Why do optional parameters in C# 4.0 require compile-time constants?

Question

Also is there a way to use run-time values for optional method parameters?

Answer 1

Optional parameters are determined at compile time, and substituted into the method if you call a method with too few parameters. They are handled via adding an attribute to the parameter in the method's IL.

As such, they need to be fully resolved at compile time (both for creation, since they're an attribute, but also when used). There is no way to use runtime values for optional method parameters.