What is an internal sealed class in C#?

I was looking through some C# code for extending language support in VS2010 (Ook example). I saw some classes called internal sealed class

What do these do? Would one use them?

Solution 1:

It is a class that:

• internal: Can only be accessed from within the assembly it is defined (or friend assemblies).
• sealed: Cannot be inherited.

Marking classes as internal is a way of preventing outside users of an assembly from using them. It's really a form of design encapsulation and IMHO it is good practice to mark types that are not part of the intended public API\object models as internal. In the long term this prevents users of your library from coupling themselves to types which you did not intend them to. This sort of unintended coupling harms your ability to change and evolve the way your libraries are implemented as you cannot change them without breaking your clients. Using internal helps to keep the public and usable surface area of a library down to what is intended.

Marking classes as sealed prevents these classes from being inherited. This is a pretty drastic design intent which is sometimes useful if a class is already so specialized that it is sensible that no other functionality should be added to it via inheritance either directly or via overriding its behaviour.

internal and sealed modify types in quite different ways, but they can be used together.

NB You have some further scoping control of internal as you can define a set of other assemblies as 'friends'. These friend assemblies may access your internal types. This can be useful for defining sets of co-operating assemblies such as production and test assemblies. It is often desirable that a test assembly can see all the types in the assembly it is testing.

Solution 2:

• internal: A class which can only be accessed inside the same assembly.

  Assembly1.dll:

  namespace test {
      internal class InternalClass {
      }
  
      public class PublicClass { 
      }
  } 
  

  Assembly2.dll:

  using test;
  ...
  InternalClass c1; // Error
  PublicClass c2; // OK
  

• sealed: A class which cannot be derived from

  sealed class SealedClass { ... }
  
  class ChildClass : SealedClass {} //ERROR