Cannot convert lambda expression to type 'System.Delegate'

52
votes

Neither of these work:

_uiDispatcher.Invoke(() => { });
_uiDispatcher.Invoke(delegate() { });

All I want to do is Invoke an inline method on my main UI thread. So I called this on the main thread:

_uiDispatcher = Dispatcher.CurrentDispatcher;

And now I want to execute some code on that thread from another thread. How do I do it? Am I using the wrong syntax?

Note that this is not a WPF application; I've referenced WindowsBase so I could get access to the Dispatcher class.

4
c#multithreadingdispatcher
What type is _uiDispatcher? Did you use the UI's synchronized object? - IAbstract
uiDispatcher is an instance of msdn.microsoft.com/en-us/library/…. I thought that was implicit from _uiDispatcher = Dispatcher.CurrentDispatcher. "The UI" doesn't have a synchronized object AFAIK....but how would I use it? What would do that do for me? - mpen
Ah ...ok, I don't think the thread dispatcher is going to work. Is this a WinForm application? - IAbstract

4 Answers

78
votes

The problem is that you aren't providing the exact type of delegate you want to invoke. Dispatcher.Invoke just takes a Delegate. Is it an Action<T>? If so, what is T? Is it a MethodInvoker? Action? What?

If your delegate takes no arguments and returns nothing, you can use Action or MethodInvoker. Try this:

_uiDispatcher.Invoke(new Action(() => { }));
5
votes
 this.Dispatcher.Invoke((Action)(() => { textBox1.Text = "Test 123"; }));
0
votes

Unless I've missed something, all you've told us is this is not a WPF application. I don't think the Dispatcher is the correct class to use.

If this is a WinForm app, your UI thread can be accessed via the WindowsFormsSynchronizationContext

0
votes

Expanding on other answers a little.

Action with no parameters:

_uiDispatcher.Invoke(new Action(() =>
{
    // Do stuff
    textBox1.Text = "Test 123";
}));

Action with 1 parameter:

_uiDispatcher.Invoke(new Action<bool>((flag) =>
{
    if (flag)
    {
        // Do stuff
        textBox1.Text = "Test 123";
    }

}));

Action with 2 parameters:

_uiDispatcher.Invoke(new Action<int, string>((id, value) =>
{
    // Do stuff
    textBox1.Text = $"{value} {id}";
}));

And so on...