Best method to check that a file hasn't been modified or altered

Question

I'm working on a solution and one of the features is to check that some files have not been tampered in other words hacked. I was planning on using the MD5 sum with a mixture of created and modified dates, but wanted to see if anybody has done something like this before. I'm using C# at the moment but you could suggest any other language. I just want to hear the technique part of it or architecture.