WPF web application contextmenu on left click

Question

I got a very strange problem. I want to open a contextmenu on left click on a button... Here is my solution but it fires an System.ComponentModel.Win32Exception.

public class ButtonKontext : System.Windows.Controls.Button
{
    public ButtonKontext()
    {
        this.Click += new System.Windows.RoutedEventHandler(this.Button_Click_1);

    }

    public void Button_Click_1(object sender, RoutedEventArgs e)
    {
        if (this.ContextMenu != null)
        {
            //ContextMenu contextMenu = this.ContextMenu;
            //this.ContextMenu.PlacementTarget = this;
            this.ContextMenu.IsOpen = true;  // System.ComponentModel.Win32Exception here
        }
    }
}

System.ComponentModel.Win32Exception: "Untergeordnetes Fenster kann nicht auf der obersten Ebene erstellt werden" --> edit: Child window can not be created at the top level – Thx Jimenemex for correct translation.

The thing is, if i right click the button the contextmenu shows up normal. So what is the problem here? Any idea?