception>
public static ValueTask<TSource?> SingleOrDefaultAsync<TSource>(
this IAsyncEnumerable<TSource> source,
Func<TSource, bool> predicate,
CancellationToken cancellationToken = default) =>
SingleOrDefaultAsync(source, predicate!, default, cancellationToken);
/// <summary>
/// Returns the only element of a sequence that satisfies a specified condition or
/// a default value if no such element exists; this method throws an exception if
/// more than one element satisfies the condition.
/// </summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}" /> to return the single element of.</param>
/// <param name="predicate">A function to test an element for a condition.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>
/// The single element of the input sequence that satisfies the condition, or the default value of
/// <typeparamref name="TSource"/> if no such element is found.
/// </returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <exception cref="ArgumentNullException"><paramref name="predicate" /> is <see langword="null" />.</exception>
/// <exception cref="InvalidOperationException">The input sequence contains more than one element.</exception>
public static ValueTask<TSource?> SingleOrDefaultAsync<TSource>(
this IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<bool>> predicate,
CancellationToken cancellationToken = default) =>
SingleOrDefaultAsync(source, predicate!, default, cancellationToken);
/// <summary>Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.</summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}" /> to return a single element from.</param>
/// <param name="predicate">A function to test an element for a condition.</param>
/// <param name="defaultValue">The default value to return if the sequence is empty.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>The single element of the input sequence that satisfies the condition, or <paramref name="defaultValue" /> if no such element is found.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <exception cref="ArgumentNullException"><paramref name="predicate" /> is <see langword="null" />.</exception>
/// <exception cref="InvalidOperationException">More than one element satisfies the condition in <paramref name="predicate" />.</exception>
public static ValueTask<TSource> SingleOrDefaultAsync<TSource>(
this IAsyncEnumerable<TSource> source,
Func<TSource, bool> predicate,
TSource defaultValue,
CancellationToken cancellationToken = default)
{
ArgumentNullException.ThrowIfNull(source);
ArgumentNullException.ThrowIfNull(predicate);
return Impl(source, predicate, defaultValue, cancellationToken);
static async ValueTask<TSource> Impl(
IAsyncEnumerable<TSource> source,
Func<TSource, bool> predicate,
TSource defaultValue,
CancellationToken cancellationToken)
{
await using IAsyncEnumerator<TSource> e = source.GetAsyncEnumerator(cancellationToken);
while (await e.MoveNextAsync())
{
TSource result = e.Current;
if (predicate(result))
{
while (await e.MoveNextAsync())
{
if (predicate(e.Current))
{
ThrowHelper.ThrowMoreThanOneMatchException();
}
}
return result;
}
}
return defaultValue;
}
}
/// <summary>Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.</summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <param name="source">An <see cref="IAsyncEnumerable{T}" /> to return a single element from.</param>
/// <param name="predicate">A function to test an element for a condition.</param>
/// <param name="defaultValue">The default value to return if the sequence is empty.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>The single element of the input sequence that satisfies the condition, or <paramref name="defaultValue" /> if no such element is found.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <exception cref="ArgumentNullException"><paramref name="predicate" /> is <see langword="null" />.</exception>
/// <exception cref="InvalidOperationException">More than one element satisfies the condition in <paramref name="predicate" />.</exception>
public static ValueTask<TSource> SingleOrDefaultAsync<TSource>(
this IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<bool>> predicate,
TSource defaultValue,
CancellationToken cancellationToken = default)
{
ArgumentNullException.ThrowIfNull(source);
ArgumentNullException.ThrowIfNull(predicate);
return Impl(source, predicate, defaultValue, cancellationToken);
static async ValueTask<TSource> Impl(
IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<bool>> predicate,
TSource defaultValue,
CancellationToken cancellationToken)
{
await using IAsyncEnumerator<TSource> e = source.GetAsyncEnumerator(cancellationToken);
while (await e.MoveNextAsync())
{
TSource result = e.Current;
if (await predicate(result, cancellationToken))
{
while (await e.MoveNextAsync())
{
if (await predicate(e.Current, cancellationToken))
{
ThrowHelper.ThrowMoreThanOneMatchException();
}
}
return result;
}
}
return defaultValue;
}
}
}
}
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