InternalsVisibleToAttribute Class

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.

[assembly:InternalsVisibleTo("Friend1a")]
[assembly:InternalsVisibleTo("Friend1b")]

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

[assembly:InternalsVisibleTo("Friend2a"), 
          InternalsVisibleTo("Friend2b")]

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 fields in the argument to the constructor; 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 Ilasm.exe (MSIL Assembler)) and the assemblies are strong-named, a MethodAccessException 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, and the corresponding --out or –o compiler option should not be used when compiling friend assemblies with the F# compiler.

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

• Friend Assemblies (C++)

• Friend Assemblies (C# and Visual Basic)

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" +
                              "26e0b3")]

public class FileUtilities
{
   internal static string AppendDirectorySeparator(string dir)
   {
      if (! dir.Trim().EndsWith(Path.DirectorySeparatorChar.ToString()))
         return dir.Trim() + Path.DirectorySeparatorChar;
      else
         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, 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);
      Console.WriteLine(dir);
   }
}
// The example displays the following output:
//       C:\Program Files\

.NET Framework

.NET Framework Client Profile

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

Reference