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.

England

Why is "Dispatcher.Invoke" in the title and in the real question you ask about "Dispatcher.BeginInvoke"? It's not the same... – Visitor

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.

Householder

I'm getting this error now: An object reference is required for the non-static field, method, or property 'System.Windows.Threading.Dispatcher.BeginInvoke(System.Delegate, params object[])'. I separated it out into 2 lines to try and see what is causing the error. var d = new Action(() => populateInbox(args.Jobs)); Dispatcher.BeginInvoke(d, args.Jobs); Error happens on the 2nd line. – England

Dispatcher.BeginInvoke(d, new object[] { args.Jobs }); Does not work either. – England

@Edward: Dispatcher is not a static class. You'll have to call this function either within the context of a UI element that has a Dispatcher property, or you'll have to pass in the dispatcher as another argument. – Householder

Yeah, you're right. I'm trying to use a WPF user control from within an Excel workbook project. The WPF user control is hosted in an ActionPane with an ElementHost. The pane is instantiated in a Ribbon control which does not have a Dispatcher property. Thanks. – England

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

Cankerous