C# Reflection unable to find NUnit attributes

Using C# on the .NET 4.5 Framework with Visual Studio 2015, I'm attempting to use Reflection to find NUnit tests and testcases from a provided .dll file. The problem I'm running into is that reflection query methods (such as IsDefined and GetCustomAttributes) require that attributes directly inherit from System.Attribute.

In NUnit 3.2.1 (Latest stable version as of June 2016), attributes inherit from an NUnitAttribute class, which in turn inherits from System.Attribute. This prevents Reflection from finding marked NUnit tests. As a temporary work around, I've downgraded to NUnit 2.6.4 where attributes directly inherit from System.Attribute. In the future, I would like to build in support for newer versions of NUnit.

Does anyone have solutions or suggestions for this issue? I've searched around, but cannot seem to find a way to make Reflection methods "realize" that System.Attribute is further up the inheritance chain.