How to keep console window open

Put a Console.Read() as the last line in your program. That will prevent it from closing until you press a key

static void Main(string[] args)
{
    StringAddString s = new StringAddString();
    Console.Read();            
}

If you want to keep it open when you are debugging, but still let it close normally when not debugging, you can do something like this:

if (System.Diagnostics.Debugger.IsAttached) Console.ReadLine();

Like other answers have stated, the call to Console.ReadLine() will keep the window open until enter is pressed, but Console.ReadLine() will only be called if the debugger is attached.