Will a DLL written in c# work with byond?
I don't see why they wouldn't, considering that it's all the same machine code in the end.
In response to Jeff8500 (#1)
I thought C-octothorpe compiled to bytecode? Can you compile it to machine code?
In response to Jp (#2)
I'd say point is that it works :)
In response to Ripiz (#3)
If it compiles to bytecode it probably /doesn't/ work, depending on how C# dlls work.
Will a DLL written in c# work with byond?

Not directly but you can build a C++ DLL that loads .NET and calls the C# dll. The only issue there is the fact that for some odd reason loading an assembly file only works with an absolute path which makes things a bit messy doing a direct approach. So you'd probably want to manually load up the dll into memory and then load the assembly from memory rather than a file.
In response to Jp (#2)
C# assemblies (DLLs etc) are compiled into CIL, JIT compiled to resolve some late dependencies, then ran on the CLR, so no, they aren't machine code and they won't natively interface with machine code. Even if they could I would throw serious concerns about calling conventions.
In response to Jp (#2)
Oh, I assumed it compiled to machine code; you know the saying, though >_>.
In response to Jeff8500 (#7)
C# is basically Microsoft's rip of java.