Question

What exactly does Control.Invoke(Delegate) do to get the delegate to run on the GUI thread? Furthermore, Its my understanding that invoke will block until the invoked function its done. How does it achieve this?

I would like some good gritty details. I’m hoping to learn something interesting.

Answer 1

Edit: The control implements ISynchronizeInvoke interface, You can make the same effect using the SynchronizationContext and call Post when you call Invoke. something like:

public object Invoke(Delegate method, object[] args)
{
    if (method == null)
    {
        throw new ArgumentNullException("method");
    }

    object objectToGet = null;

    SendOrPostCallback invoker = new SendOrPostCallback(
    delegate(object data)
    {
        objectToGet = method.DynamicInvoke(args);
    });

    _currentContext.Send(new SendOrPostCallback(invoker), method.Target);

    return objectToGet;
}

Further investigation using Reflector shows that the Invoke uses some native API calls to achieve that:

private object MarshaledInvoke(Control caller, Delegate method, object[] args, bool synchronous)
{
    int num;
    if (!this.IsHandleCreated)
    {
        throw new InvalidOperationException(SR.GetString("ErrorNoMarshalingThread"));
    }
    if (((ActiveXImpl) this.Properties.GetObject(PropActiveXImpl)) != null)
    {
        IntSecurity.UnmanagedCode.Demand();
    }
    bool flag = false;
    if ((SafeNativeMethods.GetWindowThreadProcessId(new HandleRef(this, this.Handle), out num) == SafeNativeMethods.GetCurrentThreadId()) && synchronous)
    {
        flag = true;
    }
    ExecutionContext executionContext = null;
    if (!flag)
    {
        executionContext = ExecutionContext.Capture();
    }
    ThreadMethodEntry entry = new ThreadMethodEntry(caller, this, method, args, synchronous, executionContext);
    lock (this)
    {
        if (this.threadCallbackList == null)
        {
            this.threadCallbackList = new Queue();
        }
    }
    lock (this.threadCallbackList)
    {
        if (threadCallbackMessage == 0)
        {
            threadCallbackMessage = SafeNativeMethods.RegisterWindowMessage(Application.WindowMessagesVersion + "_ThreadCallbackMessage");
        }
        this.threadCallbackList.Enqueue(entry);
    }
    if (flag)
    {
        this.InvokeMarshaledCallbacks();
    }
    else
    {
        UnsafeNativeMethods.PostMessage(new HandleRef(this, this.Handle), threadCallbackMessage, IntPtr.Zero, IntPtr.Zero);
    }
    if (!synchronous)
    {
        return entry;
    }
    if (!entry.IsCompleted)
    {
        this.WaitForWaitHandle(entry.AsyncWaitHandle);
    }
    if (entry.exception != null)
    {
        throw entry.exception;
    }
    return entry.retVal;
}