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InternalsVisibleToAttribute Class

Updated: May 2011

Specifies that types that are ordinarily visible only within the current assembly are visible to another assembly.

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

[AttributeUsageAttribute(AttributeTargets.Assembly, AllowMultiple = true, Inherited = false)]
public sealed class InternalsVisibleToAttribute : Attribute

Ordinarily, types and members with internal scope (in C#) and friend scope (in Visual Basic) are visible only in the assembly in which they are defined. The InternalsVisibleToAttribute attribute makes them also visible to the types in a specified assembly, which is known as a friend assembly.

The attribute is applied at the assembly level. This means that it can be included at the beginning of a source code file, or it can be included in the AssemblyInfo file in a Visual Studio project. You can use the attribute to specify a single friend assembly that can access the internal types and members of the current assembly. You can define multiple friend assemblies in two ways. They can appear as individual assembly-level attributes, as the following example illustrates.


They can also appear with separate InternalsVisibleToAttribute tags but a single assembly keyword, as the following example illustrates.


The friend assembly is identified by the InternalsVisibleToAttribute constructor. Both the current assembly and the friend assembly must be unsigned, or both must be signed with a strong name. If they are signed with a strong name, the argument to the InternalsVisibleToAttribute constructor must include the full public key as well as the name of the assembly. For more information about using InternalsVisibleToAttribute with strong-named assemblies, see the InternalsVisibleToAttribute constructor.

Do not include values for the CultureInfo, Version, or ProcessorArchitecture field in the argument; the Visual Basic, C#, and C++ compilers treat this as a compiler error. If you use a compiler that does not treat it as an error (such as the MSIL Assembler (ILAsm.exe)) and the assemblies are strong-named, a MethodAccessException exception is thrown the first time the specified friend assembly accesses the assembly that contains the InternalsVisibleToAttribute attribute.

If you use the C# compiler to compile the friend assembly, 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. The /out compiler option is optional for the Visual Basic compiler.

For more information about how to use this 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, the Example.Main method in Friend1 can successfully call the FileUtilities.AppendDirectorySeparator method, although 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\


Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0




May 2011

Added a discussion of compiler differences and how to specify multiple friend assemblies.

Content bug fix.

December 2009

Revised the Remarks section and added an example.

Customer feedback.

July 2009

Added links; removed incomplete example.

Information enhancement.