Interop with Other Asynchronous Patterns and Types

The .NET Framework 1.0 introduced the IAsyncResult pattern, otherwise known as the Asynchronous Programming Model (APM), or the Begin/End pattern. The .NET Framework 2.0 added the Event-based Asynchronous Pattern (EAP). Starting with the .NET Framework 4, the Task-based Asynchronous Pattern (TAP) supersedes both APM and EAP, but provides the ability to easily build migration routines from the earlier patterns.

In this topic:

Tasks and the Asynchronous Programming Model (APM)

From APM to TAP

Because the Asynchronous Programming Model (APM) pattern is very structured, it is quite easy to build a wrapper to expose an APM implementation as a TAP implementation. In fact, the .NET Framework, starting with .NET Framework 4, includes helper routines in the form of FromAsync method overloads to provide this translation.

Consider the Stream class and its BeginRead and EndRead methods, which represent the APM counterpart to the synchronous Read method:

public int Read(byte[] buffer, int offset, int count)
Public Function Read(buffer As Byte(), offset As Integer, 
                     count As Integer) As Integer
public IAsyncResult BeginRead(byte[] buffer, int offset, 
                              int count, AsyncCallback callback, 
                              object state)
Public Function BeginRead(buffer As Byte, offset As Integer, 
                          count As Integer, callback As AsyncCallback, 
                          state As Object) As IAsyncResult
public int EndRead(IAsyncResult asyncResult)
Public Function EndRead(asyncResult As IAsyncResult) As Integer

You can use the System.Threading.Tasks.TaskFactory<TResult>.FromAsync method to implement a TAP wrapper for this operation as follows:

public static Task<int> ReadAsync(this Stream stream, 
                                  byte[] buffer, int offset, 
                                  int count)
{
    if (stream == null) 
       throw new ArgumentNullException("stream");
    
    return Task<int>.Factory.FromAsync(stream.BeginRead, 
                                       stream.EndRead, buffer, 
                                       offset, count, null);
}
<Extension()>
Public Function ReadAsync(strm As Stream, 
                          buffer As Byte(), offset As Integer, 
                          count As Integer) As Task(Of Integer)
    If strm Is Nothing Then 
       Throw New ArgumentNullException("stream")
    End If   
    
    Return Task(Of Integer).Factory.FromAsync(AddressOf strm.BeginRead, 
                                              AddressOf strm.EndRead, buffer, 
                                              offset, count, Nothing)
End Function

This implementation is similar to the following:

 public static Task<int> ReadAsync(this Stream stream, 
                                   byte [] buffer, int offset, 
                                   int count)
 {
    if (stream == null) 
        throw new ArgumentNullException("stream");

    var tcs = new TaskCompletionSource<int>();
    stream.BeginRead(buffer, offset, count, iar =>
                     {
                        try { 
                           tcs.TrySetResult(stream.EndRead(iar)); 
                        }
                        catch(OperationCanceledException) { 
                           tcs.TrySetCanceled(); 
                        }
                        catch(Exception exc) { 
                           tcs.TrySetException(exc); 
                        }
                     }, null);
    return tcs.Task;
}

 <Extension()>
 Public Function ReadAsync(stream As Stream, buffer As Byte(), _
                           offset As Integer, count As Integer) _
                           As Task(Of Integer)
    If stream Is Nothing Then 
        Throw New ArgumentNullException("stream")
    End If    

    Dim tcs As New TaskCompletionSource(Of Integer)()
    stream.BeginRead(buffer, offset, count, 
                     Sub(iar)
                        Try  
                           tcs.TrySetResult(stream.EndRead(iar)) 
                        Catch e As OperationCanceledException  
                           tcs.TrySetCanceled() 
                        Catch e As Exception 
                           tcs.TrySetException(e) 
                        End Try
                     End Sub, Nothing)
    Return tcs.Task
End Function

