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Merged
SimonCropp merged 1 commit into from
Oct 11, 2026
Merged

Make Task.WhenEach observe cancellation while tasks are pending #649

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68 changes: 38 additions & 30 deletions src/Polyfill/TaskWhenEachPolyfill.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@
namespace Polyfills;

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
Expand All @@ -15,58 +16,65 @@ static partial class Polyfill
/// Creates an async-enumerable that yields completed tasks from the provided collection as they complete.
/// </summary>
//Link: https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.wheneach?view=net-11.0#system-threading-tasks-task-wheneach(system-collections-generic-ienumerable((system-threading-tasks-task)))
public static async IAsyncEnumerable<Task> WhenEach(
public static IAsyncEnumerable<Task> WhenEach(
IEnumerable<Task> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
CancellationToken cancellationToken = default)
{
if (tasks == null)
{
throw new ArgumentNullException(nameof(tasks));
}

var remaining = new List<Task>(tasks);

if (remaining.Count == 0)
{
yield break;
}

while (remaining.Count > 0)
{
cancellationToken.ThrowIfCancellationRequested();
var completed = await Task.WhenAny(remaining).ConfigureAwait(false);
remaining.Remove(completed);
yield return completed;
}
return WhenEachIterator(new List<Task>(tasks), cancellationToken);
}

/// <summary>
/// Creates an async-enumerable that yields completed tasks from the provided collection as they complete.
/// </summary>
//Link: https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.wheneach?view=net-11.0#system-threading-tasks-task-wheneach-1(system-collections-generic-ienumerable((system-threading-tasks-task((-0)))))
public static async IAsyncEnumerable<Task<TResult>> WhenEach<TResult>(
public static IAsyncEnumerable<Task<TResult>> WhenEach<TResult>(
IEnumerable<Task<TResult>> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
CancellationToken cancellationToken = default)
{
if (tasks == null)
{
throw new ArgumentNullException(nameof(tasks));
}

var remaining = new List<Task<TResult>>(tasks);
return WhenEachIterator(new List<Task<TResult>>(tasks), cancellationToken);
}
}

if (remaining.Count == 0)
{
yield break;
}
static async IAsyncEnumerable<T> WhenEachIterator<T>(
List<T> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
where T : Task
{
// Each task enqueues itself when it completes, so the next completed task is found without
// re-scanning every remaining task, and the wait for it can observe the cancellation token.
var completed = new ConcurrentQueue<T>();

while (remaining.Count > 0)
{
cancellationToken.ThrowIfCancellationRequested();
var completed = await Task.WhenAny(remaining).ConfigureAwait(false);
remaining.Remove(completed);
yield return completed;
}
// Not disposed: tasks still pending when enumeration stops will release it later.
var signal = new SemaphoreSlim(0);

foreach (var task in tasks)
{
task.ContinueWith(
_ =>
{
completed.Enqueue(task);
signal.Release();
},
CancellationToken.None,
TaskContinuationOptions.ExecuteSynchronously,
TaskScheduler.Default);
}

