Startup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;

namespace SessionExample
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            //retrieve connectoin info from environment variables in this sample
            var host = Environment.GetEnvironmentVariable("REDIS_SESSION_SERVICE_HOST");
            var port = Environment.GetEnvironmentVariable("REDIS_SESSION_SERVICE_PORT");
            var conn = $"{host}:{port}";
            
            services.AddDistributedRedisCache(option =>
            {
                option.Configuration = conn;
                option.InstanceName = "master";
            });
            services.AddSession();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
        {
            app.UseSession();

            loggerFactory.AddConsole();

            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            //please add code to confirm session is wokring. 
            //you can find full code https://github.com/tanaka-takayoshi/aspnetcore-redis-session-example
        }
    }
}