NullReferenceException in ExternallyControlledLifetimeManager

[appel1]

[TestMethod]
public void NullReferenceExceptionWhenGettingAndSettingsValuesFromMultipleThreads()
{
var e = new ExternallyControlledLifetimeManager();

Parallel.For(
  0,
  100000,
  i =>
  {
    if (i % 2 == 0)
      e.SetValue(new Foo());
    else
    {
      var foo = e.GetValue();
    }
  });

public class Foo
{
  public int[] Value { get; } = new int[1024 * 1024];
}
}

If we resolve values registered with ExternallyControlledLifetimeManager from multiple threads we quite frequently get a NullReferenceException in ExternallyControlledLifetimeManager.GetValue. There are no locks as far as I can tell anywhere so it boils down to what happens with the code above.

System.NullReferenceException: Object reference not set to an instance of an object.
   at Unity.Lifetime.ExternallyControlledLifetimeManager.GetValue(ILifetimeContainer container)
   at Unity.Strategies.LifetimeStrategy.PreBuildUp(BuilderContext& context)
   at Unity.UnityContainer.<>c.<.ctor>b__73_1(BuilderContext& context)
   at Unity.UnityContainer.Unity.IUnityContainer.Resolve(Type type, String name, ResolverOverride[] overrides)
   at Unity.UnityContainerExtensions.Resolve[T](IUnityContainer container, String name, ResolverOverride[] overrides)

This happens with Unity.Container 5.11.1.0 and Unity.Abstractions 4.2.0.0.
It did not happen with Unity.Container 5.8.13.0 and Unity.Abstractions 3.3.1.0.

[ENikS]

The externally controlled lifetime manager is not supposed to be used like that. It is initialized only once during instance registration and holds on to that value for the rest of its life.

Theoretically, if the object it holds reference to is already collected, it might create another one, but it all should happened inside the Unity.

[appel1]

This was just to show how the error occurs. The same thing happens if you resolve something registered using ExternallyControlledLifetimeManager from multiple threads as you can sort of see from the stack trace.

[ENikS]

If you could create a test case with the container, I will investigate.

I want the whole thing because resolving is a bit more involved than calling GetValue

[appel1]

Attached a small program that triggers the issue. Start the release build without a debugger attached and press the start button. On my machine it usually takes somewhere between 2-10 iterations until you get a NullReferenceException.

WpfApp2.zip

[appel1]

A possible improvement would be to make ExternallyControlledLifetimeManager a SynchronizedLifetimeManager to avoid creating multiple instances unnecessarily.

[ENikS]

I agree, I’ll make the change in next version