From TAP to APM

If your existing infrastructure expects the APM pattern, you'll also want to take a TAP implementation and use it where an APM implementation is expected. Because tasks can be composed and the Task class implements IAsyncResult, you can use a straightforward helper function to do this. The following code uses an extension of the Task<TResult> class, but you can use an almost identical function for non-generic tasks.

public static IAsyncResult AsApm<T>(this Task<T> task, 
                                    AsyncCallback callback, 
                                    object state)
{
    if (task == null) 
        throw new ArgumentNullException("task");
    
    var tcs = new TaskCompletionSource<T>(state);
    task.ContinueWith(t => 
                      {
                         if (t.IsFaulted) 
                            tcs.TrySetException(t.Exception.InnerExceptions);
                         else if (t.IsCanceled)    
                            tcs.TrySetCanceled();
                         else 
                            tcs.TrySetResult(t.Result);

                         if (callback != null) 
                            callback(tcs.Task);
                      }, TaskScheduler.Default);
    return tcs.Task;
}
<Extension()>
Public Function AsApm(Of T)(task As Task(Of T), 
                            callback As AsyncCallback, 
                            state As Object) As IAsyncResult
    If task Is Nothing Then 
        Throw New ArgumentNullException("task")
    End If
    
    Dim tcs As New TaskCompletionSource(Of T)(state)
    task.ContinueWith(Sub(antecedent) 
                         If antecedent.IsFaulted Then 
                            tcs.TrySetException(antecedent.Exception.InnerExceptions)
                         ElseIf antecedent.IsCanceled Then    
                            tcs.TrySetCanceled()
                         Else 
                            tcs.TrySetResult(antecedent.Result)
                         End If
                         
                         If callback IsNot Nothing Then 
                            callback(tcs.Task)
                         End If   
                      End Sub, TaskScheduler.Default)
    Return tcs.Task
End Function

Now, consider a case where you have the following TAP implementation:

public static Task<String> DownloadStringAsync(Uri url)
Public Shared Function DownloadStringAsync(url As Uri) As Task(Of String)

and you want to provide this APM implementation:

public IAsyncResult BeginDownloadString(Uri url, 
                                        AsyncCallback callback, 
                                        object state)
Public Function BeginDownloadString(url As Uri, 
                                    callback As AsyncCallback, 
                                    state As Object) As IAsyncResult
public string EndDownloadString(IAsyncResult asyncResult)
Public Function EndDownloadString(asyncResult As IAsyncResult) As String

The following example demonstrates one migration to APM:

public IAsyncResult BeginDownloadString(Uri url, 
                                        AsyncCallback callback, 
                                        object state)
{
   return DownloadStringAsync(url).AsApm(callback, state);
}

public string EndDownloadString(IAsyncResult asyncResult)
{
   return ((Task<string>)asyncResult).Result;
}
Public Function BeginDownloadString(url As Uri, 
                                    callback As AsyncCallback, 
                                    state As Object) As IAsyncResult
   Return DownloadStringAsync(url).AsApm(callback, state)
End Function

Public Function EndDownloadString(asyncResult As IAsyncResult) As String
   Return CType(asyncResult, Task(Of String)).Result
End Function

Tasks and the Event-based Asynchronous Pattern (EAP)

Wrapping an Event-based Asynchronous Pattern (EAP) implementation is more involved than wrapping an APM pattern, because the EAP pattern has more variation and less structure than the APM pattern. To demonstrate, the following code wraps the DownloadStringAsync method. DownloadStringAsync accepts a URI, raises the DownloadProgressChanged event while downloading in order to report multiple statistics on progress, and raises the DownloadStringCompleted event when it's done. The final result is a string that contains the contents of the page at the specified URI.

