Asynchronous operations within an asynchronous operation

Question

My multi-threading knowledge is still pretty rudimentary, so would really appreciate some pointers here. I have an interface, IOperationInvoker (from WCF) which has the following methods:

IAsyncResult InvokeBegin(object instance, object[] inputs, AsyncCallback callback, object state)
object InvokeEnd(object instance, out object[] outputs, IAsyncResult result)

Given a concrete implementation of this interface, I need to implement the same interface, whilst calling the underlying implementation in a seperate Thread. (in case you're wondering why, the concrete implmentation calls a legacy COM object which needs to be in a different apartment state).

At the moment, I'm doing something like this:

public StaOperationSyncInvoker : IOperationInvoker {
   IOperationInvoker _innerInvoker;
   public StaOperationSyncInvoker(IOperationInvoker invoker) {
       this._innerInvoker = invoker;
   } 


    public IAsyncResult InvokeBegin(object instance, object[] inputs, AsyncCallback callback, object state)
    {
        Thread t = new Thread(BeginInvokeDelegate);
        InvokeDelegateArgs ida = new InvokeDelegateArgs(_innerInvoker, instance, inputs, callback, state);
        t.SetApartmentState(ApartmentState.STA);
        t.Start(ida);
        // would do t.Join() if doing syncronously
        // how to wait to get IAsyncResult?
        return ida.AsyncResult;
    }

    public object InvokeEnd(object instance, out object[] outputs, IAsyncResult result)
    {
        // how to call invoke end on the 
        // thread? could we have wrapped IAsyncResult
        // to get a reference here?
        return null;
    }

    private class InvokeDelegateArgs {
        public InvokeDelegateArgs(IOperationInvoker invoker, object instance, object[] inputs, AsyncCallback callback, object state)
        {
            this.Invoker = invoker;
            this.Instance = instance;
            this.Inputs = inputs;
            this.Callback = callback;
            this.State = state;
        }

        public IOperationInvoker Invoker { get; private set; }
        public object Instance { get; private set; }
        public AsyncCallback Callback { get; private set; }
        public IAsyncResult AsyncResult { get; set; }
        public Object[] Inputs { get; private set; }
        public Object State { get; private set; }
    }
    private static void BeginInvokeDelegate(object data)
    {
        InvokeDelegateArgs ida = (InvokeDelegateArgs)data;
        ida.AsyncResult = ida.Invoker.InvokeBegin(ida.Instance, ida.Inputs, ida.Callback, ida.State);
    }
}

I'm thinking I need to wrap up the returned AsyncResult with my own, so I can get back to the thread we've spooled up... but honestly I'm a little out of my depth. Any pointers?

Many thanks,

James

Answer 1

The easiest way to implement a synchronous method asynchronously is to put it into a delegate, and use the BeginInvoke and EndInvoke methods on the resulting delegate. This will run the synchronous method on a threadpool thread, and BeginInvoke will return an IAsyncResult implementation, so you don't have to implement the guts of it. However, you do need to smuggle a little extra data into the IAsyncResult returned by IOperationInvoker.InvokeEnd. You could do that easily by creating an implementation of IAsyncResult that delegates everything to an inner IAsyncResult, but has an extra field to contain the delegate, so that when the IAsyncResult instance is passed to InvokeEnd, you can access the delegate to call EndInvoke on it.

However, after closer reading of your question, I see that you need to use an explicit thread with COM settings etc.

What you need to do is properly implement IAsyncResult. Almost everything follows from this, since the IAsyncResult will contain all the bits needed for synchronization.

Here's a very simple, but not terribly efficient, implementation of IAsyncResult. It encapsulates all the essential features: passing arguments, a synchronization event, callback implementation, propagating exceptions from async task and returning result.

using System;
using System.Threading;

class MyAsyncResult : IAsyncResult
{
    object _state;
    object _lock = new object();
    ManualResetEvent _doneEvent = new ManualResetEvent(false);
    AsyncCallback _callback;
    Exception _ex;
    bool _done;
    int _result;
    int _x;

    public MyAsyncResult(int x, AsyncCallback callback, object state)
    {
        _callback = callback;
        _state = state;
        _x = x; // arbitrary argument(s)
    }

    public int X { get { return _x; } }

    public void SignalDone(int result)
    {
        lock (_lock)
        {
            _result = result;
            _done = true;
            _doneEvent.Set();
        }
        // never invoke any delegate while holding a lock
        if (_callback != null)
            _callback(this); 
    }

    public void SignalException(Exception ex)
    {
        lock (_lock)
        {
            _ex = ex;
            _done = true;
            _doneEvent.Set();
        }
        if (_callback != null)
            _callback(this);
    }

    public object AsyncState
    {
        get { return _state; }
    }

    public WaitHandle AsyncWaitHandle
    {
        get { return _doneEvent; }
    }

    public bool CompletedSynchronously
    {
        get { return false; }
    }

    public int Result
    {
        // lock (or volatile, complex to explain) needed
        // for memory model problems.
        get
        {
            lock (_lock)
            {
                if (_ex != null)
                    throw _ex;
                return _result;
            }
        }
    }

    public bool IsCompleted
    {
        get { lock (_lock) return _done; }
    }
}

class Program
{
    static void MyTask(object param)
    {
        MyAsyncResult ar = (MyAsyncResult) param;
        try
        {
            int x = ar.X;
            Thread.Sleep(1000); // simulate lengthy work
            ar.SignalDone(x * 2); // demo work = double X
        }
        catch (Exception ex)
        {
            ar.SignalException(ex);
        }
    }

    static IAsyncResult Begin(int x, AsyncCallback callback, object state)
    {
        Thread th = new Thread(MyTask);
        MyAsyncResult ar = new MyAsyncResult(x, callback, state);
        th.Start(ar);
        return ar;
    }

    static int End(IAsyncResult ar)
    {
        MyAsyncResult mar = (MyAsyncResult) ar;
        mar.AsyncWaitHandle.WaitOne();
        return mar.Result; // will throw exception if one 
                           // occurred in background task
    }

    static void Main(string[] args)
    {
        // demo calling code
        // we don't need state or callback for demo
        IAsyncResult ar = Begin(42, null, null); 
        int result = End(ar);
        Console.WriteLine(result);
        Console.ReadLine();
    }
}

It's important for correctness that client code can't see the IAsyncResult implementation, otherwise they might access methods like SignalException inappropriately or read Result prematurely. The class can be made more efficient by not constructing the WaitHandle implementation (ManualResetEvent in the example) if it's not necessary, but this is tricky to get 100% right. Also, the Thread and ManualResetEvent can and should be disposed of in the End implementation, as should be done with all objects that implement IDisposable. And obviously, End should check to make sure that it has gotten an implementation of the right class to get a nicer exception than a cast exception. I've left these and other details out as they obscure the essential mechanics of the async implementation.