What's the difference between InvokeAsync and BeginInvoke for WPF Dispatcher

Question

I noticed in .NET 4.5 that the WPF Dispatcher had gotten a new set of methods to execute stuff on the Dispatcher\'s thread called InvokeAsync. Before, .NET 4.5 we had Invoke

Answer 1

The exception handling is different.

You may want to check the following:

private async void OnClick(object sender, RoutedEventArgs e)
{
    Dispatcher.UnhandledException += OnUnhandledException;
    try
    {
        await Dispatcher.BeginInvoke((Action)(Throw));
    }
    catch
    {
        // The exception is not handled here but in the unhandled exception handler.
        MessageBox.Show("Catched BeginInvoke.");
    }

    try
    {
       await Dispatcher.InvokeAsync((Action)Throw);
    }
    catch
    {
        MessageBox.Show("Catched InvokeAsync.");
    }
}

private void OnUnhandledException(object sender, DispatcherUnhandledExceptionEventArgs e)
{
    MessageBox.Show("Catched UnhandledException");
}

private void Throw()
{
    throw new Exception();
}

Answer 2

There are no differences as the BeginInvoke method calls a private LegacyBeginInvokeImpl method which itslef calls the private method InvokeAsyncImpl (the method used by InvokeAsync). So it's basically the same thing. It seems like it's a simple refactoring, however it's strange the BeginInvoke methods weren't flagged as obsolete.

BeginInvoke :

public DispatcherOperation BeginInvoke(DispatcherPriority priority, Delegate method)
{
    return this.LegacyBeginInvokeImpl(priority, method, null, 0);
}

private DispatcherOperation LegacyBeginInvokeImpl(DispatcherPriority priority, Delegate method, object args, int numArgs)
{
    Dispatcher.ValidatePriority(priority, "priority");
    if (method == null)
    {
        throw new ArgumentNullException("method");
    }
    DispatcherOperation dispatcherOperation = new DispatcherOperation(this, method, priority, args, numArgs);
    this.InvokeAsyncImpl(dispatcherOperation, CancellationToken.None);
    return dispatcherOperation;
}

InvokeAsync :

public DispatcherOperation InvokeAsync(Action callback, DispatcherPriority priority)
{
    return this.InvokeAsync(callback, priority, CancellationToken.None);
}

public DispatcherOperation InvokeAsync(Action callback, DispatcherPriority priority, CancellationToken cancellationToken)
{
    if (callback == null)
    {
        throw new ArgumentNullException("callback");
    }
    Dispatcher.ValidatePriority(priority, "priority");
    DispatcherOperation dispatcherOperation = new DispatcherOperation(this, priority, callback);
    this.InvokeAsyncImpl(dispatcherOperation, cancellationToken);
    return dispatcherOperation;
}

Answer 3

Well, one difference I've noticed is that InvokeAsync has a generic overload that returns a DispatcherOperation as a return value and accepts a Func as its delegate input parameter. Thus, you can retrieve the result of the operation via InvokeAsync in a type-safe way analogous to how you can await the result of a Task.

Answer 4

There is a difference in method signature:

BeginInvoke(Delegate, Object[])
InvokeAsync(Action)

For BeginInvoke() compiler creates array Object[] implicitly while for InvokeAsync() such array is not needed:

IL_0001:  ldarg.0
IL_0002:  call       instance class [WindowsBase]System.Windows.Threading.Dispatcher [WindowsBase]System.Windows.Threading.DispatcherObject::get_Dispatcher()
IL_0007:  ldarg.1
IL_0008:  ldc.i4.0
IL_0009:  newarr     [mscorlib]System.Object
IL_000e:  callvirt   instance class [WindowsBase]System.Windows.Threading.DispatcherOperation [WindowsBase]System.Windows.Threading.Dispatcher::BeginInvoke(class [mscorlib]System.Delegate, object[])


IL_0014:  ldarg.0
IL_0015:  call       instance class [WindowsBase]System.Windows.Threading.Dispatcher [WindowsBase]System.Windows.Threading.DispatcherObject::get_Dispatcher()
IL_001a:  ldarg.1
IL_001b:  callvirt   instance class [WindowsBase]System.Windows.Threading.DispatcherOperation [WindowsBase]System.Windows.Threading.Dispatcher::InvokeAsync(class [mscorlib]System.Action)