StephenCleary edited this page Sep 5, 2014 · 1 revision


The AsyncContext type provides a context for executing asynchronous operations. The await keyword requires a context to return back to; for most programs, this is a UI context, and you don't have to worry about it. ASP.NET also provides a proper context and you don't have to use your own.

However, Console applications and Win32 services do not have a suitable context, and AsyncContext or AsyncContextThread could be used in those situations.

AsyncContextThread is a separate thread or task that runs an AsyncContext. AsyncContextThread does not derive from the Thread class. The thread begins running its AsyncContext immediately after creation.

AsyncContextThread will stay in its loop until it is requested to exit by another thread calling JoinAsync. Disposing an AsyncContextThread will also request it to exit, but will not wait for it to do so.

Normally, AsyncContextThread is used by windows services, but it may be used by other applications that need an independent thread with an AsyncContext.

Console Example Using AsyncContext

When using AsyncContext, you normally just call the static Run method, as such:

class Program
  static async Task<int> AsyncMain()

  static int Main(string[] args)
    return AsyncContext.Run(AsyncMain);

The Run method will return when all asynchronous operations have been completed. Any exceptions will be unwrapped and propagated.

AsyncContext API

// Provides a context for asynchronous operations. This class is threadsafe.
sealed class AsyncContext : IDisposable
  // Queues an action for execution, and begins executing all actions in the queue.
  // This method returns when all actions have been completed and the outstanding asynchronous operation count is zero.
  // Returns the result of the task. This method will unwrap and propagate exceptions.
  public static void Run(Action action);
  public static TResult Run<TResult>(Func<TResult> action);
  public static void Run(Func<Task> action);
  public static TResult Run<TResult>(Func<Task<TResult>> action);

  // Gets a semi-unique identifier for this asynchronous context. This is the same identifier as the context's TaskScheduler.
  public int Id { get; }

  // Gets the current AsyncContext for this thread, or <c>null</c> if this thread is not currently running in an AsyncContext.
  public static AsyncContext Current { get; }

AsyncContextThread API

Unlike AsyncContext, AsyncContextThread provides properties that can be used to schedule tasks on that thread.

The Context property returns the AsyncContext being run. The Factory property returns a TaskFactory which can be used to queue work to the thread.

public sealed class AsyncContextThread : IDisposable
  // Initializes a new instance of the AsyncContextThread class, creating a child thread waiting for commands.
  public AsyncContextThread();

  // Gets the AsyncContext executed by this thread.
  public AsyncContext Context { get; }

  // Gets the TaskFactory for this thread, which can be used to schedule work to this thread.
  public TaskFactory Factory { get; }

  // Requests the thread to exit and returns a task representing the exit of the thread. The thread will exit when all outstanding asynchronous operations complete.
  public Task JoinAsync();

  // Requests the thread to exit.
  public void Dispose();

Platform Support

This type is available on all platforms, but may not work due to security restrictions. This is particularly a problem on Silverlight and Windows Phone platforms.

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