Well one obfuscation technique is to change the argument types. So for instance, you might have foo(string). But after obfuscation, you move the code of foo(string) into foo(object). So a decompiler ends up showing:

  String s = "123";
  foo(s);

Java will call foo(string) if this is re-compiled, which is wrong. The decompiler would need to show foo((Object)s).

On the CLR you can go even further and erase a lot of the type info and just pass objects around. That is, you can just change all your method signatures to foo(object, object, object) no matter which classes (not structs) you pass. Or have even more fun and randomize the classes. So now foo(string) becomes foo(DBConnectionInfo).