c# - Determine Whether Program is the Active Window in .NET

Question

I have a C#/.NET app and I want to implement the following behavior:

I have a popup menu. Whenever the user clicks on anything within the application that is not the popup menu, I want the popup menu to close.

However, whenever a user is not in the application I don't want anything to happen.

I'm trying to manage this through the LostFocus event, but I'm having trouble determining whether my application is the active window. The code looks something like this.

    private void Button_LostFocus(object sender, System.EventArgs e)
    {
        if (InActiveWindow()) {
           CloseMenu()
        }
        else {
           // not in active window, do nothing
        }
    }

What I need to know is how to implement the InActiveWindow() method.

Answer 1

You could P/Invoke into GetForegroundWindow(), and compare the HWND returned to the application's form.Handle property.

Once you have the handle, you can also P/Invoke GetAncestor() to get the root owner window. This should be the handle of your application's main, startup window, if this is in your application.