C++ doesn't have generics, it has templates, which are effectively a syntax-constrained form of macros. That's why it lends itself to abuse (or creative use, depending on your take on macros).
So C++ generates separate code for each template type and C# and Java do a typecheck at compile time and reuse the same codepath for all types at runtime?
.NET does a mixture of C++ and Java, meaning generic instatiations for value types get their own version of the code, instatiations for reference types get a shared version.
It's worth noting that in case of C#, it's really an implementation detail. It could just as well share all instantiations by boxing and adding a bunch of runtime checks - the language semantics wouldn't change because of it.
In C++, on the other hand, separate compilation of template for each instantiation is effectively part of language semantics, because it has numerous observable effects.
