Sometimes, just getting assemblies to cooperate the way you want them to is a real pain in the neck.
Today, I wanted to make one assembly (which we’ll call Foo) to be able to access the internal (Friend for all us VB geeks) members of another assembly (which we’ll call Bar). Now, the documented way to achieve this is by adding the InternalsVisibleTo attribute to your project. The code sample on MSDN looks like this:
[assembly:InternalsVisibleTo("AssemblyB, PublicKey=32ab4ba45e0a69a1")]For a Visual Basic application, that statement goes into the AssemblyInfo.vb file, and looks like this:
<Assembly:InternalsVisibleTo("AssemblyB, PublicKey=32ab4ba45e0a69a1")>No big deal. It’s not rocket science. You just need a strong name. To generate a strong name, you fire up the Visual Studio Command Prompt, and execute the Strong Name tool (sn.exe) and execute it as follows:
C:\Program Files\Microsoft Visual Studio 9.0\VC>sn.exe -k bar.snk
Microsoft (R) .NET Framework Strong Name Utility Version 3.5.30729.1
Copyright (c) Microsoft Corporation. All rights reserved.
Key pair written to bar.snk
As you can see, this creates a .SNK file, which contains a public and a private key. However, its contents are not human readable. Further, the InternalsVisibleTo attribute requires that you provide the public key in the constructor. (Don’t even think about trying it without it.)
Here’s where things get tricky: The code samples on MSDN do not provide a public key to the constructor; they provide a public key token. There’s a huge difference between the two, and it’s very misleading. A strong name token is much shorter than a public key; the former is only about 16 characters long, the other well in excess of 140 characters. If you rely on the code sample from MSDN to get you where you want to be, you’ll be pulling your hair out in no time.
But how do you get the public key token?
You can retrieve the public key token as follows:
sn –p foo.snk barpublic.snk
This creates a new file that contains only the public key. It removes the private key information from the file.
sn –tp > barkey.txt
This creates a text file that contains the full dump of the key information, including the public key. We redirect it to a text file so that you can open it in the editor of your choice (because you’ll have to do some cleanup to get the key onto one line). You’ll want to select the public key and paste it into the PublicKey portion of the constructor for the InternalsVisible attribute.
So here’s everything we did at the command prompt:
C:\Program Files\Microsoft Visual Studio 9.0\VC>sn -k bar.snk
Microsoft (R) .NET Framework Strong Name Utility Version 3.5.30729.1
Copyright (c) Microsoft Corporation. All rights reserved.
Key pair written to bar.snk
C:\Program Files\Microsoft Visual Studio 9.0\VC>sn -p bar.snk barpublic.snk
Microsoft (R) .NET Framework Strong Name Utility Version 3.5.30729.1
Copyright (c) Microsoft Corporation. All rights reserved.
Public key written to barpublic.snk
C:\Program Files\Microsoft Visual Studio 9.0\VC>sn -tp barpublic.snk > bar.txt
C:\Program Files\Microsoft Visual Studio 9.0\VC>
And here’s the contents of our text file:
Microsoft (R) .NET Framework Strong Name Utility Version 3.5.30729.1
Copyright (c) Microsoft Corporation. All rights reserved.
Public key is
0024000004800000940000000602000000240000525341310004000001000100e13cb392af5437279736fc3c33fe237242d0f6301fafb01c5cbc719d84102c2d8b30a148600997ed53d99624b5d0eab37fd6b24cca3ce7f7b62ae99f961e148d5421576bade0ac8ab1187a3eee318ca20026ffe9b56b8a63156f817cef49998633867ae547684e8e59c0fe0b68ab29dffa749340dc6cfdd18071f1b69c6772ac
Public key token is db218359dd8997df
So, when we finally add that attribute to our AssemblyInfo.vb file, it looks like this:
<Assembly:InternalsVisibleTo("Bar, PublicKey=0024000004800000940000000602000000240000525341310004000001000100e13cb392af5437279736fc3c33fe237242d0f6301fafb01c5cbc719d84102c2d8b30a148600997ed53d99624b5d0eab37fd6b24cca3ce7f7b62ae99f961e148d5421576bade0ac8ab1187a3eee318ca20026ffe9b56b8a63156f817cef49998633867ae547684e8e59c0fe0b68ab29dffa749340dc6cfdd18071f1b69c6772ac")>
Once this stuff is in place, Bar should be able to access any members in Foo that are marked internal/Friend. It should be smooth sailing from there.
Good luck!
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