Question

Why does the delegate need to call the EndInvoke before the method fires? If i need to call the EndInvoke (which blocks the thread) then its not really an asynchronous call is it?

Here is the code im trying to run.

class Program
    {
        private delegate void GenerateXmlDelegate();

        static void Main(string[] args)
        {
            GenerateXmlDelegate worker = new GenerateXmlDelegate(GenerateMainXml);
            IAsyncResult result = worker.BeginInvoke(null, null);
        }

        private static void GenerateMainXml()
        {
            Thread.Sleep(10000);
            Console.WriteLine("GenerateMainXml Called by delegate");
        }
    }

Answer 1

The reason you need to call EndInvoke is to avoid memory leaks; .Net will store information about the function s result (or exception) until you call EndInvoke.

You can call EndInvoke in the completion handler that you give to BeginInvoke and retain the asyncronous nature.

EDIT:

For example:

class Program {
    private delegate void GenerateXmlDelegate();

    static void Main(string[] args) {
        GenerateXmlDelegate worker = new GenerateXmlDelegate(GenerateMainXml);
        IAsyncResult result = worker.BeginInvoke(delegate {
            try {
                worker.EndInvoke();
            } catch(...) { ... }
        }, null);
    }

    private static void GenerateMainXml() {
        Thread.Sleep(10000);
        Console.WriteLine("GenerateMainXml Called by delegate");
    }
}

If you want to fire an async call and forget about it, you can use the ThreadPool, like this:

ThreadPool.QueueUserWorkItem(delegate { GenerateMainXml(); });

Answer 2

As SLaks said, EndInvoke insures against memory leaks.

BeginInvoke is still asynchronous; consider the following code:

static void Main() {
    Func<double> slowCalculator = new Func<double>(PerformSlowCalculation);
    IAsyncResult slowCalculation = slowCalculator.BeginInvoke(null, null);

    // lots of stuff to do while slowCalculator is doing its thing

    Console.WriteLine("Result is {0}", slowCalculator.EndInvoke(slowCalculation));
}

static double PerformSlowCalculation() {
    double result;

    // lots and lots of code

    return result;
}

If this code were written without the BeginInvoke/EndInvoke calls, PerformSlowCalculation would have to finish before Main could do the rest of its "lots of stuff"; this way, the two can be happening at the same time.

Now, in your example using a GenerateXmlDelegate, you still need EndInvoke even though you re not returning anything. The way to do this is:

static void Main(string[] args) {
    GenerateXmlDelegate worker = new GenerateXmlDelegate(GenerateMainXml);
    IAsyncResult result = worker.BeginInvoke(GenerateXmlComplete, null);
}

private static void GenerateXmlComplete(IAsyncResult result) {
    AsyncResult realResult = result as AsyncResult;
    GenerateXmlDelegate worker = result.AsyncDelegate as GenerateXmlDelegate;
    worker.EndInvoke();
}

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