Why are static classes considered “classes” and “reference types”?

Question

I’ve been pondering about the C# and CIL type system today and I’ve started to wonder why static classes are considered classes. There are many ways in which they are not really

Answer 1

Although class types, value types, and interfaces behave in many regards as though they are in three different kinds of things, they are in fact all described using the same kind of Type object; the parentage of a type determines which kind of thing it is. In particular, all types in .NET are class types except for the following:

• Types other than System.Object which inherit from null; those are interfaces.

• Types other than System.ValueType or System.Enum which inherit from System.ValueType or System.Enum; those are value types.

• A few types like pointers and byrefs, which may be identified by Type objects (necessary for things like parameter types) but don't have members the way other types do.

Every type which has members, and whose parentage does not meet either of the above criteria, is considered to be a class. Static classes aren't really classes because of any particular quality they have, but rather because they don't have any quality that would make them be some other named kind of thing, and calling them "static classes" seems easier than inventing some other term to describe them.

Answer 2

Your question is "why do I have to type the words static class X rather than foobar X". The answer is, because programmers already associate the word 'class' with 'a bundle of tightly packed encapsulated functionality someone wrote for me'. Which, coincidentally, fits perfectly with the definition of static classes.

They could've used namespaces instead, yes. That's what happens in C++. But the term 'static class' has an advantage here: it implies a smaller and much more tightly coupled group of functionality. For example, you can have a namespace called Qt or boost::asio but a static class called StringUtils or KWindowSystem (to borrow one from KDE).

Answer 3

I don't know if this qualifies as an answer, but I would point out that "static classes" are more of a language concept and less of a CLR concept. From the point of view of the CLR, they are just classes, like any other. It's up to the language to enforce all the rules you described.

As such, one advantage of the current implementation is that it does not add further complexity to the CLR, which all CLR-targeting languages would have to understand and model.

Answer 4

Static classes and "normal" classes (and structs) are containers for executable code (members fields, properties, methods) and they declare a Type. If they had a separate word for this then we would ask the opposite ("if they are so similar, why did you not use the kayword class?"). I'd suggest "CLR via C#", where it's well explained how type resolving, method calling, etc occurs. It works in the same way for "both" classes, just instance members have additional parameter passed in for the instance object.

Classes are not like namespaces because they are only for naming and referencing. They do not affect the functionality of the class.

Classes are also different from interfaces, because interfaces are merely compile-time verification tools and do not have functionality of their own.