Until C# 8.0, an interface in C# could not contain method definitions. You could only include method declarations in an interface, and the members of an interface were public and abstract by default.

In C#, however, multiple inheritance is not allowed for classes, but rather only for interfaces in a limited way, so that does not contain state. Figure -3- Diamond Problem dependencies ...

A controversial new proposal for .NET suggests the introduction of a limited form of multiple inheritance via abstract interfaces. This feature was inspired by Java’s default methods.