https://msdn.microsoft.com/en-us/lib...vs.110%29.aspx
If the Ctrl+Z character is pressed when the method is reading input from the console, the method returns null. This enables the user to prevent further keyboard input when the ReadLine method is called in a loop. The following example illustrates this scenario.
Non è che hai creato un eseguibile senza console (o hai premuto Ctrl-D da console, equivalente di Ctrl-Z su Linux)?