Send an Enter command using the SendMessage or PostMessage DLL

I am trying to send an enter command using c #, I can write to another process using this dll, but when I am trying to send the enter command it is written the following "{ENTER}" ...

  [DllImport("user32.dll", CharSet = CharSet.Auto)]
  public static extern string SendMessage(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam);

When I try to use a PostMessage nothing happens ..

 public void SendMessage(string message, IntPtr WindowHandle)
 {
     foreach (char c in message)
     {
         int charValue = c;
         IntPtr val = new IntPtr((Int32)c);
         Win32.SendMessage(WindowHandle, WM_CHAR, val, new IntPtr(0));
     }
 }

Could someone help me?