Try this:
using Microsoft.Win32;
[DllImport("user32.dll")]
public static extern int FindWindow(string lpClassName,string lpWindowName);
[DllImport("user32.dll")]
public static extern int SendMessage(int hWnd, uint Msg, int wParam, int lParam);
public const int WM_SYSCOMMAND = 0x0112;
public const int SC_CLOSE = 0xF060;
private void closeWindow()
{
int iHandle = FindWindow("Notepad", "Untitled - Notepad");
if (iHandle > 0)
{
SendMessage(iHandle, WM_SYSCOMMAND, SC_CLOSE, 0);
}
}