In Mvc6, where is WriteAsync function?

one package is missing. please add this package "Microsoft.AspNetCore.Http.Abstractions" and use the namespace "using Microsoft.AspNet.Http;" to get the intellisense in your code.

Add dependency:

<PackageReference Include="Microsoft.AspNetCore.Http.Abstractions" Version="1.1.1" />

Then in source code add:

using Microsoft.AspNetCore.Http;