InternalsVisibleTo attribute isn't working

I am trying to use the InternalsVisibleTo assembly attribute to make my internal classes in a .NET class library visible to my unit test project. For some reason, I keep getting an error message that says:

>'MyClassName' is inaccessible due to its protection level

Both assemblies are signed and I have the correct key listed in the attribute declaration. Any ideas?

Are you absolutely sure you have the correct public key specified in the attribute? Note that you need to specify the full public key, not just the public key token. It looks something like:

[assembly: InternalsVisibleTo("MyFriendAssembly,
PublicKey=0024000004800000940000000602000000240000525341310004000001000100F73
F4DDC11F0CA6209BC63EFCBBAC3DACB04B612E04FA07F01D919FB5A1579D20283DC12901C8B66
A08FB8A9CB6A5E81989007B3AA43CD7442BED6D21F4D33FB590A46420FB75265C889D536A9519
674440C3C2FB06C5924360243CACD4B641BE574C31A434CE845323395842FAAF106B234C2C140
6E2F553073FF557D2DB6C5")]

It's 320 or so hex digits. Not sure why you need to specify the full public key - possibly with just the public key token that is used in other assembly references it would be easier for someone to spoof the friend assembly's identity.

Another possible "gotcha": The name of the friend assembly that you specify in the InternalsVisibleToAttribute must exactly match the name of your friend assembly as shown in the friend's project properties (in the Application tab).

In my case, I had a project Thingamajig and a companion project ThingamajigAutoTests (names changed to protect the guilty) that both produced unsigned assemblies. I duly added the attribute [assembly: InternalsVisibleTo( "ThingamajigAutoTests" )] to the Thingamajig\AssemblyInfo.cs file, and commented out the AssemblyKeyFile and AssemblyKeyName attributes as noted above. The Thingamajig project built just fine, but its internal members stubbornly refused to show up in the autotest project.

After much head scratching, I rechecked the ThingamajigAutoTests project properties, and discovered that the assembly name was specified as "ThingamajigAutoTests.dll". Bingo - I added the ".dll" extension to the assembly name in the InternalsVisibleTo attribute, and the pieces fell into place.

Sometimes it's the littlest things...

If your assemblies aren't signed, but you are still getting the same error, check your AssemblyInfo.cs file for either of the following lines:

[assembly: AssemblyKeyFile("")]
[assembly: AssemblyKeyName("")]

The properties tab will still show your assembly as unsigned if either (or both) of these lines are present, but the InternalsVisibleTo attribute treats an assembly with these lines as strongly signed. Simply delete (or comment out) these lines, and it should work fine for you.

It is worth noting that if the "friend" (tests) assembly is written in C++/CLI rather than C#/VB.NET, you need to use the following:

#using "AssemblyUnderTest.dll" as_friend

instead of a project reference or the usual #using statement. For some reason, there is no way to do this in the project reference UI.

You can use AssemblyHelper tool that will generate InternalsVisibleTo syntax for you. Here's the link to the latest version. Just note that it only works for strongly-named assemblies.

Here's a macro I use to quickly generate this attribute. Its a bit hacky, but it works. On my machine. When the latest signed binary is in /bin/debug. Etc equivocation etc. Anyhow, you can see how it gets the key, so that'll give you a hint. Fix/improve as your time permits.

Sub GetInternalsVisibleToForCurrentProject()
    Dim temp = "[assembly:  global::System.Runtime.CompilerServices." + _
               "InternalsVisibleTo(""{0}, publickey={1}"")]"
    Dim projs As System.Array
    Dim proj As Project
    projs = DTE.ActiveSolutionProjects()
    If projs.Length < 1 Then
        Return
    End If

