.Net, Coding, TDD

Testing internals members with InternalsVisibleTo attribute.

I posted previously about testing non-public members with MbUnit.  I recently found out about InternalsVisibleTo attribute by reading Roy Osherove‘s book The Art of Unit Testing.

To use this attribute, your test assembly must be strong-named.  Then you put InternalsVisibleTo attribute with public key of test assembly into AssemblyInfo.cs file of assembly under test.

 1: [assembly: InternalsVisibleTo("MyAssembly.Tests, PublicKey=XXX")]

I created an Assembly Helper tool that creates the syntax for you.  The tool is working (or should work) with following languages:

  • C#AssemblyHelper
  • Visual Basic
  • Delphi
  • MS C++
  • Chrome

You can get the tool from here.

Steps to use InternalsVisibleTo attribute with the tool.

Let assume that you have Assm_A and Assm_ATests.

  1. Sign Assm_A and Assm_ATests with a Strong Name. You can read about that here.
  2. Compile your assemblies.
  3. Launch Assembly Helper tool .
  4. Select desired language and press on ‘Get Assemblies’ button and navigate to Assm_ATests.dll.
  5. Copy the text into AssemblyInfo file of Assm_A assembly.

Now your tests can access internals members of the assembly under test.

Note: With Assembly Helper tool you can select multiple assemblies at once.

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13 thoughts on “Testing internals members with InternalsVisibleTo attribute.

  1. You can actually use this attribute with unsigned assemblies as well.
    There is one (undocumented) pitfall you should be aware of:
    You can’t have the following attributes in your assemblies:

    [assembly: AssemblyDelaySign(false)]
    [assembly: AssemblyKeyFile(“”)]
    [assembly: AssemblyKeyName(“”)]

    Those were used to be included by default in the AssemblyInfo.cs by VS2003. You just need to remove them in order to use the InternalsVisibleTo attribute (called friend assemblies).

  2. Omer,
    I don’t think that you can use InternalsVisibleTo attribute without strong-named assembly. Microsoft documentation specifically says: “This attribute can only be applied to strong-named assemblies”.

  3. Hello,

    I’m using your tool with a Net 2 mixed model DLL and when I try to use an internal method I get the usual error (OnReceivedMessage is internal).

    1>.\ComTests.cpp(166) : error C3767: ‘PimsApi::ComInterface::EventChannelImpCOM::OnReceivedMessage’: candidate function(s) not accessible

    I’m putting the attribute

    [assembly:InternalsVisibleTo(“PcsPims500.IntegrationTests, PublicKey=002400000480000094000000060200000024000052534131000400000100010069EB6CD4E0E36EF1992E8DA1646C97270C48FE5DBF6936655757D545B71FF22AD01BF2F470EABBB02D7B66CA9DD58D920FA62E4C46D4F721616B1680D6098D51ED81D8ED2107AE533151D13B86165B806AAF25E878A56BF8A23F3E86FAFC3E6F016F2CF8B82A9F887068C21F37D5616F9314D2D170781ED2915BF56BB1F6A5CD”)]

    at the AssemblyInfo file of the assembly that must open its internals to a integrationtests DLL.

    Any hint? I’m lost with this one.

      1. Hello Vadim, thanks for your answer. I thougth on this one and I added the attribute to PimsApi because this is the one that must expose its internal methods. Is that rigth?

        Thanks again for your time.

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