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//------------------------------------------------------------------------------
// <copyright file="AspNetSynchronizationContextBase.cs" company="Microsoft">
// Copyright (c) Microsoft Corporation. All rights reserved.
// </copyright>
//------------------------------------------------------------------------------
namespace System.Web {
using System;
using System.Runtime.ExceptionServices;
using System.Threading;
using System.Threading.Tasks;
using System.Web.Util;
// Provides an abstraction around the different SynchronizationContext-derived types that the
// ASP.NET runtime might use. Consumers can target this abstraction rather than coding against
// the various concrete types directly.
internal abstract class AspNetSynchronizationContextBase : SynchronizationContext {
private AllowAsyncOperationsBlockDisposable _allowAsyncOperationsBlockDisposable;
internal abstract bool AllowAsyncDuringSyncStages { get; set; }
internal abstract bool Enabled { get; }
internal Exception Error {
get {
ExceptionDispatchInfo dispatchInfo = ExceptionDispatchInfo;
return (dispatchInfo != null) ? dispatchInfo.SourceException : null;
}
}
internal abstract ExceptionDispatchInfo ExceptionDispatchInfo { get; }
internal abstract int PendingOperationsCount { get; }
internal abstract void ClearError();
internal abstract void Disable();
internal abstract void Enable();
internal abstract bool PendingCompletion(WaitCallback callback);
// A helper method which provides a Task-based wrapper around the PendingCompletion method.
// NOTE: The caller should verify that there are never outstanding calls to PendingCompletion
// or to WaitForPendingOperationsAsync, since each call replaces the continuation that will
// be invoked.
internal Task WaitForPendingOperationsAsync() {
TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
WaitCallback callback = _ => {
Exception ex = Error;
if (ex != null) {
// We're going to observe the exception in the returned Task. We shouldn't keep
// it around in the SynchronizationContext or it will fault future Tasks.
ClearError();
tcs.TrySetException(ex);
}
else {
tcs.TrySetResult(null);
}
};
if (!PendingCompletion(callback)) {
// If PendingCompletion returns false, there are no pending operations and the
// callback will not be invoked, so we should just signal the TCS immediately.
callback(null);
}
return tcs.Task;
}
// These methods are used in the synchronous handler execution step so that a synchronous IHttpHandler
// can call asynchronous methods without locking on the HttpApplication instance (possibly causing
// deadlocks).
internal abstract void SetSyncCaller();
internal abstract void ResetSyncCaller();
// These methods are used for synchronization, e.g. to create a lock that is tied to the current
// thread. The legacy implementation locks on the HttpApplication instance, for example.
internal abstract void AssociateWithCurrentThread();
internal abstract void DisassociateFromCurrentThread();
// These methods are used for telling the synchronization context when it is legal for an application
// to kick off async void methods. They are used by the "AllowAsyncDuringSyncStages" setting to
// determine whether kicking off an operation should throw.
internal virtual void AllowVoidAsyncOperations() { /* no-op by default */ }
internal virtual void ProhibitVoidAsyncOperations() { /* no-op by default */ }
// helper method for wrapping AllowVoidAsyncOperations / ProhibitVoidAsyncOperations in a using block
internal IDisposable AllowVoidAsyncOperationsBlock() {
if (_allowAsyncOperationsBlockDisposable == null) {
_allowAsyncOperationsBlockDisposable = new AllowAsyncOperationsBlockDisposable(this);
}
AllowVoidAsyncOperations();
return _allowAsyncOperationsBlockDisposable;
}
// Helper method to wrap Associate / Disassociate calls in a using() statement
internal IDisposable AcquireThreadLock() {
AssociateWithCurrentThread();
return new DisposableAction(DisassociateFromCurrentThread);
}
private sealed class AllowAsyncOperationsBlockDisposable : IDisposable {
private readonly AspNetSynchronizationContextBase _syncContext;
public AllowAsyncOperationsBlockDisposable(AspNetSynchronizationContextBase syncContext) {
_syncContext = syncContext;
}
public void Dispose() {
_syncContext.ProhibitVoidAsyncOperations();
}
}
}
}
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