How to plug method parameters into custom attribute

Making vcsjones' comment an answer, this is not possible.

Attributes are metadata; they are compiled into the assembly at compile-time and do not change during runtime. As such, any parameters you pass into an attribute must be constants; literals, constant variables, compiler defines, etc.

The one way this would work is to make the attribute an AOP element, using a framework like PostSharp or rolling your own with the Unity Framework etc. This would allow you to attach an "interceptor" to the method by decorating it with an attribute, which will then run code in the attribute and will also have knowledge about exactly how the method was called including parameter values. Check out this blog: http://www.progware.org/Blog/post/Interception-and-Interceptors-in-C-(Aspect-oriented-programming).aspx

There is a way to do this _in ASP.NET MVC_ with action-methods (not with attributes in general)

public class CustomAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        int userId = (int)filterContext.ActionParameters["userId"];
    }
}