SEHException was unhandeled by user code

I am trying to RUN a Unit Test Method which calls CommonUtility.cs which calls " return RoleEnvironment.GetConfigurationSettingValue(key); "

I have Mocked GetConfigurationSettingValue like below,

var objDataMock = Rhino.Mocks.MockRepository.GenerateStub<ICommonUtility>();
objDataMock.Stub(y => y.GetConfigurationSettingValue(Arg<string>.Is.Anything)).Return("true");

Still this mocking does not seem working.

I have tried removing in ServiceDefination.csdef but still that does not seem to work.

Can I please get any other help on this issue. I am unable to run Unit Test Methods.