Including all of the following removed the errors I was getting:

using UnityEngine;
using Vuforia;
using System.Collections.Generic;

I have update my project today to the last version and  now the next code

trackableStateMng = TrackerManager.Instance.GetStateManager ();
    
 IEnumerable<TrackableBehaviour> activeTrackables = trackableStateMng.GetActiveTrackableBehaviours ();

do not work any more, activeTrackables is always empty even when the OnTrackableFound() is called.

Any idea or solution??

Thank you

StateManager sm = TrackerManager.Instance.GetStateManager();

IList<TrackableBehaviour> activeTrackables = (IList<TrackableBehaviour>) sm.GetActiveTrackableBehaviours();

if(activeTrackables.Count==0)
{
     //CURRENTLY NOT TRACKING
}
else
{
     //CURRENTLY IN TRACKING
}

(In fact, I use the Alessandro solution, but I use TrackableBehaviour instead of TrackableResult  )

I have yet to get the built-in active trackables code working, but I did create a workaround:

Bascially I created a list of DefaultTrackableEventHandler scripts. When a particular instance acquires a taget, it gets added. When it loses it, it gets removed. When the list length is 0, I know nothing is being tracked, so I can show my info screen. It's redundant since Vuforia is already doing that internally, but works for my needs.

Re-imported Vuforia, then added the lines of code back, and same error....

Verified on all three. 

I don't think I've modified any other code, but where does TrackableResult come from? Should I verify something in another script?

I'll also try re-importing Vuforia just to make sure.

Just to summarize, verify that:

• you have these statements:

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

• that your class extends MonoBehaviour
• that you don't have any other errors in your Project, as sometimes this error could a consequence of other errors in other scripts

Sure did. I apologize that i don't have a better grasp of what's going on...not sure how to troubleshoot this.

Did you also add 

using UnityEngine;

?

That's all you need normally.

Thanks! That fixed that error.

Now I have this one: The type or namespace name `TrackableResult' could not be found. Are you missing a using directive or an assembly reference?

Any thoughts?

You should be able to fix this error by adding this statement at the top of the script:

using System.Collections.Generic;

So this is what I have inside the OnTrackingLost function of DefaultTrackableEventHandler:

	StateManager sm = TrackerManager.Instance.GetStateManager();
	IEnumerable<TrackableResult> activeTrackables = sm.GetActiveTrackableResults();
		
	if (defaultText && activeTrackables==0) defaultText.SetIsOut(true);

And I get the error: The type or namespace name `IEnumerable`1' could not be found. Are you missing a using directive or an assembly reference?

Do I need an include or using line? Any ideas?

dp

You can achieve that by retrieving the number of trackable behaviours (in Unity), using the State Manager API:

StateManager sm = TrackerManager.Instance.GetStateManager();

IEnumerable<TrackableResult> activeTrackables = sm.GetActiveTrackableResults();

Then if the number of activeTrackables (i.e. the number of targets currently being tracked) is zero, you will known that there are no targets tracked in your view.