    proj = CType(projs.GetValue(0), EnvDTE.Project)
    Dim path, dir, filename As String
    path = proj.FullName
    dir = System.IO.Path.GetDirectoryName(path)
    filename = System.IO.Path.GetFileNameWithoutExtension(path)
    filename = System.IO.Path.ChangeExtension(filename, "dll")
    dir += "\bin\debug\"
    filename = System.IO.Path.Combine(dir, filename)
    If Not System.IO.File.Exists(filename) Then
        MsgBox("Cannot load file " + filename)
        Return
    End If
    Dim assy As System.Reflection.Assembly
    assy = System.Reflection.Assembly.Load(filename)
    Dim pk As Byte() = assy.GetName().GetPublicKey()
    Dim hex As String = BitConverter.ToString(pk).Replace("-", "")
    System.Windows.Forms.Clipboard.SetText(String.Format(temp, assy.GetName().Name, hex))
    MsgBox("InternalsVisibleTo attribute copied to the clipboard.")
End Sub

In addition to all of the above, when everything seems to be correct, but the friend assembly stubbornly refuses to see any internals, reloading the solution or restarting Visual Studio can solve the problem.

You need to use the /out: compiler switch when compiling the friend assembly (the assembly that does not contain the InternalsVisibleTo attribute).

The compiler needs to know the name of the assembly being compiled in order to determine if the resulting assembly should be considered a friend assembly.

You are required to generate an new full public key for the assembly and then specify the attribute to assembly.

[assembly: InternalsVisibleTo("assemblyname,
PublicKey="Full Public Key")]

Follow the below MSDN steps to generate new full public key for the assembly from visual studio.

To add a Get Assembly Public Key item to the Tools menu

In Visual Studio, click External Tools on the Tools menu.

In the External Tools dialog box, click Add and enter Get Assembly Public Key in the Title box.

Fill the Command box by browsing to sn.exe. It is typically installed at the following location: C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0a\Bin\x64\sn.exe.

In the Arguments box, type the following (case sensitive): -Tp $(TargetPath). Select the Use Output window check box.

Click OK. The new command is added to the Tools menu.

Whenever you need the Public Key Token of the assembly you are developing, click the Get Assembly Public Key command on the Tools menu, and the public key token appears in the Output window.

In my case using VS.Net 2015, I needed to sign BOTH assemblies (if at least 1 assembly shall be signed or you want to reference on the public key of your assembly).

My project didn't use signing at all. So I started adding a sign key to my test library and useing the InternalsVisibleTo-Attribute at my project's base library. But VS.Net always explained it couldn't access the friend methods.

When I started to sign the base library (it can be the same or another sign key - as long as you do sign the base library), VS.Net was immediately able to work as expected.

Previous answers with PublicKey worked: (Visual Studio 2015: NEED to be on one line, otherwise it complains that the assembly reference is invalid or cannot referenced. PublicKeyToken didn't worked)

[assembly: InternalsVisibleTo("NameSpace.MyFriendAssembly, PublicKey=0024000004800000940000000602000000240000525341310004000001000100F73F4DDC11F0CA6209BC63EFCBBAC3DACB04B612E04FA07F01D919FB5A1579D20283DC12901C8B66A08FB8A9CB6A5E81989007B3AA43CD7442BED6D21F4D33FB590A46420FB75265C889D536A9519674440C3C2FB06C5924360243CACD4B641BE574C31A434CE845323395842FAAF106B234C2C1406E2F553073FF557D2DB6C5")]

Thanks to @Joe

To get the public key of the friend assembly:

sn -Tp path\to\assembly\MyFriendAssembly.dll

Inside a Developper command prompt (Startup > Programs > Visual Studio 2015 > Visual Studio Tools > Developer Command Prompt for VS2015). Thanks to @Ian G.

Although, the final touch that made it work for me after the above was to sign my friend library project the same way the project of the library to share is signed. Since it was a new Test library, it wasn't signed yet.

Another possibility that may be tricky to track down, depending on how your code is written.

1. You're invoking an internal method defined in X from another assembly Y
2. The method signature uses internal types defined in Z
3. You then have to add [InternalsVisibleTo] in X AND in Z

For example:

// In X
internal static class XType
{
    internal static ZType GetZ() { ... }
}

// In Y:
object someUntypedValue = XType.GetZ();

// In Z:
internal class ZType { ... }

If you have it written like above, where you're not referring to ZType directly in Y, after having added Y as a friend of X, you may be mystified why your code still doesn't compile.

