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Add NonConcurrentSynchronizationContext
This `SynchronizationContext` schedules work on the threadpool in such a way as to ensure that no posted delegate is concurrent with another posted delegate. It provides a "logically single thread" environment without actually having to create a dedicated thread.
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‎src/Microsoft.VisualStudio.Threading.Tests/NonConcurrentSynchronizationContextTests.cs‎

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using System;
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using System.Collections.Generic;
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using System.Linq;
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using System.Text;
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using System.Threading;
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using System.Threading.Tasks;
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using Microsoft.VisualStudio.Threading;
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using Microsoft.VisualStudio.Threading.Tests;
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using Xunit;
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using Xunit.Abstractions;
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public class NonConcurrentSynchronizationContextTests : TestBase
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{
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private readonly NonConcurrentSynchronizationContext nonSticky = new NonConcurrentSynchronizationContext(sticky: false);
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public NonConcurrentSynchronizationContextTests(ITestOutputHelper logger)
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: base(logger)
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{
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}
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[Fact]
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public void CreateCopy()
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{
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var copy = this.nonSticky.CreateCopy();
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Assert.NotSame(this.nonSticky, copy);
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ConfirmNonConcurrentPost(this.nonSticky, copy);
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}
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[Fact]
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public void Post_NonConcurrentExecution()
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{
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ConfirmNonConcurrentPost(this.nonSticky, this.nonSticky);
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}
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[Fact]
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public void Send_NonConcurrentExecution()
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{
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ConfirmNonConcurrentSend(this.nonSticky);
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}
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[Fact]
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public void UnhandledException_WithNoHandler()
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{
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// Verifies that no crash occurs when an exception is thrown without an event handler attached.
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this.nonSticky.Post(s => throw new InvalidOperationException(), null);
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}
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[Fact]
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public async Task UnhandledException_WithHandler()
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{
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var eventArgs = new TaskCompletionSource<(object?, Exception)>();
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this.nonSticky.UnhandledException += (s, e) => eventArgs.SetResult((s, e));
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this.nonSticky.Post(s => throw new InvalidOperationException(), null);
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var (sender, ex) = await eventArgs.Task.WithCancellation(this.TimeoutToken);
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Assert.Same(this.nonSticky, sender);
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Assert.IsType<InvalidOperationException>(ex);
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}
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[Fact]
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public async Task NonSticky_HasNoCurrentSyncContext()
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{
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Assert.Null(await GetCurrentSyncContextDuringPost(this.nonSticky));
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}
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[Fact]
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public async Task Sticky_SetsCurrentSyncContext()
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{
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var sticky = new NonConcurrentSynchronizationContext(sticky: true);
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Assert.Same(sticky, await GetCurrentSyncContextDuringPost(sticky));
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}
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[Fact]
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public async Task InlinedSendReappliesSyncContextWhenSticky()
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{
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var sticky = new NonConcurrentSynchronizationContext(sticky: true);
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var tcs = new TaskCompletionSource<object?>();
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sticky.Post(_ =>
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{
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try
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{
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SynchronizationContext.SetSynchronizationContext(new SynchronizationContext());
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sticky.Send(_ =>
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{
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Assert.Same(sticky, SynchronizationContext.Current);
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}, null);
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Assert.IsType<SynchronizationContext>(SynchronizationContext.Current);
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tcs.TrySetResult(null);
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}
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catch (Exception ex)
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{
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tcs.TrySetException(ex);
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}
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}, null);
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await tcs.Task.WithCancellation(this.TimeoutToken);
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}
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[Fact]
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public async Task InlinedSendIgnoresSyncContextWhenNotSticky()
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{
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var tcs = new TaskCompletionSource<object?>();
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this.nonSticky.Post(_ =>
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{
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try
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{
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SynchronizationContext.SetSynchronizationContext(new SynchronizationContext());
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this.nonSticky.Send(_ =>
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{
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Assert.IsType<SynchronizationContext>(SynchronizationContext.Current);
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}, null);
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Assert.IsType<SynchronizationContext>(SynchronizationContext.Current);
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tcs.TrySetResult(null);
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}
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catch (Exception ex)
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{
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tcs.TrySetException(ex);
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}
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}, null);
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await tcs.Task.WithCancellation(this.TimeoutToken);
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}
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[Fact]
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public async Task SyncContextIsNotInherited()
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{
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// Set a SyncContext when creating the context, and ensure the current context is never used.
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SynchronizationContext.SetSynchronizationContext(new ThrowingSyncContext());
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var nonConcurrentContext = new NonConcurrentSynchronizationContext(sticky: false);
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// Also confirm the current context doesn't impact the Post method.
