This is a slightly over-engineered solution to our Calculator problem. :)

The whole solution was developed using strict TDD approach, but even though some of the cases slipped through my net and I ended up doing few characterization tests. Mainly for protecting the inputs. In the end the code nears 100% test-coverage.

The tests are using multiple variations of auto-mocking and auto-fixturing to avoid newing-up instances of SUT. This keeps our hands free when constructor signatures change, a natural side-effect of constructor injection.

The whole solution was also developed with DI in mind, althought there are very few places where DI is beneficial in this problem. There is one example though, the FileWrapper, which wraps the System.IO.File, and implements IFile to make testing simple.

I am not very happy with the IFile interface though, it's leaky, but I did not want to over-complicate the solution even more, by providing an IDataSource interface or somthing.

Even though you recommend using external libraries, I did not find their use warranted. The .NET Framework provides a solid base for all the file operation I needed, as well as regual expression support which made the parsing of the input trivial.

The solution is executed through command line interface. You can find the given examples pre-canned in the Examples folder of the solution.