Having done this from both sides of the table my guess would be that they want to see you program. Likely they will have some sort of simple problem you will have to solve.
The best prep you can do is to practice. Create a solution, add class libs, test projects and an app, web/wpf/wcf write some code covering the technologies they mention in the job app, Linq, PTL, REST, whatever, ensure you write unit tests and run them etc
A good question to ask is which version of Visual Studio they are using. You don't want to get tripped up because you are used to 2010 and they are on 2013 or vice versa. Similarly, do they use resharper? if you are used to that.
During the int, talk constantly through what you are typing, do the test first and build and run frequently so you pick up on any bugs/typos, ask the interviewer how they would do it "would you use dependency injection here?" and stuff as this indicates you know about the topic without having to do it/expressing an opinion about best practice