Question

I have the following class which tries to act as a simple asynchronous operation :

public class AsyncLineWriter
{
    private delegate void SynchronousWriteLineDelegate(string message);
    private SynchronousWriteLineDelegate DoWriteLine;
    private void SynchronousWriteLine(string message)
    {
        Console.WriteLine(message);
    }
    public AsyncLineWriter()
    {
        DoWriteLine = new SynchronousWriteLineDelegate(SynchronousWriteLine);

    public IAsyncResult BeginWriteLine(string message, AsyncCallback callback, object state)
    {
        return DoWriteLine.BeginInvoke(message,callback,state);
    }
    public void EndWriteLine(IAsyncResult asyncResult)
    {
        DoWriteLine.EndInvoke(asyncResult);
    }
}

The following unit test is intermittently failing, but I don’t understand where the race condition is:

[TestMethod]
public void Callback_is_called()
{
    // Arrange
    AsyncLineWriter lineWriter = new AsyncLineWriter();
    object state = new object();
    object callbackState = null;
    AsyncCallback callback = (r) =>
        {
            callbackState = r.AsyncState;
        };

    // Act
    IAsyncResult asyncResult = lineWriter.BeginWriteLine("test", callback, state);
    lineWriter.EndWriteLine(asyncResult);

    // Assert
    Assert.AreSame(state, callbackState);
}

Answer 1

In this pattern, the callback is run on a thread pool thread and you are supposed to call EndInvoke from within the callback.

EndInvoke doesn’t wait for the callback to complete ( because this would cause a deadlock ), so you have a race between the callback and your test method.

---

EDIT: The wait handle can be set before the callback has completed too. Try this:

[TestMethod]
public void Callback_is_called()
{
    // Arrange
    var lw = new AsyncLineWriter();

    object state = new object();
    object callbackState = null;

    var mre = new ManualResetEvent( false );

    AsyncCallback callback = r =>
        {
            callbackState = r.AsyncState;

            lw.EndWriteLine( r );

            mre.Set();
        };

    // Act
    var ar = lw.BeginWriteLine( "test", callback, state );
    mre.WaitOne();

    // Assert
    Assert.AreSame( state, callbackState );
}