ally,
There are two things you need to debug source code for a class. The
first is the source code itself. If you don't have this, then you can't
step through line by line. The second thing you need is a build of the
assembly with the debug symbols in it. If the assembly doesn't have these,
then you can't step through it. If you have just the assembly with the
debug symbols, you could step through the IL still, but not the actual
source that was written.
Hope this helps.