What is the difference between OptionalAttribute and optional parameters in C# 4.0

Question

I'm researching someone else's code and there is a method like this:

public SomeClass DoSomething(string param1, [Optional, DefaultParameterValue("")] string optional)

Why would someone use these attributes instead of

public SomeClass DoSomething(string param1, string optional = "")

Is there any difference in the behavior, etc.?

Answer 1

If they weren't using C# 4, for example? I believe the second version will compile into exactly the first version...

(I've compiled them both and run them through ILDASM - there were no significant differences.)