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InternalsVisibleToAttribute Constructor

Initializes a new instance of the InternalsVisibleToAttribute class with the name of the specified friend assembly.

Namespace:  System.Runtime.CompilerServices
Assembly:  mscorlib (in mscorlib.dll)

public InternalsVisibleToAttribute(
	string assemblyName


Type: System.String
The name of a friend assembly.

The InternalsVisibleToAttribute constructor defines a friend assembly, which is an assembly that has access to the internal types and members of the current assembly.

Both the current assembly and the friend assembly must be unsigned, or both must be signed with a strong name. (For more information about strong-named assemblies, see Creating and Using Strong-Named Assemblies.) If both are unsigned, the assemblyName parameter consists of the name of the friend assembly, specified without a directory path or file extension. If both are signed, assemblyName consists of the name of the friend assembly along with its full public key (but not its public key token). The other components of a strong name, such as those that provide culture, version, or processor architecture information, cannot be specified in the assemblyName parameter.

You can use Sn.exe (Strong Name Tool) to retrieve the full public key from a strong-named key (.snk) file. To do this, you perform the following steps:

  1. Extract the public key from the strong-named key file to a separate file:

    Sn -p snk_file outfile

  2. Display the full public key to the console:

    Sn -tp outfile

  3. Copy and paste the full public key value into your source code.

For more information about how to use the InternalsVisibleToAttribute attribute, see the following topics:

The following example defines a FileUtilities class that includes an internal AppendDirectorySeparator method. The InternalsVisibleToAttribute attribute is applied to the assembly that contains the FileUtilities class. The attribute allows an assembly named Friend1 to access this internal member.

// The source code should be saved in a file named Example1.cs. It 
// can be compiled at the command line as follows:
//    csc /t:library /keyfile:<snkfilename> Assembly1.cs
// The public key of the Friend1 file should be changed to the full
// public key stored in your strong-named key file.
using System;
using System.IO;
using System.Runtime.CompilerServices;

[assembly: InternalsVisibleTo("Friend1, PublicKey=002400000480000094" + 
                              "0000000602000000240000525341310004000" +
                              "001000100bf8c25fcd44838d87e245ab35bf7" +
                              "3ba2615707feea295709559b3de903fb95a93" +
                              "3d2729967c3184a97d7b84c7547cd87e435b5" +
                              "6bdf8621bcb62b59c00c88bd83aa62c4fcdd4" +
                              "712da72eec2533dc00f8529c3a0bbb4103282" +
                              "f0d894d5f34e9f0103c473dce9f4b457a5dee" +
                              "fd8f920d8681ed6dfcb0a81e96bd9b176525a" +

public class FileUtilities
   internal static string AppendDirectorySeparator(string dir)
      if (! dir.Trim().EndsWith(Path.DirectorySeparatorChar.ToString()))
         return dir.Trim() + Path.DirectorySeparatorChar;
         return dir;

If the following example is compiled into a strong-named assembly named Friend1, it can successfully call the FileUtilities.AppendDirectorySeparator method, even though the method is internal to the Assembly1 assembly.

// The source code should be saved in a file named Friend1.cs. It 
// can be compiled at the command line as follows:
//    csc /r:Assembly1.dll /keyfile:<snkfilename> /out:Friend1.dll Friend1.cs
// The public key of the Friend1 assembly should correspond to the public key
// specified in the class constructor of the InternalsVisibleTo attribute in the
// Assembly1 assembly.
using System;

public class Example
   public static void Main()
      string dir = @"C:\Program Files";
      dir = FileUtilities.AppendDirectorySeparator(dir);
// The example displays the following output:
//       C:\Program Files\

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.