I don't use ";" in the end.
I use Console.Write() and not Console.WriteLine().
According to the interpreter my PIBAS programm is correct.
Still I've got WA#1.

I'd like to have some hints from people with C# AC.

Disregard that. I don't understand how I checked my program a dozen times and didn't see the obviously wrong length of last substring!

Funny thing about the interpreter provided by DR. Zhihua Lai: it does not react to substring length which is higher than the length of an actual string. That's why my PIBAS programm worked well in interpreter but gave me WA1 here.