Obfuscar: The Story Behind 'Inconsistent virtual method obfuscation state detected'

“Inconsistent virtual method obfuscation state detected” has been a well-known issue of Obfuscar for a while but users do not seem to fully understand the rationale behind it. This post is going to cover the technical details.

The Scenario

Assume that we have the following elements defined in a C# project,

namespace test
{
  internal interface MainInterface
  {
    void Test();
  }
  public class MainClass : MainInterface
  {
    public void Test(){
    }
  }
}

Then when we use Obfuscar to obfuscate the code, and use KeepPublicApi=true and HidePrivateApi=true, we will see the issue reported by Obfuscar.

An error occurred during processing:
Inconsistent virtual method obfuscation state detected. Abort. Please review the
following methods,
[test]test.MainInterface::Test[0]->WillRename:A
[test]test.MainClass::Test[0]->Skipped:KeepPublicApi option in configuration

So, how to understand the cause? MainInterface is going to be obfuscated, as it is internal. That obfuscation also applies to MainInterface.Test. That’s why a WillRename action is set for it. However, MainClass is a public class, and its public Test method should not be obfuscated in this case. A Skipped action is marked. Obfuscar decides to break at this stage, because it cannot fulfill the two actions due to the relationship between the functions.

The Thoughts

Do you know that before that Obfuscar silently changes actions of Rename to Skipped? Do you want to see the things you want to hide revealed unexpectedly? I personally go against silent operations.

Thus, when I determined to change Obfuscar in this way in this February, I was trying to make things simple and fully under control. The fact is that when Obfuscar reports such inconsistency to the developer, the issue can be easily fixed. Possible approaches include but are not limited to,

• Use attribute to force obfuscation of MainClass.Test.
• Use <ForceMethod> tag in configuration to force obfuscation of MainClass.Test.
• Use attribute to skip obfuscation of MainInterface.Test.
• Use <SkipMethod> tag in configuration to skip obfuscation of MainInterface.Test.

Anyway the developer controls what to hide or not.

Hope this post can help if you meet exactly this issue.