WinForms, MDI, multithreading - delegate to send events from thread to UI thread

[jmcilhinney]

I can't figure out how to send events from a worker thread to the UI thread.

That's because there is no such thing. Events are defined like so:

public event EventHandler SomethingHappended;
 
protected void OnSomethingHappened(EventArgs e)
{
    SomethingHappened?.Invoke(this, e);
}

The OnX method is where the X event is actually raised. When you want to raise that event, you call that method:

// Do something here.
 
// Raise the event.
OnSomethingHappened(EventArgs.Empty);

In code that can or will execute on a secondary thread, you can marshal a method call to the UI thread to execute code on the UI thread. That method can do whatever you want. It's just a method. If you want to raise your event in that method, do so, just as I showed above:

private void RaiseSomethingHappened()
{
    OnSomethingHappened(EventArgs.Empty);
}

and:

Invoke(RaiseSomethingHappened);

Alternatively, you could use a lambda to call the method that raises the event directly:

Invoke(() => OnSomethingHappened(EventArgs.Empty));

In summary, you "send" a method call to the UI thread, just as you always do, then raise the event in that method, just as you always do.