asynchronous programming APM vs EAP

Question

What\'s actually difference between Asynchronous Programming Model and Event-based Asychronous Pattern?

Which approach to use and when

Answer 1

The Asynchronous Programming Model (APM) is the model you see with BeginMethod(...) and EndMethod(...) pairs.

For example here is a Socket using the APM implementation:

 var socket = new Socket(AddressFamily.InterNetwork, 
                        SocketType.Stream, ProtocolType.Tcp);

 // ...

 socket.BeginReceive(recvBuffer, 0, recvBuffer.Length, 
                     SocketFlags.None, ReceiveCallback, null);

 void ReceiveCallback(IAsyncResult result)
 {
   var bytesReceived = socket.EndReceive(result);

   if (bytesReceived > 0) { // Handle received data here. }

   if (socket.Connected)
   {
     // Keep receiving more data...
     socket.BeginReceive(recvBuffer, 0, recvBuffer.Length, 
                         SocketFlags.None, ReceiveCallback, null);
   }
 }

The Event-based Asynchronous Pattern (EAP) is the model you see with MethodAsync(...) and CancelAsync(...) pairs. There's usually a Completed event. BackgroundWorker is a good example of this pattern.

As of C# 4.5, both have been replaced by the async/await pattern, which is using the Task Parallelism Library (TPL). You will see them marked with Async after the method name and usually returning an awaitable Task or Task. If you are able to target .NET 4.5, you should definitely use this pattern over the APM or EAP design.

For example, compressing a (potentially large) file asynchronously:

public static async Task CompressFileAsync(string inputFile, string outputFile)
{
  using (var inputStream = File.Open(inputFile, FileMode.Open, FileAccess.Read))
  using (var outputStream = File.Create(outputFile))
  using (var deflateStream = new DeflateStream(outputStream, CompressionMode.Compress))
  {
    await inputStream.CopyToAsync(deflateStream);

    deflateStream.Close();
    outputStream.Close();
    inputStream.Close();
  }
}