Add support for virtual extension methods - methods in interfaces with concrete implementations. A class that implements such an interface is required to have a single most specific implementation for the interface method inherited from its base classes or interfaces. Virtual extension methods enable an API author to add methods to an interface in future versions without breaking source or binary compatibility with existing implementations of that interface.

These are similar to Java's "Default Methods".

This feature requires corresponding support in the CLI/CLR. Programs that take advantage of this feature cannot run on earlier versions of the platform.