If it is a managed DLL (.NET), you can open it with tools such as Reflector or ILDASM , and you will see the IL code. (Editing 2017-02-03: after 7 years, .NET parsers, of course, have advanced a lot and can now create very decent C # code)
If this is an unmanaged DLL (native), you're out of luck. The best you can do is load the disassembler. It leads you almost nowhere unless you know exactly where to go.
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