Pattern

Uses two methods: BeginOperationName and EndOperationName

BeginOperationName

• Contains the same parameters as the synchronous version of the method and two additional parameters:
  1. An AsyncCallback delegate that references a method that is called when the async op completes
  2. A user-defined object
• Returns an IAsyncResult

IAsyncResult

• Members:
  • AsyncState
  • AsyncWaitHandle
  • CompletedSynchronously
  • IsCompleted

EndOperationName

• Contains the same parameters as the synchronous counterpart and an IAsyncResult parameter
• The IAsyncResult passed in must be the same one that originated from the BeginOperationName method
  • Passing other IAsyncResult objects is undefined.
  • Calling EndOperationName multiple times with the same IAsyncResult object is undefined.

Sequencing

1. Call BeginOperationName
2. If there is additional work to perform (blocking is not needed), either:
   1. Poll for operation completion status by checking IsCompleted periodically, then call EndOperationName when op is complete
   2. Use an AsyncCallback
3. If blocking is needed (no additional work to be performed), either:
   1. Call EndOperationName from the app’s main thread. This blocks execution until the op is complete.
   2. Use the AsyncWaitHandle to block until one or more ops completes.

Performing additional work (no blocking needed)

Polling for operation completion

IAsyncResult result = BeginOperationName(…, null, null);

while (result.IsCompleted != true)
{
    // Do additional work
}

var returnValue = EndOperationName(result);

Use AsyncCallback

Block until async op completes (no additional work to perform)

Call EndOperationName

Use AsyncWaitHandle