Category name:TDD

InternalsVisibleTo and PublicKey or PublicKeyToken

A long time since my previous blog as nowadays I often tweet my ramblings but this one does not fit a tweet 🙂

Sometimes you are working with strong named assemblies and when you are having unit tests and want to access internals then you have to use the InternalsVisibleTo assembly attribute. So to discover the public key token I ran “sn.exe –tp project.publickey” and then you get the public key (long) and the public key  token (short).


Microsoft (R) .NET Framework Strong Name Utility  Version 4.0.30319.1
Copyright (c) Microsoft Corporation.  All rights reserved.

Public key is

Public key token is 67178dccc283ce39


So I used the following attribute:


[assembly: InternalsVisibleTo("My.Project.Tests, PublicKeyToken=67178dccc283ce39")]


And got this nice compiler error:

Friend assembly reference is invalid. Strong-name signed assemblies must specify a public key in their InternalsVisibleTo declarations.

Then I pasted the long variant in the InternalsVisibleTo attribute and it compiled but I knew for 100% that the short version had to work. After investigation there seem two ways to pass the required strong name public key information. You can choose if you want to pass the whole public key or the public key token.

When both assemblies are signed then you need to pass the full PublicKey.

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