public void ConfigureServices(IServiceCollection services)
    {
        services.AddDistributedMemoryCache();
        services.AddSession(options =>
        {
            options.IdleTimeout = TimeSpan.FromDays(10);
            options.CookieHttpOnly = true;
        });
        services.AddMvc();
    }

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        app.UseSession();
        app.UseStaticFiles();
        app.UseDeveloperExceptionPage();
        app.UseMvc(routes =>
        {
            routes.MapRoute(
                name: "default",
                template: "{controller=Home}/{action=Index}/{id?}");
        });
    }

@inject ISession Session

@inject Microsoft​.AspNetCore​.Http.IHttpContextAccessor HttpContextAccessor

@if(HttpContextAccessor.HttpContext.Session ...)

services.AddHttpContextAccessor();

@inject Microsoft​.AspNetCore​.Http.IHttpContextAccessor HttpContextAccessor
@{
    var session = HttpContextAccessor.HttpContext.Session;
}