internal (C# Reference) 

The internal keyword is an access modifier for types and type members. Internal types or members are accessible only within files in the same assembly, as in this example:

public class BaseClass 
{
    // Only accessible within the same assembly
    internal static int x = 0;
}

For a comparison of internal with the other access modifiers, see Accessibility Levels and Access Modifiers (C# Programming Guide).

For more information on assemblies, see Assemblies and the Global Assembly Cache (C# Programming Guide).

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 type or a member with internal access outside the assembly within which it was defined.

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 instantiate BaseClass will produce an error.

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

// Assembly1_a.cs
// compile with: /reference:Assembly1.dll
class TestAccess 
{
   static void Main() 
   {
      BaseClass myBase = new BaseClass();   // CS0122
   }
}

In this example, use the same files you used in example 1, and change the accessibility level of BaseClass to public. Also change the accessibility level of the member IntM to internal. In this case, you can instantiate the class, but you cannot access the internal member.

// Assembly2.cs
// compile with: /target:library
public class BaseClass 
{
   internal static int intM = 0;
}

// Assembly2_a.cs
// compile with: /reference:Assembly1.dll
public class TestAccess 
{
   static void Main() 
   {
      BaseClass myBase = new BaseClass();   // Ok.
      BaseClass.intM = 444;    // CS0117
   }
}

C# Language Specification

For more information, see the following sections in the C# Language Specification:

• 3.5.1 Declared Accessibility

• 3.5.4 Accessibility constraints

• 10.2.3 Access Modifiers

• 10.2.6.2 Declared Accessibility

See Also

Reference

C# Keywords
Access Modifiers (C# Reference)
Accessibility Levels (C# Reference)
Modifiers (C# Reference)
public (C# Reference)
private (C# Reference)
protected (C# Reference)

Concepts

C# Programming Guide

Other Resources

C# Reference