How to return a value with Dispatcher.Invoke?

Question

Anyone knows how to return a value from Dispatcher.Invoke in wpf? I want to return the selected index for a ComboBox.

Thanks!

Answer 1

I have solved this. The solution is create a custom delegate that returns the desired type like this:

    private object GetValueCB(System.Windows.Controls.ComboBox cb)
    {
        object obj = null;


            if (!cb.Dispatcher.CheckAccess())
            {
                obj = cb.Dispatcher.Invoke(
                  System.Windows.Threading.DispatcherPriority.Normal,
                  (MyDelegate)
                    delegate()
                    {
                        return (obj = cb.SelectedValue);
                    }
                );

                return obj;
            }
            else
            {
                return obj = cb.SelectedValue;
            }

    }

    public delegate object MyDelegate();

Answer 2

There's another way that returns value from Invoke():

object oIsLoaded = container.Dispatcher.Invoke( new Func<bool> ( () =>
    {
        return container.IsLoaded;
    })
);

And by the way, chances are that the initial code (which is working with delegate) won't modify oIsLoaded at all; So I'd rather use a Func<> for returning a value from that kind of function.

Answer 3

int result = -1;

// this is synchronous
myCombo.Invoke(() => 
{
  result = myCombo.SelectedIndex;
});

return result;

This is, of course, kind of clunky. Better design would be to implement INotifyPropertyChanged in your VM, create a SelectedIndex property and bind the SelectedIndex property of your combo box to it. INPC binds are thread-insensitive (3.5 or 4.0+, I don't remember which), so you can read and update these properties from different threads in your VM without worry.

Answer 4

You can't do this directly but you can do this.

Dispatcher.Invoke() actually returns the return value from the delegate you call, so alter your delegate accordingly.

Return Value

Type: System.Object The return value from the delegate being invoked or null if the delegate has no return value.

Source

Answer 5

This is my method to retrieve selected value for a combobox, how can I say delegate to return value?

    private object getValueCB(System.Windows.Controls.ComboBox cb)
    {
        object obj;


            if (!cb.Dispatcher.CheckAccess())
            {
                obj = cb.Dispatcher.Invoke(
                  System.Windows.Threading.DispatcherPriority.Normal,
                  new Action(
                    delegate()
                    {
                        obj = cb.SelectedValue;
                    }
                ));

                return obj;
            }
            else
            {
                return obj = cb.SelectedValue;
            }

    }