How to create an IAsyncResult that immediately completes?

I am not sure if I am missing something but today you can just return Task.CompletedTask/Task.FromResult. Task implements IAsyncResult

protected override IAsyncResult BeginDoSomething(int a, int b, AsyncCallback callback, object state)
{
     return Task.FromResult(a > b);
}

The IAsyncResult.IsCompleted is rightly true here. The Func.BeginInvoke approach wouldn't result in true.

The quick hack way of doing it is to use a delegate:

protected override IAsyncResult BeginDoSomething(int a, int b, AsyncCallback callback, object state)
{
     bool returnValue = a > b;
     Func<int,int,bool> func = (x,y) => x > y;
     return func.BeginInvoke(a,b,callback,state);
}

The downside of this approach, is that you need to be careful if two threads will be calling this method concurrently you'll get an error.

This is a little quick and dirty, but you can implement a class that implements IAsyncResult like so:

    public class MyAsyncResult : IAsyncResult
    {
        bool _result;

        public MyAsyncResult(bool result)
        {
            _result = result;
        }

        public bool IsCompleted
        {
            get { return true; }
        }

        public WaitHandle AsyncWaitHandle
        {
            get { throw new NotImplementedException(); }
        }

        public object AsyncState
        {
            get { return _result; }
        }

        public bool CompletedSynchronously
        {
            get { return true; }
        }
    }

Then use it in your BeginDoSomething like this:

    return new MyAsyncResult(a > b);