Does foreach() iterate by reference?

Yes, until you change the generic type from List to IEnumerable..

Yes, obj is a reference to the current object in the collection (assuming MyClass is in fact a class). If you change any properties via the reference, you're changing the object, just like you would expect.

Be aware however, that you cannot change the variable obj itself as it is the iteration variable. You'll get a compile error if you try. That means that you can't null it and if you're iterating value types, you can't modify any members as that would be changing the value.

The C# language specification states (8.8.4)

"The iteration variable corresponds to a read-only local variable with a scope that extends over the embedded statement."