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

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.

We have an application that checks file validity for safety reasons. The CRC32 checksums are stored in a separate file using a simple dictionary lookup. Which of CRC32, MD5, or any other hashing/checksumming feature is purely choice: you simply need to know if the file has changed (at least that's what you've said). As each byte of the file is included in the calculation, any changes will be picked up, including simple swapping of bytes.

Don't use file dates: too unreliable and can be easily changed.