When I declare a class as internal, why does the IL show it as private?

Question

If I declare a class as internal, why does the IL show it as private?

internal class Thing

.class private auto ansi beforefieldinit Thing.Thing
       exten         


        

Answer 1

From IL's point of view, private means private to the assembly, i.e. internal in C#.

In C#, it is not possible to mark types as private if they are not nested. IL's equivalent accessibility for such types is nested private.

So we have:

• C#'s internal -> IL's private (to the assembly)
• C#'s private -> IL's nested private (to the enclosing type)

Answer 2

The mapping of C# keywords to IL keywords isn't always a logical one. Ecma-335, section II.23.1.15 shows what flags are valid for a type. You'll see that it only defines Public, NotPublic and a set of NestedXxx flags. Nothing similar to "internal". So your class is actually NotPublic, displayed as "private" in ildasm.

It is easy to see a side-effect of this: try this declaration in your C# code:

private class DoesNotWork {
}

You'll get:

error CS1527: Elements defined in a namespace cannot be explicitly declared as private, protected, or protected internal

Answer 3

On MSDN, it says:

The C# keywords protected and internal have no meaning in IL and are not used in the reflection APIs. The corresponding terms in IL are Family and Assembly. To identify an internal method using reflection, use the IsAssembly property. To identify a protected internal method, use the IsFamilyOrAssembly.

So I guess it just makes them private, since they can't be accessed from elsewhere.

EDIT: I see how my answer might not be completely correct, I just thought it was something worth noting. The MSDN article I linked has some interesting stuff on this "what code we write" - "what it becomes" relationship.