The thing you need to be careful with in that case it what CPU you're C# assembly is set to. Since your .dll is obviously 32-bit, it will appear to work fine if the C# is set to AnyCPU and you test on a 32-bit system. If you subsequently move to a 64-bit machine, it'll suddenly break. However if you explicitly set the CPU to 32-bit in the project, it'll always work (though obviously won't take full advantage of a 64-bit CPU).
IIRC the default CPU type in Visual Studio changed around VS2010 from AnyCPU to 32-bit, precisely because people weren't realising this.