C# 8 & C# 11 brought new features to interfaces, but many where not well publicized -- including default implementation, member access modifiers, and static members. The Microsoft documentation is correct, but many other C# resources have outdated information. So let's look at these changes and see where they are useful and where they should be avoided. The world of interfaces has changed; the line between interfaces and abstract classes has blurred; and C# now has multiple inheritance (sort of). With some practical tips, "gotchas", and plenty of examples, we'll see how to use these features effectively (and safely) in our code.