 public static Task<string> DownloadStringAsync(Uri url)
 {
     var tcs = new TaskCompletionSource<string>();
     var wc = new WebClient();
     wc.DownloadStringCompleted += (s,e) =>
         {
             if (e.Error != null) 
                tcs.TrySetException(e.Error);
             else if (e.Cancelled) 
                tcs.TrySetCanceled();
             else 
                tcs.TrySetResult(e.Result);
         };
     wc.DownloadStringAsync(url);
     return tcs.Task;
}
 Public Shared Function DownloadStringAsync(url As Uri) As Task(Of String)
     Dim tcs As New TaskCompletionSource(Of String)()
     Dim wc As New WebClient()
     AddHandler wc.DownloadStringCompleted, Sub(s,e) 
             If e.Error IsNot Nothing Then 
                tcs.TrySetException(e.Error)
             ElseIf e.Cancelled Then 
                tcs.TrySetCanceled()
             Else 
                tcs.TrySetResult(e.Result)
             End If   
         End Sub
     wc.DownloadStringAsync(url)
     Return tcs.Task
End Function

Tasks and Wait Handles

From Wait Handles to TAP

Although wait handles don't implement an asynchronous pattern, advanced developers may use the WaitHandle class and the System.Threading.ThreadPool.RegisterWaitForSingleObject method for asynchronous notifications when a wait handle is set. You can wrap the RegisterWaitForSingleObject method to enable a task-based alternative to any synchronous wait on a wait handle:

public static Task WaitOneAsync(this WaitHandle waitHandle)
{
    if (waitHandle == null) 
        throw new ArgumentNullException("waitHandle");

    var tcs = new TaskCompletionSource<bool>();
    var rwh = ThreadPool.RegisterWaitForSingleObject(waitHandle, 
        delegate { tcs.TrySetResult(true); }, null, -1, true);
    var t = tcs.Task;
    t.ContinueWith( (antecedent) => rwh.Unregister(null));
    return t;
}
<Extension()>
Public Function WaitOneAsync(waitHandle As WaitHandle) As Task
    If waitHandle Is Nothing Then 
        Throw New ArgumentNullException("waitHandle")
    End If
    
    Dim tcs As New TaskCompletionSource(Of Boolean)()
    Dim rwh As RegisteredWaitHandle = ThreadPool.RegisterWaitForSingleObject(waitHandle, 
        Sub(state, timedOut) 
           tcs.TrySetResult(True)
        End Sub, Nothing, -1, True)
    Dim t = tcs.Task
    t.ContinueWith( Sub(antecedent) 
                       rwh.Unregister(Nothing)
                    End Sub)
    Return t
End Function

With this method, you can use existing WaitHandle implementations in asynchronous methods. For example, if you want to throttle the number of asynchronous operations that are executing at any particular time, you can utilize a semaphore (a System.Threading.SemaphoreSlim object). You can throttle to N the number of operations that run concurrently by initializing the semaphore’s count to N, waiting on the semaphore any time you want to perform an operation, and releasing the semaphore when you’re done with an operation:

static int N = 3;

static SemaphoreSlim m_throttle = new SemaphoreSlim(N, N);

static async Task DoOperation()
{
    await m_throttle.WaitAsync();
    // do work
    m_throttle.Release();
}
Shared N As Integer = 3

Shared m_throttle As New SemaphoreSlim(N, N)

Shared Async Function DoOperation() As Task
    Await m_throttle.WaitAsync()
    ' Do work.
    m_throttle.Release()
End Function

You can also build an asynchronous semaphore that does not rely on wait handles and instead works completely with tasks. To do this, you can use techniques such as those discussed in Consuming the Task-based Asynchronous Pattern for building data structures on top of Task.

From TAP to Wait Handles

As previously mentioned, the Task class implements IAsyncResult, and that implementation exposes an IAsyncResult.AsyncWaitHandle property that returns a wait handle that will be set when the Task completes. You can get a WaitHandle for a Task as follows:

WaitHandle wh = ((IAsyncResult)task).AsyncWaitHandle;
Dim wh As WaitHandle = CType(task, IAsyncResult).AsyncWaitHandle

See Also

Task-based Asynchronous Pattern (TAP)
Implementing the Task-based Asynchronous Pattern
Consuming the Task-based Asynchronous Pattern