Friend assemblies

A friend assembly is an assembly that can access another assembly's internal (C#) or Friend (Visual Basic) types and members. If you add an assembly attribute to AssemblyA to identify AssemblyB as a friend assembly, you no longer have to mark types and members in AssemblyA as public in order for them to be accessed by AssemblyB. This is especially convenient in the following scenarios:

  • During unit testing, when test code runs in a separate assembly but requires access to members in the assembly being tested that are marked as internal in C# or Friend in Visual Basic.

  • When you are developing a class library and additions to the library are contained in separate assemblies but require access to members in existing assemblies that are marked as internal in C# or Friend in Visual Basic.


You can use the InternalsVisibleToAttribute attribute to identify one or more friend assemblies for a given assembly. The following example uses the InternalsVisibleToAttribute attribute in AssemblyA and specifies assembly AssemblyB as a friend assembly. This gives assembly AssemblyB access to all types and members in Assembly A that are marked as internal in C# or Friend in Visual Basic.


When you compile an assembly like AssemblyB that will access internal types or internal members of another assembly like AssemblyA, you must explicitly specify the name of the output file (.exe or .dll) by using the -out compiler option. This is required because the compiler has not yet generated the name for the assembly it is building at the time it is binding to external references. For more information, see OutputAssembly (C#) or -out (Visual Basic).

using System.Runtime.CompilerServices;
using System;

[assembly: InternalsVisibleTo("AssemblyB")]

// The class is internal by default.
class FriendClass
    public void Test()
        Console.WriteLine("Sample Class");

// Public class that has an internal method.
public class ClassWithFriendMethod
    internal void Test()
        Console.WriteLine("Sample Method");

Imports System.Runtime.CompilerServices
<Assembly: InternalsVisibleTo("AssemblyB")>

' Friend class.
Friend Class FriendClass
    Public Sub Test()
        Console.WriteLine("Sample Class")
    End Sub
End Class

' Public class with a Friend method.
Public Class ClassWithFriendMethod
    Friend Sub Test()
        Console.WriteLine("Sample Method")
    End Sub
End Class

Only assemblies that you explicitly specify as friends can access internal (C#) or Friend (Visual Basic) types and members. For example, if AssemblyB is a friend of Assembly A and Assembly C references AssemblyB, Assembly C does not have access to internal (C#) or Friend (Visual Basic) types in Assembly A.

The compiler performs some basic validation of the friend assembly name passed to the InternalsVisibleToAttribute attribute. If Assembly A declares AssemblyB as a friend assembly, the validation rules are as follows:

  • If Assembly A is strong named, AssemblyB must also be strong named. The friend assembly name that is passed to the attribute must consist of the assembly name and the public key of the strong-name key that is used to sign AssemblyB.

    The friend assembly name that is passed to the InternalsVisibleToAttribute attribute cannot be the strong name of AssemblyB. Don't include the assembly version, culture, architecture, or public key token.

  • If Assembly A is not strong named, the friend assembly name should consist of only the assembly name. For more information, see How to: Create unsigned friend assemblies.

  • If AssemblyB is strong named, you must specify the strong-name key for AssemblyB by using the project setting or the command-line /keyfile compiler option. For more information, see How to: Create signed friend assemblies.

The StrongNameIdentityPermission class also provides the ability to share types, with the following differences:

  • StrongNameIdentityPermission applies to an individual type, while a friend assembly applies to the whole assembly.

  • If there are hundreds of types in Assembly A that you want to share with AssemblyB, you have to add StrongNameIdentityPermission to all of them. If you use a friend assembly, you only need to declare the friend relationship once.

  • If you use StrongNameIdentityPermission, the types you want to share have to be declared as public. If you use a friend assembly, the shared types are declared as internal (C#) or Friend (Visual Basic).

For information about how to access an assembly's internal (C#) or Friend (Visual Basic) types and methods from a module file (a file with the .netmodule extension), see ModuleAssemblyName (C#) or -moduleassemblyname (Visual Basic).

See also