How Can I Avoid NullReferenceExceptions When Unit Testing Web Services That Use HttpContext.Current.Session?

Mock HttpContext.Current.Session in unit tests

When writing unit tests for a web service that accesses a HttpContext.Current.Session value, a null reference exception may occur if the session is not initialized. To solve this problem, you can use mock sessions for unit testing.

One way is to create a mock HttpSessionStateContainer with a new HttpContext. Here’s how to do it:

public static HttpContext FakeHttpContext()
{
    var httpRequest = new HttpRequest("", "http://example.com/", "");
    var stringWriter = new StringWriter();
    var httpResponse = new HttpResponse(stringWriter);
    var httpContext = new HttpContext(httpRequest, httpResponse);

    var sessionContainer = new HttpSessionStateContainer("id", new SessionStateItemCollection(),
                                            new HttpStaticObjectsCollection(), 10, true,
                                            HttpCookieMode.AutoDetect,
                                            SessionStateMode.InProc, false);

    httpContext.Items["AspSession"] = typeof(HttpSessionState).GetConstructor(
                                BindingFlags.NonPublic | BindingFlags.Instance,
                                null, CallingConventions.Standard,
                                new[] { typeof(HttpSessionStateContainer) },
                                null)
                        .Invoke(new object[] { sessionContainer });

    return httpContext;
}

Alternatively, as suggested by Brent M. Spell, you can append HttpSessionStateContainer directly to HttpContext:

SessionStateUtility.AddHttpSessionStateToContext(httpContext, sessionContainer);

Use this mock session in unit tests:

HttpContext.Current = MockHelper.FakeHttpContext();

With this approach, the value can be set to HttpContext.Current.Session during unit testing without encountering a null reference exception, allowing the session-dependent functionality of the web service to be thoroughly tested.

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