c# - "Cross-thread operation not valid" exception on inner controls

Question

I've been struggling with this for quite a while: I have a function designed to add control to a panel with cross-thread handling, the problem is that though the panel and the control are in "InvokeRequired=false" - I get an exception telling me that one of the controls inner controls are accessed from a thread other than the thread it was created on, the snippet goes like this:

public delegate void AddControlToPanelDlgt(Panel panel, Control ctrl);
    public void AddControlToPanel(Panel panel, Control ctrl)
    {
        if (panel.InvokeRequired)
        {
            panel.Invoke(new AddControlToPanelDlgt(AddControlToPanel),panel,ctrl);
            return;
        }
        if (ctrl.InvokeRequired)
        {
            ctrl.Invoke(new AddControlToPanelDlgt(AddControlToPanel),panel,ctrl);
            return;
        }
        panel.Controls.Add(ctrl); //<-- here is where the exception is raised
    }

the exception message goes like this:

"Cross-thread operation not valid: Control 'pnlFoo' accessed from a thread other than the thread it was created on"

('pnlFoo' is under ctrl.Controls)

How can I add ctrl to panel?!

---

When the code reaches the "panel.Controls.Add(ctrl);" line - both panel and ctrl "InvokeRequired" property is set to false, the problem is the the controls inside ctrl has "InvokeRequired" set to true. To clarify things: "panel" is created on the base thread and "ctrl" on the new thread, therefore, "panel" has to be invoked (causing "ctrl" to need invoke again). Once both of the invokes are done, it reaches the panel.Controls.Add(ctrl) command (both "panel" and "ctrl" doesn't need invocation in this state)

Here is a small snippet of the full program:

public class ucFoo : UserControl
{
    private Panel pnlFoo = new Panel();

    public ucFoo()
    {
        this.Controls.Add(pnlFoo);
    }
}

public class ucFoo2 : UserControl
{
    private Panel pnlFooContainer = new Panel();

    public ucFoo2()
    {
         this.Controls.Add(pnlFooContainer);
         Thread t = new Thread(new ThreadStart(AddFooControlToFooConatiner());
         t.Start()
    }

    private AddFooControlToFooConatiner()
    {
         ucFoo foo = new ucFoo();
         this.pnlFooContainer.Controls.Add(ucFoo); //<-- this is where the exception is raised
    }
}

Answer 1

As an aside - to save yourself having to create countless delegate types:

if (panel.InvokeRequired)
{
    panel.Invoke((MethodInvoker) delegate { AddControlToPanel(panel,ctrl); } );
    return;
}

Additionally, this now does regular static checks on the inner call to AddControlToPanel, so you can't get it wrong.