Compatibility of .NET csc and Mono mcs

Question

I know .NET and Mono are binary compatible but given a set of source code, will csc and mcs produce the exact same 100% identical binary CLI executable? Would one be able to tell whether an executable was compiled with csc or mcs?

Answer 1

A lot of things are not fully defined in the spec, or are implementation-specific extensions.

Examples of not-fully-specified:

• the synchronisation semantics of field-like events; this is explicitly open to implementations in the ecma spec; it is stricty defined in the MS spec, but uses a different version in c# 4.0 which does not yet appear in the formal spec, IIRC
• Expression construction (from lambdas); is simply "defined elsewhere" (actually, it isn't)

Examples of implementation extensions:

• P/Invoke
• COM interface handling (i.e. how you can call new on an interface)

So no: it is not guarantees to have the same IL, either between csc or [g]mcs - but even between different versions of csc.

Even more: depending on debug settings, optimizations being enabled or not, and some compilation constants being defined (such as DEBUG or TRACE), the same compiler will generate different code.