Unit testing mvc model with HttpContext.Current.User.Identity.GetUserId() - unit-testing

Question

In my MVC5 application, I have a Log entity, that is used to log any call to any controler. This log uses: HttpContext.Current.User.Identity.GetUserId() to determine the identity of the user accessing the controller.
public class Log
{
public Log()
{
TS = DateTime.Now;
UserId = HttpContext.Current.User.Identity.GetUserId();
}
[Required]
public Int32 Id { get; set; }
public string UserId { get; set; }
[Display(Name = "TS", ResourceType = typeof(Resources.Models.Log.Log))]
[Required(ErrorMessageResourceType = typeof(Resources.Models.Log.Log), ErrorMessageResourceName = "RequiredTS")]
public DateTime TS { get; set; }
[Required]
public short LogTypeId { get; set; }
[Display(Name = "LogText", ResourceType = typeof(Resources.Models.Log.Log))]
public string LogText { get; set; }
public ApplicationUser User { get; set; }
}
When I try to unit test a controller and crating an instance of the log class I get this error:
threw exception: System.NullReferenceException: Object reference not
set to an instance of an object.
at DASU.Core.Models.Log..ctor()
I know this is because the context is not set.
So my question is how do I set the context, or how do I mock the context, so I can create the Log for my test?

Answer 1

You should avoid coupling to HttpContext. Like suggested in the comments you could simplify your log my injecting the UserId into the dependent Log class
public class Log
{
public Log(string userId)
{
TS = DateTime.Now;
UserId = userId;
}
//...other code removed for brevity
}
or abstracting away the calls to HttpContext so that you can mock your abstract and inject that instead of trying and mock HttpContext
public interface IUserProvider {
string GetUserId();
}
You production implementations can wrap calls to HttpContext and you can easily create mock implementations for your unit tests.
public class Log
{
public Log(IUserProvider userProvider)
{
TS = DateTime.Now;
UserId = userProvider.GetUserId();
}
//...other code removed for brevity
}