Yes it is possible.  Since you seem to have a security related purpose
to this then I would sugget one of the cryptographic hash functions,
either SHA256 or SHA512.  Both of these are available in .NET:
System.Security.Cryptography.SHA256Managed and
System.Security.Cryptography.SHA512Managed

If you need some general background then the Wikipedia article is a
good introduction:
http://en.wikipedia.org/wiki/Cryptographic_hash_function

Probably not, this is one of the problems that cryptographic hash
functions are designed to solve.

rossum

Did you compile the DLL yourself? If you did, you could sign the DLL which
will give you a signature. Then using reflection you can check the
signatures:

http://groups.google.com/group/microsoft.public.dotnet.framework/msg/7ccd12
b8770b0714

Depending on what you're doing, I think this check can be done
declaratively in the .config file.

I got a few questions:
- tampered by who?
- and why?
- and when?

The answers to those questions would be handy to have because... what if
the same person modifies your program too? Replacing that hash for instance?

You could, technically, produce a cryptographic hash of the file, and
sign it with an asymmetric cryptography algorithm, which would make it
impossible for someone without your private key to change the file and
then produce a new hash.

However, that same person could just as easily change the program so
that it either didn't check the dll, or just used his/her crypto keys
instead.

So it all depends on how paranoid you intend to be :)