From Eric Lippert, blogger of all things internal to C#: http://blogs.msdn.com/ericlippert/archive/2010/02/04/how-many-passes.aspx:
The C# language does not require that
declarations occur before usages,
which has two impacts, again, on the
user and on the compiler writer. [...]
The impact on the compiler writer is
that we have to have a “two pass”
compiler. In the first pass, we look
for declarations and ignore bodies.
Once we have gleaned all the
information from the declarations that
we would have got from the headers in
C++, we take a second pass over the
code and generate the IL for the
bodies.
To sum up, using something does not require declaring it in C#, whereas it does in C++. That means that in C++, you need to explicitly declare things, and it's more convenient and safe to do that with header files so you don't violate the One Definition Rule.
