Dispatcher.Invoke with anonymous delegate works in Silverlight but not WPF

Question

In Silverlight 4 I have a custom service class which has an asynchronous Completed event. Inside the Completed event I take the returned data and invoke a populate method via something like this:

private void service_Completed(object sender, CompletedEventArgs args)
{
    Dispatcher.BeginInvoke(() => populateInbox(args.Jobs));
}

private void populateInbox(List<JobViewModel> jobs)
{
    inbox.DataContext = jobs;
}

The BeginInvoke works in SL4, however when I ported it to WPF I get the following error:

Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type

I tried changing it to an in-line, anonymous, paramaterized delegate:

Dispatcher.BeginInvoke(delegate(List<JobViewModel> jobs)
{
    inbox.DataContext = jobs;
});

However, that yields the same compile-time error.

Any idea how to get this to work in WPF? Refactoring to use the BackgroundWorker is not an option for me.

Answer 1

You need to specify an explicit delegate type. Just use an Action.

Dispatcher.BeginInvoke(new Action(() => populateInbox(args.Jobs));

You could, however, avoid having to close over the args.Jobs value like this:

Dispatcher.BeginInvoke(new Action((jobs) => populateInbox(jobs)), jobs);

This is because the single-parameter version of Dispatcher.BeginInvoke has a different signature in Silverlight than in WPF. In Silverlight, it takes an Action, which allows the C# compiler to implicitly type your lambda as an Action. In WPF, it takes a Delegate (like its Control.BeginInvoke analog in Winforms), so the C# compiler has to have a delegate type explicitly specified.

Answer 2

In WPF and winforms you must cast it to a MethodInvoker first, otherwise you will get the error Cannot convert anonymous method to type 'System.Delegate' because it is not a delegate type.

Dispatcher.BeginInvoke((MethodInvoker) delegate(List<JobViewModel> jobs)
{
   inbox.DataContext = jobs;
});

For more info: http://msdn.microsoft.com/en-us/library/system.windows.forms.methodinvoker.aspx