.net - Accessing newly signed third party DLL gives error

Question

I have a signed application that uses third party DLLs. These DLLs were not signed. - So far no problem for the first step: I just signed them (getting *.il with ildasm.exe, ajust publickeytoken in the *.il 's because they have interdependencies, and made the *.dll's with ilasm.exe)

The project now compiles fine and also starts up.

But when in my code, a class constructor of the 3rd-party-DLL is called (or something else? - was just the first thing I did), I get the error "Strong-name signed assemblies must specify a public key in their InternalsVisibleTo declarations"

It seems there won't be a problem if you have the source of the DLL and can ajust in AssemblyInfo.cs by setting

[assembly: InternalsVisibleTo("MyProject.Domain.Tests, PublicKey=..."]

But: As mentioned above I have a third-party DLL I don't have the source. So no way to solve the problem like this.

Any suggestions to get this running?

Answer 1

I had the exact same issue.

Why it happens

• The 3rd-party assembly is declared with InternalsVisibleTo to make it "friend" to other assemblies, e.g. InternalsVisibleTo("OtherAssembly")
• .NET requires that strong-name assembly can only be "friend" to other strong-name assemblies, in which case the InternalsVisibleTo attribute must specify the public keys of those other assemblies, e.g. InternalsVisibleTo("OtherAssembly, PublicKey=[key]")
• At runtime, the CLR sees that InternalsVisibleTo is not properly declared for the assembly in question, so it throws the exception.

How to fix

If the "friend" assemblies aren't needed for the program execution (e.g. it's a Test assembly, which isn't deployed in production), follow these steps:

• Disassemble the assembly in question: ildasm.exe ThirdParty.dll /OUTPUT=ThirdParty.il
• Use a text editor to edit the IL file, remove any declaration of InternalsVisibleTo
• Assemble and sign the IL: ilasm.exe ThirdParty.il /DLL /OUTPUT=ThirdParty.modified.dll /KEY=key.snk
• Note: generate a key by: sn.exe -k key.snk

If the "friend" assemblies are needed for the program execution, you have to sign all those friend assemblies. Then follow similar steps as above, except instead of removing InternalsVisibleTo, you have to amend each declaration with the correct public key.