Hi,
The Swap.NextInstance() API is used to handle 'new' calls in the code under test - the instance you provide to .With() will be returned when you call new. Because you asked to do this for an interface type, which cannot be instantiated (i.e. you will never see 'new IContentService') you the error.
What are you trying to test? Can you use a concrete class instead of the interface with NextInstance()?
Doron
Typemock Support