for (var index = 0; index < tasks.Count; index++)
{
await signal.WaitAsync(cancellationToken).ConfigureAwait(false);
completed.TryDequeue(out var next);
yield return next!;
}
}
}
Expand Down
61 changes: 33 additions & 28 deletions src/Split/net461/TaskWhenEachPolyfill.cs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
#if FeatureAsyncInterfaces
namespace Polyfills;
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
Expand All @@ -14,50 +15,54 @@ static partial class Polyfill
/// <summary>
/// Creates an async-enumerable that yields completed tasks from the provided collection as they complete.
/// </summary>
public static async IAsyncEnumerable<Task> WhenEach(
public static IAsyncEnumerable<Task> WhenEach(
IEnumerable<Task> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
CancellationToken cancellationToken = default)
{
if (tasks == null)
{
throw new ArgumentNullException(nameof(tasks));
}
var remaining = new List<Task>(tasks);
if (remaining.Count == 0)
{
yield break;
}
while (remaining.Count > 0)
{
cancellationToken.ThrowIfCancellationRequested();
var completed = await Task.WhenAny(remaining).ConfigureAwait(false);
remaining.Remove(completed);
yield return completed;
}
return WhenEachIterator(new List<Task>(tasks), cancellationToken);
}
/// <summary>
/// Creates an async-enumerable that yields completed tasks from the provided collection as they complete.
/// </summary>
public static async IAsyncEnumerable<Task<TResult>> WhenEach<TResult>(
public static IAsyncEnumerable<Task<TResult>> WhenEach<TResult>(
IEnumerable<Task<TResult>> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
CancellationToken cancellationToken = default)
{
if (tasks == null)
{
throw new ArgumentNullException(nameof(tasks));
}
var remaining = new List<Task<TResult>>(tasks);
if (remaining.Count == 0)
{
yield break;
}
while (remaining.Count > 0)
{
cancellationToken.ThrowIfCancellationRequested();
var completed = await Task.WhenAny(remaining).ConfigureAwait(false);
remaining.Remove(completed);
yield return completed;
}
return WhenEachIterator(new List<Task<TResult>>(tasks), cancellationToken);
}
}
static async IAsyncEnumerable<T> WhenEachIterator<T>(
List<T> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
where T : Task
{
var completed = new ConcurrentQueue<T>();
var signal = new SemaphoreSlim(0);
foreach (var task in tasks)
{
task.ContinueWith(
_ =>
{
completed.Enqueue(task);
signal.Release();
},
CancellationToken.None,
TaskContinuationOptions.ExecuteSynchronously,
TaskScheduler.Default);
}
for (var index = 0; index < tasks.Count; index++)
{
await signal.WaitAsync(cancellationToken).ConfigureAwait(false);
completed.TryDequeue(out var next);
yield return next!;
}
}
}
Expand Down
61 changes: 33 additions & 28 deletions src/Split/net462/TaskWhenEachPolyfill.cs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
#if FeatureAsyncInterfaces
namespace Polyfills;
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
Expand All @@ -14,50 +15,54 @@ static partial class Polyfill
/// <summary>
/// Creates an async-enumerable that yields completed tasks from the provided collection as they complete.
/// </summary>
public static async IAsyncEnumerable<Task> WhenEach(
public static IAsyncEnumerable<Task> WhenEach(
IEnumerable<Task> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
CancellationToken cancellationToken = default)
{
if (tasks == null)
{
throw new ArgumentNullException(nameof(tasks));
}
var remaining = new List<Task>(tasks);
if (remaining.Count == 0)
{
yield break;
}
while (remaining.Count > 0)
{
cancellationToken.ThrowIfCancellationRequested();
var completed = await Task.WhenAny(remaining).ConfigureAwait(false);
remaining.Remove(completed);
yield return completed;
}
return WhenEachIterator(new List<Task>(tasks), cancellationToken);
}
/// <summary>
/// Creates an async-enumerable that yields completed tasks from the provided collection as they complete.
/// </summary>
public static async IAsyncEnumerable<Task<TResult>> WhenEach<TResult>(
public static IAsyncEnumerable<Task<TResult>> WhenEach<TResult>(
IEnumerable<Task<TResult>> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
CancellationToken cancellationToken = default)
{
if (tasks == null)
{
throw new ArgumentNullException(nameof(tasks));
}
var remaining = new List<Task<TResult>>(tasks);
if (remaining.Count == 0)
{
yield break;
}
while (remaining.Count > 0)
{
cancellationToken.ThrowIfCancellationRequested();
var completed = await Task.WhenAny(remaining).ConfigureAwait(false);
remaining.Remove(completed);
yield return completed;
}
return WhenEachIterator(new List<Task<TResult>>(tasks), cancellationToken);
}
}
static async IAsyncEnumerable<T> WhenEachIterator<T>(
List<T> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
where T : Task
{
var completed = new ConcurrentQueue<T>();
var signal = new SemaphoreSlim(0);
foreach (var task in tasks)
{
task.ContinueWith(
_ =>
{
completed.Enqueue(task);
signal.Release();
},
CancellationToken.None,
TaskContinuationOptions.ExecuteSynchronously,
TaskScheduler.Default);
}
for (var index = 0; index < tasks.Count; index++)
{
await signal.WaitAsync(cancellationToken).ConfigureAwait(false);
completed.TryDequeue(out var next);
yield return next!;
}
}
}
Expand Down
61 changes: 33 additions & 28 deletions src/Split/net47/TaskWhenEachPolyfill.cs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
#if FeatureAsyncInterfaces
namespace Polyfills;
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
Expand All @@ -14,50 +15,54 @@ static partial class Polyfill
/// <summary>
/// Creates an async-enumerable that yields completed tasks from the provided collection as they complete.
/// </summary>
public static async IAsyncEnumerable<Task> WhenEach(
public static IAsyncEnumerable<Task> WhenEach(
IEnumerable<Task> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
CancellationToken cancellationToken = default)
{
if (tasks == null)
{
throw new ArgumentNullException(nameof(tasks));
}
var remaining = new List<Task>(tasks);
if (remaining.Count == 0)
{
yield break;
}
while (remaining.Count > 0)
{
cancellationToken.ThrowIfCancellationRequested();
var completed = await Task.WhenAny(remaining).ConfigureAwait(false);
remaining.Remove(completed);
yield return completed;
}
return WhenEachIterator(new List<Task>(tasks), cancellationToken);
}
/// <summary>
/// Creates an async-enumerable that yields completed tasks from the provided collection as they complete.
/// </summary>
public static async IAsyncEnumerable<Task<TResult>> WhenEach<TResult>(
public static IAsyncEnumerable<Task<TResult>> WhenEach<TResult>(
IEnumerable<Task<TResult>> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
CancellationToken cancellationToken = default)
{
if (tasks == null)
{
throw new ArgumentNullException(nameof(tasks));
}
var remaining = new List<Task<TResult>>(tasks);
if (remaining.Count == 0)
{
yield break;
}
while (remaining.Count > 0)
{
cancellationToken.ThrowIfCancellationRequested();
var completed = await Task.WhenAny(remaining).ConfigureAwait(false);
remaining.Remove(completed);
yield return completed;
}
return WhenEachIterator(new List<Task<TResult>>(tasks), cancellationToken);
}
}
static async IAsyncEnumerable<T> WhenEachIterator<T>(
List<T> tasks,
[EnumeratorCancellation] CancellationToken cancellationToken = default)
where T : Task
{
var completed = new ConcurrentQueue<T>();
var signal = new SemaphoreSlim(0);
foreach (var task in tasks)
{
task.ContinueWith(
_ =>
{
completed.Enqueue(task);
signal.Release();
},
CancellationToken.None,
TaskContinuationOptions.ExecuteSynchronously,
TaskScheduler.Default);
}
for (var index = 0; index < tasks.Count; index++)
{
await signal.WaitAsync(cancellationToken).ConfigureAwait(false);
completed.TryDequeue(out var next);
yield return next!;
}
}
}
Expand Down
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