internal class in C#

The internal keyword is an access modifier for types and type members. Internal members are accessible only within files in the same assembly.

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.

It is an error to reference a member with internal access outside the assembly within which it was defined.

Caution   An internal virtual method can be overridden in some languages, such as textual Microsoft intermediate language (MSIL) using Ilasm.exe, even though it cannot be overridden using C#.

Example

This example contains two files, Assembly1.cs and Assembly2.cs. The first file contains an internal base class, BaseClass. In the second file, an attempt to access the member of the base class will produce an error.

File Assembly1.cs:

// Assembly1.cs
// compile with: /target:library
internal class BaseClass 
{
   public static int IntM = 0;
}

File Assembly2.cs

// Assembly2.cs
// compile with: /reference:Assembly1.dll
// CS0122 expected
class TestAccess 
{
   public static void Main() 
   {
      BaseClass myBase = new BaseClass();   // error, BaseClass not visible outside assembly
   }
}

http://msdn.microsoft.com/en-us/library/7c5ka91b(VS.71).aspx