Listing 1: An implementation of IAsynchResult

public interface IAsyncResult
{
object AsyncState { get; }
WaitHandle AsyncWaitHandle { get; }
bool CompletedSynchronously { get; }
bool IsCompleted { get; }
}
¬ public class AsynchResult<TArg, TResult> : IAsyncResult
{
private readonly AsyncCallback m_asynchCallBack;
private readonly Object m_asychState;
private int m_completedState;
private ManualResetEvent m_asynchWaitHandle;
private Exception m_exception;
private TResult m_result = default(TResult);
private TArg m_arg = default(TArg);

public TArg Arg { get { return m_arg; } }

public AsynchResult(TArg arg, AsyncCallback callback, Object state)
{
m_arg = arg;
m_asynchCallBack = callback;
m_asychState = state;
}

public void SetAsCompleted(TResult result, bool completedSynchronously, Exception exception)
{
m_result = result;
m_exception = exception;

int previous = Interlocked.Exchange(ref m_completedState,
completedSynchronously ? (int) AsynchronousState.CompletedSychronously : (int) AsynchronousState.
CompletedAsynchronously);

if (previous != (int)AsynchronousState.Pending)
{
throw new InvalidOperationException("A result has already been set");
}

if (m_asynchWaitHandle != null)
{
m_asynchWaitHandle.Set();
}

if (m_asynchCallBack != null)
{
m_asynchCallBack(this);
}
}

public TResult EndInvoke()
{
if (!IsCompleted)
{
AsyncWaitHandle.WaitOne();
AsyncWaitHandle.Close();
m_asynchWaitHandle = null;
}

if (m_exception != null)
{
throw m_exception;
}

return m_result;
}

#region IAsyncResult Members

public object AsyncState
{
get
{
return m_asychState;
}
}

public System.Threading.WaitHandle AsyncWaitHandle
{
get
{
if (m_asynchWaitHandle == null)
{
bool done = IsCompleted;
ManualResetEvent resetEvent = new ManualResetEvent(done);

if (Interlocked.CompareExchange(ref m_asynchWaitHandle, resetEvent, null)
!= null)
{
resetEvent.Close();
}
else
{
if (!done && IsCompleted)
{
m_asynchWaitHandle.Set();
}
}
}

return m_asynchWaitHandle;
}
}

public bool CompletedSynchronously
{
get
{
return Thread.VolatileRead(ref m_completedState) == (int)AsynchronousState.CompletedSychronously;
}
}

public bool IsCompleted
{
get { return Thread.VolatileRead(ref m_completedState) != (int)AsynchronousState.Pending; }
}

#endregion
}

About the Author

Eric Vogel is a Senior Software Developer for Red Cedar Solutions Group in Okemos, Michigan. He is the president of the Greater Lansing User Group for .NET. Eric enjoys learning about software architecture and craftsmanship, and is always looking for ways to create more robust and testable applications. Contact him at vogelvision@gmail.com.