Default interface methods are included in a new feature proposal for C# 8, which will allow developers to use the traits programming technique. Based on an existing language feature found in Java ...

By using interfaces, you can, for example, include behavior from multiple sources in a class. That capability is important in C# because the language doesn’t support multiple inheritance of classes.

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.