How to get and modify a property value through a custom Attribute?

Question

I want to create a custom attribute that can be used on a property like:

[TrimInputString]
public string FirstName { get; set; }

that will

Answer 1

As Matti pointed out, this is not how attributes work. However, you could use the PostSharp AOP framework to accomplish this, probably overriding OnMethodBoundaryAspect. But this is not trivial.