Overview

Delegates enable calling any synchronous method in an asynchronous manner.

How it Works

The CLR automatically defines BeginInvoke and EndInvoke methods for a delegate with the appropriate signature. This can then be used like the APM model.

BeginInvoke

• Has same parameters as method to be executed asynchronously, plus:
  1. an AsyncCallback delegate that references the method to be called when the async call completes
  2. a user-defined object that passes information into the callback method
• Returns an IAsyncResult

EndInvoke

• If called before async call is complete, blocks calling thread until it is.
• Returns the result of the async call.

Process

1. Call BeginInvoke
2. Either:
   1. Do some work and then call EndInvoke
   2. Wait
      1. Obtain a WaitHandle from IAsyncResult.AsyncWaitHandle
      2. Use it WaitOne method to block until the WaitHandle is signaled
      3. Call EndInvoke
   3. Poll
      1. Poll IAsyncResult to determine when the async call has completed
      2. Call EndInvoke
   4. Pass a delegate for a callback method to BeginInvoke
      1. This method is executed when the async call completes.
      2. Have the callback method call EndInvoke.

Example

public class SomeClass
{
    public string SomeLongRunningMethod(int callDuration, out int threadId)
    {
        Thread.Sleep(callDuration);
        threadId = Thread.CurrentThread.ManagedThreadId;
        return $"Call time {callDuration.ToString()}";
    }

    public delegate string SomeDelegate(int callDuration, out int threadId);
}

public class SomeOtherClass
{
    public static void Main()
    {
    int threadId;

    SomeClass c = new();

    // Create the delegate:
    SomeDelegate caller = new(c.SomeLongRunningMethod);

    // Initiate the async call:
    IAsyncResult result = caller.BeginInvoke(3000, out threadId, null, null);

    // End the async call:
    string returnValue = caller.EndInvoke(out threadId, result);
    }
}