messagebox yn - displays the message with (YES NO).
messagebox ync - output the message from (YES NO CANCEL).

Return:
1 - yes
-1 - no
0 - cancellation

Example
int a = messageboxync ("Select");
if (a == 1) messagebox ("Yes");
if (a == - 1) messagebox ("No");
if (a == 0) messagebox ("Cancel");

or
if (messageboxyn ("Selection") == 1)
{
messagebox ("Yes");
}
else
{
messagebox ("No");
}