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