29.4. Attributes

In the previous example you saw how to choose which types and methods to obfuscate within Dotfuscator. Of course, if you were to start using a different obfuscating product you would have to configure it to exclude the public members. It would be more convenient to be able to annotate your code with attributes indicating whether a symbol should be obfuscated. You can do this by using the Obfuscation and ObfuscationAssemblyAttribute attributes.

The default behavior in Dotfuscator is to ignore the obfuscation attributes in favor of any exclusions specified in the project. In Figure 29-4 there are a series of checkboxes for each assembly added to the project, of which the top checkbox is Honor Obfuscation Attributes. A limitation with the Community edition of Dotfuscator is that you can't control this feature for each assembly. You can apply this feature to all assemblies using the second button from the right on the Toolbar.

29.4.1. ObfuscationAssemblyAttribute

The ObfuscationAssemblyAttribute attribute can be applied to an assembly to control whether it should be treated as a class library or as a private assembly. The distinction is that with a class library it is expected that other assemblies will be referencing the public types and methods it exposes. As such, the obfuscation tool needs to ensure that these symbols are not renamed. Alternatively, as a private assembly, every symbol can be potentially renamed. The following is the Visual Basic syntax for ObfuscationAssemblyAttribute:

[assembly: Reflection.ObfuscateAssemblyAttribute(false, StripAfterObfuscation=true)]

The two arguments that this attribute takes indicate whether it is a private assembly and whether to strip the attribute off after obfuscation. The preceding snippet indicates that this is not a private assembly, and that public symbols should not be renamed. In addition, the snippet indicates that the obfuscation attribute should be stripped off after obfuscation — after all, the less information available to anyone wishing to decompile the assembly, the better.

Adding this attribute to the assemblyinfo.vb file will automatically preserve the names of all public symbols in the ObfuscationSample application. This means that you can remove the exclusion you created earlier for the GenerateMagicNumber method.

Within Dotfuscator you can specify that you want to run all assemblies in library mode. Enabling this option has the same effect as applying this attribute to the assembly.

29.4.2. ObfuscationAttribute

The downside of the ObfuscationAssemblyAttribute attribute is that it will expose all the public types and methods regardless of whether they existed for internal use only. On the other hand, the ObfuscationAttribute attribute can be applied to individual types and methods, so it provides a much finer level of control over what is obfuscated. To illustrate the use of this attribute, extend the example to include an additional public method, EvaluatePerson, and place the logic into another class, HiddenGenius:

namespace ObfuscationSample
{
    [System.Reflection.ObfuscationAttribute(ApplyToMembers=true, Exclude=true)]
    public class MathematicalGenius
    {
        public static Int32 GenerateMagicNumber(Int32 age, Int32 height)
        {
            return HiddenGenius.CalculateMagicNumber(age, height);
        }

        private static Boolean EvaluatePerson(Int32 age, Int32 height)
        {
            return HiddenGenius.QualifyPerson(age, height);
        }
    }

    [System.Reflection.ObfuscationAttribute(ApplyToMembers=false, Exclude=true)]
    public class HiddenGenius
    {
        public static Int32 CalculateMagicNumber(Int32 age, Int32 height)
        {
            return age * height;
        }

        [System.Reflection.ObfuscationAttribute(Exclude=true)]
        public static Boolean QualifyPerson(Int32 age, Int32 height)
        {
            return (age / height) > 3;
        }
    }
}

In this example, the MathematicalGenius class is the class that you want to expose outside of this library. As such, you want to exclude this class and all its methods from being obfuscated. You do this by applying the ObfuscationAttribute attribute with both the Exclude and ApplyToMembers parameters set to True.

The second class, HiddenGenius, has mixed obfuscation. As a result of some squabbling among the developers who wrote this class, the QualifyPerson method needs to be exposed, but all other methods in this class should be obfuscated. Again, the ObfuscationAttribute attribute is applied to the class so that the class does not get obfuscated. However, this time you want the default behavior to be such that symbols contained in the class are obfuscated, so the ApplyToMembers parameter is set to False. In addition, the Obfuscation attribute is applied to the QualifyPerson method so that it will still be accessible.