Unity MVC GetService In Background Thread throws NullReferenceException

Question

As far as I know the DependencyResolver is Thread-Safe, however, running the following code throws a null reference exception in a background thread.

public interface ITest {

}
public class Test : ITest {

}

//this works fine
var service = DependencyResolver.Current.GetService<ITest>();

var t1 = Task.Run(() => {
   //This throws a Null Reference exception.
   // note that DependencyResolver.Current is NOT null.  
   // The exception occurs in GetService
   var s1 = DependencyResolver.Current.GetService<ITest>();
});
Task.WaitAll(t1);

Here's the stack trace:

at Unity.Mvc4.UnityDependencyResolver.get_ChildContainer()
at Unity.Mvc4.UnityDependencyResolver.IsRegistered(Type typeToCheck)
at Unity.Mvc4.UnityDependencyResolver.GetService(Type serviceType)
at System.Web.Mvc.DependencyResolverExtensions.GetService[TService](IDependencyResolver resolver)
at System.Threading.Tasks.Task.InnerInvoke()
at System.Threading.Tasks.Task.Execute()

I'm aware that the "Service Locator" pattern is an anti-pattern. At this point I'm just trying to understand why this doesn't work.

Any insights would be appreciated.

Thanks!

Answer 1

From the stack trace it is clear that you are using the the Unity.Mvc4 NuGet package, which is some unofficial package and is not published by Microsoft. This package contains a bug. Its UnityDependencyResolver.ChildContainer property calls HttpContext.Current.Items without checking whether HttpContext.Current is null and it causes a NullReferenceException when instances are resolved outside the context of a web request.

So instead of using that unofficial NuGet package, I think you're better off using the official NuGet package.