What is an internal sealed class in C#?

An internal sealed class is one that is:

internal - Only accessible from within the same assembly
sealed - Cannot be subclassed

In other words, there's no way for you to use it directly.

• 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
  

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.

Internal means the member is accessible to other types that are defined in the same assembly. A Sealed class is sort of the oppositie of abstract. It can be instantiated but cannot serve as a base class. The primary reason to seal a class is to prevent your users from fiddling around with it and breaking it. It’s also the case that sealing a class permits certain compiler optimizations that are not possible with non-sealed classes.