'IServiceCollection' does not contain a definition for 'AddSession'

I know this is a bit late, but did you try to install the Microsoft.AspNetCore.Session package from nuget?

https://www.nuget.org/packages/Microsoft.AspNetCore.Session/

In Visual Studio: Install-Package Microsoft.AspNetCore.Session -Version 1.1.2(in my case)

It worked for me!

The following way I fixed the Issue.

1. Clean and Rebuild the solution.
2. Restart Visual Studio 2017.

Thanks @HenkMollema