Default value for attribute constructor?

Question

I'm getting this error,

An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

When I try to write something like this

[AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
public class UrlAttribute : Attribute
{
    public UrlAttribute(string pattern, string name=null)
    {
        // ...

It doesn't even show a line number, but it disappears when I take out that =null bit.

Actually, the error only occurs both when I provide a default value and rely on it (i.e., I omit it) like so

    [Url("/index")]

I'm curious to know why this? How is "null" not a constant expression?

Answer 1

Attributes already provide default-able arguments. You simply create public properties on the attribute and those can be assigned in the attribute instantiation process. It already works, it's already well-understand and it's consistent with how framework attributes work. So...why not just use that mechanism instead of trying to redundantly add default parameters to the constructor?