Cannot use interface default methods

Question

I have installed new version of visual studio and set the project to C# 8. When I tried to use new interface feature like below it said that I cannot implement in interface whic

Answer 1

Default interface methods aren't available in .NET Core 3.0 Preview 1. This really is a preview preview - even async enumerables won't work without some manual fixes. The Roslyn team decided to release the preview even though some things wouldn't work right out :

Should we tell users to skip this feature until preview 2?

This is a preview, not RTM. If we told users to skip preview features because they had bugs we should just stop shipping previews altogether.