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();
  ...
}