Practical uses for the "internal" keyword in C#

Utility or helper classes/methods that you would like to access from many other classes within the same assembly, but that you want to ensure code in other assemblies can't access.

From MSDN (via archive.org):

A common use of internal access is in component-based development because it enables a group of components to cooperate in a private manner without being exposed to the rest of the application code. For example, a framework for building graphical user interfaces could provide Control and Form classes that cooperate using members with internal access. Since these members are internal, they are not exposed to code that is using the framework.

You can also use the internal modifier along with the InternalsVisibleTo assembly level attribute to create "friend" assemblies that are granted special access to the target assembly internal classes.

This can be useful for creation of unit testing assemblies that are then allowed to call internal members of the assembly to be tested. Of course no other assemblies are granted this level of access, so when you release your system, encapsulation is maintained.

If Bob needs BigImportantClass then Bob needs to get the people who own project A to sign up to guarantee that BigImportantClass will be written to meet his needs, tested to ensure that it meets his needs, is documented as meeting his needs, and that a process will be put in place to ensure that it will never be changed so as to no longer meet his needs.

If a class is internal then it doesn't have to go through that process, which saves budget for Project A that they can spend on other things.

The point of internal is not that it makes life difficult for Bob. It's that it allows you to control what expensive promises Project A is making about features, lifetime, compatibility, and so on.

Another reason to use internal is if you obfuscate your binaries. The obfuscator knows that it's safe to scramble the class name of any internal classes, while the name of public classes can't be scrambled, because that could break existing references.