如何创建从动态类名访问变量的方法？

I have an "Extension" class for handling general functions that's designed to be used dynamically - that is, I want to be able to use it with many different classes and variables (kind of like how you would use something like the Math class). But I've encountered a little problem that I can't seem to figure out (I've looked at quite a few forums and websites, but no luck).I have an "Extension" class for handling genera