Hi,
strings don't care about their content.
When converting a string to a number using the Convert class or the Parse/TryParse methods of numeric types, then such method expects an exact number, no more, no less.
And you have
Serial.print("\n ");
Serial.print(outputValue);
which clearly contains a space, something conversion methods do not understand.
I suggest you:
(1) change your arduino code to not include a space, and to send the newline AFTER te value, not before;
(2) and then to change your C# code to use int.TryParse (or int.Parse inside a try-catch block), so you can survive a badly formed number, which may occur when something goes wrong in the data transmission, something I/O should always allow for.
Sending \n after the value is relevant for the first value (your receiver currently gets an empty line when the system starts up); and it makes sure the last value sent (assuming your peripheral might be modified later to make it shut up at some point) is also received.
:)