Question

I want my c# winform application to switch to another running instance if a certain event occurs.

For example if I have a application with just a button and three instances are running at the moment. Now if I

1. press the button in first instance, focus to second instance
2. press the button in second instance, focus to third instance
3. press the button in third instance, focus to first instance

How do i do that?

Answer 1

if you know the handle of the other instances you should just call the Windows API: SetForegroundWindow:

[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool SetForegroundWindow(IntPtr hWnd);

[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

you can use the FindWindow API call to get the handle of the other instances, for example:

 public static int FindWindow(string windowName)
    {
        int hWnd = FindWindow(null, windowName);

        return hWnd;
    }

you can search for those api calls here in SO for more examples, for example found this one:

How do I focus a foreign window?