// Pinvoke Quit
internal const uint WM_QUIT = 0x12;
[DllImport("user32.dll", EntryPoint = "PostThreadMessage", CharSet = CharSet.Unicode)]
[return: MarshalAs(UnmanagedType.Bool)]
internal static extern bool PostThreadMessage(int idThread, uint Msg, IntPtr wParam, IntPtr lParam);

foreach (Process process in Process.GetProcessesByName("APPNAME"))
{
    if (process.Id != Process.GetCurrentProcess().Id)
    {
        try
        {
            foreach (ProcessThread thread in process.Threads)
            {
                PostThreadMessage(thread.Id, WM_QUIT, IntPtr.Zero, IntPtr.Zero);
            }
        }
        catch (Exception ex)
        {
            ErrorHandler(ex);
        }
    }
}




// or pinvoke closing
internal const int WM_CLOSE = 0x0010;
[DllImport("user32.dll", CharSet = CharSet.Auto)]
internal static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);

NativeMethods.SendMessage(hWindow, NativeMethods.WM_CLOSE, IntPtr.Zero, IntPtr.Zero);



// or Process
Process[] processes = Process.GetProcessesByName("APPNAME");
foreach (Process process in processes)
{
    if (process.Id != Process.GetCurrentProcess().Id)
    {
        process.CloseMainWindow();
    }
}