The compilation error could definitely be more helpful in this case.

I'm writing this out of frustration. Make sure the assembly you are granting access to is named as you expect.

I renamed my project but this does not automatically update the Assembly Name. Right click your project and click Properties. Under Application, ensure that the Assembly Name and Default Namespace are what you expect.

Applies only if you like to keep unsigned assemblies as unsigned assembly (and don't want to sign it for several reasons):

There is still another point: if you compile your base library from VS.Net to a local directory, it may work as expected.

BUT: As soon as you compile your base library to a network drive, security policies apply and the assembly can't be successfully loaded. This again causes VS.NET or the compiler to fail when checking for the PublicKey match.

FINALLY, it's possible to use unsigned assemblies: https://msdn.microsoft.com/en-us/library/bb384966.aspx You must ensure that BOTH assemblies are NOT SIGNED And the Assembly attribute must be without PublicKey information:

<Assembly: InternalsVisibleTo("friend_unsigned_B")>

If you have more than 1 referenced assembly - check that all necessary assemblies have InternalsVisibleTo attribute. Sometimes it's not obviously, and no message that you have to add this attribute into else one assembly.

I had the same problem. None of the solutions worked.

Eventually discovered the issue was due to class X explicitly implementing interface Y, which is internal.

the method X.InterfaceMethod was unavailable, though I have no idea why.

The solution was to cast (X as YourInterface).InterfaceMethod in the test library, and then things worked.

1- Sign the test project: In Visual Studio go to the properties window of the test project and Sign the assembly by checking the checkbox with the same phrase in the Signing tab.

2- Create a PublicKey for the test project: Open Visual Studio Command Prompt (e.g. Developer Command Prompt for VS 2017). Go to the folder where the .dll file of the test project exists. Create a Public Key via sn.exe:

sn -Tp TestProject.dll

Note that the argument is -Tp, but not -tp.

3- Introduce the PublicKey to the project to be tested: Go to the AssemblyInfo.cs file in the project to be tested and add this line with the PublicKey created in the previous step:

[assembly: InternalsVisibleTo("TestProjectAssemblyName, PublicKey=2066212d128683a85f31645c60719617ba512c0bfdba6791612ed56350368f6cc40a17b4942ff16cda9e760684658fa3f357c137a1005b04cb002400000480000094000000060200000024000052534131000400000100010065fe67a14eb30ffcdd99880e9d725f04e5c720dffc561b23e2953c34db8b7c5d4643f476408ad1b1e28d6bde7d64279b0f51bf0e60be2d383a6c497bf27307447506b746bd2075")]

Don't forget to replace the above PublicKey with yours.

4- Make the private method internal: In the project to be tested change the access modifier of the method to internal.

internal static void DoSomething(){...}

Although using an AssemblyInfo file and adding InternalsVisibleTo still works, the preferred approach now (as far as I can tell) is to use an ItemGroup in your Project's csproj file, like so:

	<ItemGroup>
		<InternalsVisibleTo Include="My.Project.Tests" />
	</ItemGroup>

If a PublicKey is required, this attribute may also be added.

As a side note, if you want to easily get the public key without having to use sn and figure out its options you can download the handy program here. It not only determines the public key but also creates the "assembly: InternalsVisibleTo..." line ready to be copied to the clipboard and pasted into your code.

I just resolved a similar problem with the InternalsVisibleTo Attribute. Everything seemed right and I couldn't figure out why the internal class I was aiming still wasn't accessible.

Changing the case of the key from upper to lower case fixed the problem.

InternalsVisibleTo is often used in the context of testing. If you're using a mocking framework such as Moq, you will need to expose the internals of your project to Moq as well. In the particular case of that framework, I also needed to add
[assembly: InternalsVisibleTo("DynamicProxyGenAssembly2")]
to my AssemblyInfo.