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title Friend Assemblies (Visual Basic)
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ms.date 07/20/2015
ms.prod .net
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devlang-visual-basic
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ms.topic article
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Friend Assemblies (Visual Basic)

A friend assembly is an assembly that can access another assembly's Friend types and members. If you identify an assembly as a friend assembly, you no longer have to mark types and members as public in order for them to be accessed by other assemblies. 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 Friend.

  • 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 Friend.

Remarks

You can use the xref:System.Runtime.CompilerServices.InternalsVisibleToAttribute attribute to identify one or more friend assemblies for a given assembly. The following example uses the xref:System.Runtime.CompilerServices.InternalsVisibleToAttribute attribute in assembly A 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 Friend.

Note

When you compile an assembly (assembly AssemblyB) that will access internal types or internal members of another assembly (assembly A), 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 /out (Visual Basic).

Imports System.Runtime.CompilerServices  
Imports System  
<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 Friend types and members. For example, if assembly B is a friend of assembly A and assembly C references assembly B, C does not have access to Friend types in A.

The compiler performs some basic validation of the friend assembly name passed to the xref:System.Runtime.CompilerServices.InternalsVisibleToAttribute attribute. If assembly A declares B as a friend assembly, the validation rules are as follows:

  • If assembly A is strong named, assembly B 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 assembly B.

    The friend assembly name that is passed to the xref:System.Runtime.CompilerServices.InternalsVisibleToAttribute attribute cannot be the strong name of assembly B: do not 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 (Visual Basic).

  • If assembly B is strong named, you must specify the strong-name key for assembly B by using the project setting or the command-line /keyfile compiler option. For more information, see How to: Create Signed Friend Assemblies (Visual Basic).

The xref:System.Security.Permissions.StrongNameIdentityPermission class also provides the ability to share types, with the following differences:

  • xref:System.Security.Permissions.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 assembly B, you have to add xref:System.Security.Permissions.StrongNameIdentityPermission to all of them. If you use a friend assembly, you only need to declare the friend relationship once.

  • If you use xref:System.Security.Permissions.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 Friend.

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

See Also

xref:System.Runtime.CompilerServices.InternalsVisibleToAttribute
xref:System.Security.Permissions.StrongNameIdentityPermission
How to: Create Unsigned Friend Assemblies (Visual Basic)
How to: Create Signed Friend Assemblies (Visual Basic)
Assemblies and the Global Assembly Cache (Visual Basic)
Programming Concepts