Recipe 5.17 Handling Exceptions Thrown from an Asynchronous Delegate

Problem

When using a delegate asynchronously, you want to be notified in the calling thread if the delegate has thrown any exceptions.

Solution

Wrap the EndInvoke method of the delegate in a try / catch block:

 using System; using System.Threading; public class AsyncAction {     public void PollAsyncDelegate( )     {         // Create the async delegate to call Method1 and call its BeginInvoke method         AsyncInvoke MI = new AsyncInvoke(TestAsyncInvoke.Method1);         IAsyncResult AR = MI.BeginInvoke(null, null);         // Poll until the async delegate is finished         while (!AR.IsCompleted)         {             System.Threading.Thread.Sleep(100);             Console.Write('.');         }         Console.WriteLine("Finished Polling");         // Call the EndInvoke method of the async delegate  try   {   int RetVal = MI.EndInvoke(AR);   Console.WriteLine("RetVal: " + RetVal);   }   catch (Exception e)   {   Console.WriteLine(e.ToString( ));   }  } }

The following code defines the AsyncInvoke delegate and the asynchronously invoked static method TestAsyncInvoke.Method1 :

 public delegate int AsyncInvoke( ); public class TestAsyncInvoke {     public static int Method1( )     {         throw (new Exception("Method1"));    // Simulate an exception being thrown     } }

Discussion

If the code in the PollAsyncDelegate method did not contain a call to the delegate's EndInvoke method, the exception thrown in Method1 would simply be discarded and never caught. This behavior is by design; for all unhandled exceptions that occur within the thread, the thread immediately returns to the thread pool and the exception is lost.

If a method that was called asynchronously through a delegate throws an exception, the only way to trap that exception object is to include a call to the delegate's EndInvoke method and wrap this call in an exception handler. The EndInvoke method must be called to retrieve the results of the asynchronous delegate; in fact, the EndInvoke method must be called even if there are no results. These results can be obtained through a return value or any ref or out parameters of the delegate.

Recipe 5.17 Handling Exceptions Thrown from an Asynchronous Delegate