// Licensed to the .NET Foundation under one or more agreements. // The .NET Foundation licenses this file to you under the MIT license. // See the LICENSE file in the project root for more information. // =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ // // // // Compiler-targeted type for switching back into the current execution context, e.g. // // await Task.Yield(); // ===================== // var $awaiter = Task.Yield().GetAwaiter(); // if (!$awaiter.IsCompleted) // { // $builder.AwaitUnsafeOnCompleted(ref $awaiter, ref this); // return; // Label: // } // $awaiter.GetResult(); // // =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- using System; using System.Security; using System.Diagnostics; using System.Diagnostics.Contracts; using System.Diagnostics.Tracing; using System.Threading; using System.Threading.Tasks; namespace System.Runtime.CompilerServices { // NOTE: YieldAwaitable currently has no state; while developers are encouraged to use Task.Yield() to produce one, // no validation is performed to ensure that the developer isn't doing "await new YieldAwaitable()". Such validation // would require additional, useless state to be stored, and as this is a type in the CompilerServices namespace, and // as the above example isn't harmful, we take the cheaper approach of not validating anything. /// Provides an awaitable context for switching into a target environment. /// This type is intended for compiler use only. public struct YieldAwaitable { /// Gets an awaiter for this . /// An awaiter for this awaitable. /// This method is intended for compiler user rather than use directly in code. public YieldAwaiter GetAwaiter() { return new YieldAwaiter(); } /// Provides an awaiter that switches into a target environment. /// This type is intended for compiler use only. public struct YieldAwaiter : ICriticalNotifyCompletion { /// Gets whether a yield is not required. /// This property is intended for compiler user rather than use directly in code. public bool IsCompleted { get { return false; } } // yielding is always required for YieldAwaiter, hence false /// Posts the back to the current context. /// The action to invoke asynchronously. /// The argument is null (Nothing in Visual Basic). public void OnCompleted(Action continuation) { QueueContinuation(continuation, flowContext: true); } /// Posts the back to the current context. /// The action to invoke asynchronously. /// The argument is null (Nothing in Visual Basic). public void UnsafeOnCompleted(Action continuation) { QueueContinuation(continuation, flowContext: false); } /// Posts the back to the current context. /// The action to invoke asynchronously. /// true to flow ExecutionContext; false if flowing is not required. /// The argument is null (Nothing in Visual Basic). private static void QueueContinuation(Action continuation, bool flowContext) { // Validate arguments if (continuation == null) throw new ArgumentNullException(nameof(continuation)); Contract.EndContractBlock(); if (TplEtwProvider.Log.IsEnabled()) { continuation = OutputCorrelationEtwEvent(continuation); } // Get the current SynchronizationContext, and if there is one, // post the continuation to it. However, treat the base type // as if there wasn't a SynchronizationContext, since that's what it // logically represents. var syncCtx = SynchronizationContext.CurrentNoFlow; if (syncCtx != null && syncCtx.GetType() != typeof(SynchronizationContext)) { syncCtx.Post(s_sendOrPostCallbackRunAction, continuation); } else { // If we're targeting the default scheduler, queue to the thread pool, so that we go into the global // queue. As we're going into the global queue, we might as well use QUWI, which for the global queue is // just a tad faster than task, due to a smaller object getting allocated and less work on the execution path. TaskScheduler scheduler = TaskScheduler.Current; if (scheduler == TaskScheduler.Default) { if (flowContext) { ThreadPool.QueueUserWorkItem(s_waitCallbackRunAction, continuation); } else { ThreadPool.UnsafeQueueUserWorkItem(s_waitCallbackRunAction, continuation); } } // We're targeting a custom scheduler, so queue a task. else { Task.Factory.StartNew(continuation, default(CancellationToken), TaskCreationOptions.PreferFairness, scheduler); } } } private static Action OutputCorrelationEtwEvent(Action continuation) { int continuationId = Task.NewId(); Task currentTask = Task.InternalCurrent; // fire the correlation ETW event TplEtwProvider.Log.AwaitTaskContinuationScheduled(TaskScheduler.Current.Id, (currentTask != null) ? currentTask.Id : 0, continuationId); return AsyncMethodBuilderCore.CreateContinuationWrapper(continuation, () => { var etwLog = TplEtwProvider.Log; etwLog.TaskWaitContinuationStarted(continuationId); // ETW event for Task Wait End. Guid prevActivityId = new Guid(); // Ensure the continuation runs under the correlated activity ID generated above if (etwLog.TasksSetActivityIds) EventSource.SetCurrentThreadActivityId(TplEtwProvider.CreateGuidForTaskID(continuationId), out prevActivityId); // Invoke the original continuation provided to OnCompleted. continuation(); // Restore activity ID if (etwLog.TasksSetActivityIds) EventSource.SetCurrentThreadActivityId(prevActivityId); etwLog.TaskWaitContinuationComplete(continuationId); }); } /// WaitCallback that invokes the Action supplied as object state. private static readonly WaitCallback s_waitCallbackRunAction = RunAction; /// SendOrPostCallback that invokes the Action supplied as object state. private static readonly SendOrPostCallback s_sendOrPostCallbackRunAction = RunAction; /// Runs an Action delegate provided as state. /// The Action delegate to invoke. private static void RunAction(object state) { ((Action)state)(); } /// Ends the await operation. public void GetResult() { } // Nop. It exists purely because the compiler pattern demands it. } } }