How to access the Session in ASP.NET Core via static variable?

Revised approach 1/17/17 to fix bug

First, I'll assume that you have your ASP.NET Core application configured to use session state. If not see @slfan's answer How to access the Session in ASP.NET Core via static variable?

How to access session in different class called via controller, without passing the session property as an additional parameter in the constructor of all classes

Asp.Net Core is designed around dependency injection and in general the designers didn't provide much static access to context information. More specifically there is no equivalent to System.Web.HttpContext.Current.

In Controllers you can get access to Session vars via this.HttpContext.Session but you specifically asked how to get access to the session from methods called by the controller without passing the session property as a parameter.

So, to do this we need to setup our own static class to provide access to session and we need some code to initialize that class at startup. Since it's likely that a person may want static access to the whole HttpContext object and not just the Session I took that approach.

So first we need the static class:

using Microsoft.AspNetCore.Http; 
using System;
using System.Threading;

namespace App.Web {

    public static class AppHttpContext {
        static IServiceProvider services = null;

        /// <summary>
        /// Provides static access to the framework's services provider
        /// </summary>
        public static IServiceProvider Services {
            get { return services; }
            set {
                if(services != null) {
                    throw new Exception("Can't set once a value has already been set.");
                }
                services = value;
            }
        }

        /// <summary>
        /// Provides static access to the current HttpContext
        /// </summary>
        public static HttpContext Current {
            get {
                IHttpContextAccessor httpContextAccessor = services.GetService(typeof(IHttpContextAccessor)) as IHttpContextAccessor;
                return httpContextAccessor?.HttpContext;
            }
        } 

    }
}

Next we need to add a service to the DI container that can provide access to the current HttpContext. This service ships with the Core MVC framework but is not installed by default. So we need to "install" it with a single line of code. This line goes in the ConfigureServices method of the Startup.cs file and can be located anywhere in that method:

//Add service for accessing current HttpContext
services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();    

Next we need to setup our static class so that it has access to the DI container to obtain the service we just installed. The code below goes in the Configure method of the Startup.cs file. This line can be be located anywhere in that method:

AppHttpContext.Services = app.ApplicationServices; 

Now any method called by the Controller, even via the async await pattern, can access the current HttpContext via AppHttpContext.Current

So, if we use the Session extension methods in the Microsoft.AspNetCore.Http namespace, we can save an int called "Count" to session could be done like this:

AppHttpContext.Current.Session.SetInt32("Count", count);

And retrieving an int called "Count" from session could be done like this:

int count count = AppHttpContext.Current.Session.GetInt32("Count");

Enjoy.