Unit-testing an action which calls session object

How can I unit test a method which uses a session object inside of its body?

Let us say I have the following action:

[HttpPost]
public JsonResult GetSearchResultGrid(JqGridParams gridParams, Guid campaignId, string queryItemsString)
{
    var queryItems = new JavaScriptSerializer().Deserialize<IList<FilledQueryItem>>(queryItemsString);
    IPageData pageData = gridParams.ToPageData();
    var extraFieldLinker = SessionHandler.CurrentExtraFieldsLinker;
    var searchParams = new SearchParamsModel(extraFieldLinker, queryItems);
    IList<CustomerSearchResultRow> searchResults = null;
    searchResults = _customerService.SearchCustomersByUrlAndCampaign(campaignId,
        searchParams.SearchString,
        searchParams.AddressFilterPredicate,
        pageData);
    return GetGridData<CustomerSearchResultGridDefinition, CustomerSearchResultRow>(searchResults, pageData);
}

I made the following unit tests which fails so far because of the session thing:

[Test]
public void CanGetSearchResultGrid()
{
    //Initialize
    var mockJqGridParams = new Mock<JqGridParams>();
    var mockPageData = new Mock<IPageData>();
    IPagedList<CustomerSearchResultRow> mockPagedResult = new PagedList<CustomerSearchResultRow>(mockPageData.Object);
    var guid= Guid.NewGuid();
    const string searchString =
        "[{\"Caption\":\"FirstName\",\"ConditionType\":\"contains\",\"Value\":\"d\",\"NextItem\":\"开发者_Go百科Last\"}]";
    Func<Address,bool> addressFilterPredicate = (x => true);

    //Setup
    mockJqGridParams.Setup(x => x.ToPageData()).Returns(mockPageData.Object);
    _customerService.Setup(x => x.SearchCustomersByUrlAndCampaign(guid, searchString, addressFilterPredicate, mockPageData.Object))
        .Returns(mockPagedResult);

    //Call
    var result = _homeController.GetSearchResultGrid(mockJqGridParams.Object, guid, searchString);

    mockJqGridParams.Verify(x => x.ToPageData(), Times.Once());
    _customerService.Verify(x => x.SearchCustomersByUrlAndCampaign(guid, searchString, addressFilterPredicate, mockPageData.Object)
        , Times.Once());

    //Verify
    Assert.That(result, Is.Not.Null);
    Assert.That(result, Is.TypeOf(typeof(JsonResult)));
}

And the method from the helper of course:

   public static ExtraFieldsLinker CurrentExtraFieldsLinker
    {
        get
        {
            object extraFieldLinker = GetSessionObject(EXTRA_FIELDS_LINKER);
            return extraFieldLinker as ExtraFieldsLinker;
        }
        set { SetSessionObject(EXTRA_FIELDS_LINKER, value); }
    }

I've solved similar issues (use of static data accessors that aren't mock friendly - in particular, HttpContext.Current) by wrapping the access in another object, and accessing it through an interface. You could do something like:

pubic interface ISessionData
{
    ExtraFieldsLinker CurrentExtraFieldsLinker { get; set; }
}

public class SessionDataImpl : ISessionData
{
    ExtraFieldsLinker CurrentExtraFieldsLinker
    {
        // Note: this code is somewhat bogus,
        // since I think these are methods of your class.
        // But it illustrates the point.  You'd put all the access here
        get { return (ExtraFieldsLinker)GetSessionObject(EXTRA_FIELDS_LINKER); }
        set { SetSessionObject(EXTRA_FIELDS_LINKER, value); }
    }
}

public class ClassThatContainsYourAction
{
    static ClassThatContainsYourAction()
    {
        SessionData = new SessionDataImpl();
    }

    public static ISessionData SessionData { get; private set; }

    // Making this access very ugly so you don't do it by accident
    public void SetSessionDataForUnitTests(ISessionData sessionData)
    {
        SessionData = sessionData;
    }

    [HttpPost]
    public JsonResult GetSearchResultGrid(JqGridParams gridParams,
        Guid campaignId, string queryItemsString)
    {
        var queryItems = // ...
        IPageData pageData = // ...

        // Access your shared state only through SessionData
        var extraFieldLinker = SessionData.CurrentExtraFieldsLinker;

        // ...
    }
}

Then your unit test can set the ISessionData instance to a mock object before calling GetSearchResultGrid.

Ideally you'd use a Dependency Injection library at some point, and get rid of the static constructor.

If you can figure out a way to make your ISessionData an instanced object instead of static, even better. Mock object frameworks tend to like to create a new mock type for every test case, and having mocks lying around from previous tests is kind of gross. I believe session state is going to be global to your session anyway, so you might not have to do anything tricky to make a non-static object work.