C# 4 and .NET 4

event-Based asynchronous Pattern ❘ 541
delegate are all input parameters from the caller plus the AsyncOperation parameter for getting the status
and mapping the result of the operation.
public void LongTaskAsync(string input, object taskId)
{
AsyncOperation asyncOp = AsyncOperationManager.CreateOperation(taskId);
lock (userStateDictionary)
{
if (userStateDictionary.ContainsKey(taskId))
throw new ArgumentException("taskId must be unique", "taskId");
}
userStateDictionary[taskId] = asyncOp;
}
Action longTaskDelegate = DoLongTask;
longTaskDelegate.BeginInvoke(input, asyncOp, null, null);
The method DoLongTask() is now called asynchronously by using the delegate. The synchronous method
LongTask() can now be invoked to get the output value.
Because an exception that might happen inside the synchronous method should not just blow up the
background thread, any exception is caught and remembered with the variable e of type Exception.
Finally, the CompletionMethod() is invoked to inform the caller about the result.
// running in a background thread
private void DoLongTask(string input, AsyncOperation asyncOp)
{
Exception e = null;
string output = null;
try
{
output = LongTask(input);
}
catch (Exception ex)
{
e = ex;
}
}
this.CompletionMethod(output, e, false, asyncOp);
With the implementation of the CompletionMethod, the userStateDictionary is cleaned up as the
operation is removed. The PostOperationCompleted() method of the AsyncOperation object ends
the lifetime of the asynchronous operation and informs the caller, using the onCompletedDelegate
method. This method ensures that the delegate is invoked on the thread as needed for the application type.
To get information to the caller, an object of type LongTaskCompletedEventArgs is created and passed to
the method PostOperationCompleted().
private void CompletionMethod(string output, Exception ex,
bool cancelled, AsyncOperation asyncOp)
{
lock (userStateDictionary)
{
userStateDictionary.Remove(asyncOp.UserSuppliedState);
}
}
}
// results of the operation
asyncOp.PostOperationCompleted(onCompletedDelegate,
new LongTaskCompletedEventArgs(output, ex, cancelled,
asyncOp.UserSuppliedState));
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