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Assert.Null(await GetCurrentSyncContextDuringPost(nonConcurrentContext).ConfigureAwait(false));
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}
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[Fact]
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public void Send_RethrowsExceptionFromDelegate()
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{
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bool handlerInvoked = false;
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this.nonSticky.UnhandledException += (s, e) => handlerInvoked = true;
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Assert.Throws<InvalidOperationException>(() => this.nonSticky.Send(_ => throw new InvalidOperationException(), null));
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Assert.False(handlerInvoked);
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}
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[Fact]
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public void Send_RethrowsExceptionFromDelegate_WhenInlined()
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{
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bool handlerInvoked = false;
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this.nonSticky.UnhandledException += (s, e) => handlerInvoked = true;
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this.nonSticky.Send(_ => Assert.Throws<InvalidOperationException>(() => this.nonSticky.Send(_ => throw new InvalidOperationException(), null)), null);
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Assert.False(handlerInvoked);
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}
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[Fact]
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public void SendWithinSendDoesNotDeadlock()
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{
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Task.Run(delegate
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{
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bool reachedInnerDelegate = false;
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this.nonSticky.Send(_ =>
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{
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this.nonSticky.Send(_ =>
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{
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reachedInnerDelegate = true;
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}, null);
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}, null);
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Assert.True(reachedInnerDelegate);
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}).WithCancellation(this.TimeoutToken).GetAwaiter().GetResult();
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}
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[Fact]
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public async Task SendWithinPostDoesNotDeadlock()
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{
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var reachedInnerDelegate = new TaskCompletionSource<bool>();
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this.nonSticky.Post(_ =>
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{
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this.nonSticky.Send(_ =>
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{
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reachedInnerDelegate.SetResult(true);
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}, null);
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}, null);
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Assert.True(await reachedInnerDelegate.Task.WithCancellation(this.TimeoutToken));
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}
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[Fact]
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public void CannotFoolSendBySettingSyncContext()
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{
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using var releaseFirst = new ManualResetEventSlim();
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this.nonSticky.Post(s =>
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{
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releaseFirst.Wait(UnexpectedTimeout);
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}, null);
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using var secondEntered = new ManualResetEventSlim();
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Task sendTask = Task.Run(delegate
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{
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SynchronizationContext.SetSynchronizationContext(this.nonSticky);
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this.nonSticky.Send(s =>
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{
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secondEntered.Set();
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}, null);
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});
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Assert.False(secondEntered.Wait(ExpectedTimeout));
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// Now that we've proven the second one hasn't started, allow the first to finish and confirm the second one could then execute.
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releaseFirst.Set();
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Assert.True(secondEntered.Wait(UnexpectedTimeout));
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sendTask.Wait(UnexpectedTimeout);
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}
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private static Task<SynchronizationContext?> GetCurrentSyncContextDuringPost(SynchronizationContext synchronizationContext)
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{
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var observed = new TaskCompletionSource<SynchronizationContext?>();
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synchronizationContext.Post(
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s =>
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{
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observed.SetResult(SynchronizationContext.Current);
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},
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null);
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return observed.Task;
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}
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private static void ConfirmNonConcurrentSend(SynchronizationContext ctxt)
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{
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using var releaseFirst = new ManualResetEventSlim();
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ctxt.Post(s =>
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{
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releaseFirst.Wait(UnexpectedTimeout);
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}, null);
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using var secondEntered = new ManualResetEventSlim();
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Task sendTask = Task.Run(delegate
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{
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ctxt.Send(s =>
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{
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secondEntered.Set();
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}, null);
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});
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Assert.False(secondEntered.Wait(ExpectedTimeout));
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// Now that we've proven the second one hasn't started, allow the first to finish and confirm the second one could then execute.
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releaseFirst.Set();
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Assert.True(secondEntered.Wait(UnexpectedTimeout));
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sendTask.Wait(UnexpectedTimeout);
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}
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private static void ConfirmNonConcurrentPost(SynchronizationContext a, SynchronizationContext b)
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{
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// Verify that the two instances share a common non-concurrent queue
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// by scheduling something on each one, and confirming that the second can't start
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// before the second one completes.
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using var releaseFirst = new ManualResetEventSlim();
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a.Post(s =>
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{
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releaseFirst.Wait(UnexpectedTimeout);
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}, null);
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using var secondEntered = new ManualResetEventSlim();
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b.Post(s =>
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{
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secondEntered.Set();
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}, null);
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Assert.False(secondEntered.Wait(ExpectedTimeout));
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// Now that we've proven the second one hasn't started, allow the first to finish and confirm the second one could then execute.
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releaseFirst.Set();
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Assert.True(secondEntered.Wait(UnexpectedTimeout));
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}
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private class ThrowingSyncContext : SynchronizationContext
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{
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public override void Post(SendOrPostCallback d, object? state) => throw new NotSupportedException();
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public override void Send(SendOrPostCallback d, object? state) => throw new NotSupportedException();
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}
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}

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