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//------------------------------------------------------------------------------
// <copyright file="TaskAsyncHelper.cs" company="Microsoft">
// Copyright (c) Microsoft Corporation. All rights reserved.
// </copyright>
//------------------------------------------------------------------------------
/*
* Assists in converting an method written using the Task Asynchronous Pattern to a Begin/End method pair.
*
* Copyright (c) 2010 Microsoft Corporation
*/
namespace System.Web {
using System;
using System.Threading.Tasks;
internal static class TaskAsyncHelper {
private static readonly Task s_completedTask = Task.FromResult<object>(null);
internal static IAsyncResult BeginTask(Func<Task> taskFunc, AsyncCallback callback, object state) {
Task task = taskFunc();
if (task == null) {
// Something went wrong - let our caller handle it.
return null;
}
// We need to wrap the inner Task so that the IAsyncResult exposed by this method
// has the state object that was provided as a parameter. We could be a bit smarter
// about this to save an allocation if the state objects are equal, but that's a
// micro-optimization.
TaskWrapperAsyncResult resultToReturn = new TaskWrapperAsyncResult(task, state);
// Task instances are always marked CompletedSynchronously = false, even if the
// operation completed synchronously. We should detect this and modify the IAsyncResult
// we pass back to our caller as appropriate. Only read the 'IsCompleted' property once
// to avoid a race condition where the underlying Task completes during this method.
bool actuallyCompletedSynchronously = task.IsCompleted;
if (actuallyCompletedSynchronously) {
resultToReturn.ForceCompletedSynchronously();
}
if (callback != null) {
// ContinueWith() is a bit slow: it captures execution context and hops threads. We should
// avoid calling it and just invoke the callback directly if the underlying Task is
// already completed. Only use ContinueWith as a fallback. There's technically a ---- here
// in that the Task may have completed between the check above and the call to
// ContinueWith below, but ContinueWith will do the right thing in both cases.
if (actuallyCompletedSynchronously) {
callback(resultToReturn);
}
else {
task.ContinueWith(_ => callback(resultToReturn));
}
}
return resultToReturn;
}
// The parameter is named 'ar' since it matches the parameter name on the EndEventHandler delegate type,
// and we expect that most consumers will end up invoking this method via an instance of that delegate.
internal static void EndTask(IAsyncResult ar) {
if (ar == null) {
throw new ArgumentNullException("ar");
}
// Make sure the incoming parameter is actually the correct type.
TaskWrapperAsyncResult taskWrapper = ar as TaskWrapperAsyncResult;
if (taskWrapper == null) {
// extraction failed
throw new ArgumentException(SR.GetString(SR.TaskAsyncHelper_ParameterInvalid), "ar");
}
// The End* method doesn't actually perform any actual work, but we do need to maintain two invariants:
// 1. Make sure the underlying Task actually *is* complete.
// 2. If the Task encountered an exception, observe it here.
// (TaskAwaiter.GetResult() handles both of those, and it rethrows the original exception rather than an AggregateException.)
taskWrapper.Task.GetAwaiter().GetResult();
}
internal static Task CompletedTask {
get {
return s_completedTask;
}
}
}
}
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