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

Based on this:

Microsoft has fleshed out more details about C# Version 8.0, the next planned major release of the language. C# 8.0 is expected to arrive in 2019, concurrent with the arrival of .Net Core 3.0.

All of the C# 8 features are still early in development and are likely to change. So it would be better to wait until the features end up in the final release. The following blog could be also helpful:

Any estimation for C# 8.0 release date?

Not all C# 8.0 features are available yet (Preview 1 of Visual Studio 2019). You can find the list of current available features in the following link:

Take C# 8.0 for a spin

Answer 2

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.