I would say following but it would apply for other languages as well.
a. Does the software run if various services are not ON.What is the behavior?Is behavior acceptable.It could be as simple as displaying an error message or something complex that show data from cache.
b. Conduct concurrency tests i.e. same transactions performed by many users to ensure that it can handle multi threading.
c. I normally conduct a test by executing some tests by putting machine date ahead by one year.We once discovered that application stopped running because developer was using an evaluation version of IDE.
d. If web services are used.Use tools like Soap Sonar to see how robust web services are.
hey it depends on your application but normally all application should support Unicode so why not inject some crazy strings (such as Russian characters or Japanese characters) via search (post method). Also inject some SQL script to attempt to retrieve data (which should not be allowed by any app).
install old version of .NET and run your app and then upgrade to new version of .NET and then run your application and see if any thing is different.
I believe your C-sharp app might have services running at the background stop and start the services and then launch your app again see if thatís going to make any difference.