为什么使用HttpContext的MVC 4会话助手不能工作，而HttpContextBase却不能呢？

Question

I implemented a session helper to save and retrieve session variables based on the following example: (I try to minimize the use of session variables) Stackoverflow question

I use MVC 4 and target .NET 4.5 on Visual Studio 2012.

I implemented a setter. This one uses HttpContextBase (ie controller.HttpContext):

public class HttpContextBaseSessionHelper : ISessionHelper
{
    private readonly HttpContextBase _context;

    public HttpContextBaseSessionHelper(HttpContextBase context)
    {
        _context = context;
    }

    public T Get<T>(string key)
    {
        object value = _context.Session[key];
        return value == null ? default(T) : (T)value;
    }

    public void Set<T>(string key, T value)
    {
        _context.Session[key] = value;
    }
}

This implementation uses HttpContext (ie System.Web.HttpContext.Current):

public class HttpContextSessionHelper : ISessionHelper
{
    private readonly HttpContext _context;

    public HttpContextSessionHelper(HttpContext context)
    {
        _context = context;
    }

    public T Get<T>(string key)
    {
        object value = _context.Session[key];
        return value == null ? default(T) : (T)value;
    }

    public void Set<T>(string key, T value)
    {
        _context.Session[key] = value;
    }
}

In the controller the HttpContext property is of type HttpContextBase (Controller.HttpContext). I can mock (using Moq) the ISessionHelper based on HttpContextBase.

I use the following two controller actions to see in the running app (not the unit test) if the correct values are set and retrieved:

public ActionResult SessionSet()
{
    _sessionHelper.Set<string>("TestKey", "TestValue");
    ViewBag.SessionValue = (string)HttpContext.Session["TestKey"];
    return View();
}

public ActionResult SessionGet()
{
    HttpContext.Session["TestKey"] = "TestValue";
    ViewBag.SessionValue = _sessionHelper.Get<string>("TestKey");
    return View();
}

This implementation throws a NullReference exception when using the above actions:

_sessionHelper = new HttpContextBaseSessionHelper(HttpContext);

But this implementation works just fine:

_sessionHelper = new HttpContextSessionHelper(System.Web.HttpContext.Current);

My question is why does this happen? Shouldn't the implementation using HttpContextBase work and the one using HttpContext give a problem seeing that Controller.HttpContext returns a HttpContextBase type?

Answer 1

This implementation throws a NullReference exception when using the above actions:

_sessionHelper = new HttpContextBaseSessionHelper(HttpContext);

You have placed this code in the constructor of your controller, haven't you? That won't work because the HttpContext property is not yet initialized at this stage. You should put this code in the Initialize method if you want to access any HttpContext related properties:

protected override void Initialize(RequestContext requestContext)
{
    base.Initialize(requestContext);
    _sessionHelper = new HttpContextBaseSessionHelper(HttpContext);
}

Remark about the following sentence from your question:

I try to minimize the use of session variables

You should not be trying to minimize the use of session variables. You should try to completely get rid of any ASP.NET Session from your application.