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 required to be constants because they are written out as values of an attribute. Hence they inherit all of the restrictions that an attribute value has.

There is no way to directly encode a runtime value. However you can get close with the following pattern

public void MyApi(SomeType type = null) {
  type = type ?? new SomeType();
  ...
}

Answer 2

Optional parameters are compiled into the assembly and as such (just like anything that is designated as const) they must be a compile-time constant.

And no, you cannot use execution-time values as optional parameters.

Answer 3

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.