The mutation technique inserts faults in a program under test in order to assess or generate test cases, or evaluate the reliability of the program. Faults introduced into the source code are defined using mutation operators. They should be related to different, also object-oriented features of a program. The most research on OO mutations was devoted to Java programs. This paper describes analytical and empirical study performed to evaluate the quality of advanced mutation operators for C# programs. Experimental results demonstrate effectiveness of different mutation operators. Unit tests suites and functional tests were used in experiments. A detailed analysis was conducted on mutation operators dealing with delegates and exception handling.