Operation is not valid due to the current state of the object - Linq on List

Question

This error is being thrown when running a Linq query over a List.

I am using Unity3D 3.0 with C# (Unity3D uses Mono 2.6). Unity3D, as far as I know, is single-threaded. It works by attaching "scripts" (c# .cs files) that inherit a baseclass, to a "GameObject". Also, Unity controls instantiation and serialization of scripts so you can't use constructors.

I have a RoadNetwork script that holds a reference to RoadNodes and RoadCurves, both of which locate RoadNetwork via a singleton and register/deregister themselves. I've put "mini-factories" in RoadNode and RoadCurve that do the hard work of hooking themselves up to a gameobject.

RoadNode first checks with RoadNetwork to make sure there isnt already a node at that same position, by doing this:

public static RoadNode2 New(float x, float y, float z)
{
    //First try to find an existing one
    var rn = RoadNetwork.Instance.GetNodeAtPosition(new Vector3(x, y, z))
             ?? UnityReferenceHelper.GetNewGameObjectFor<RoadNode2>(
                 "RoadNode_" + (RoadNetwork.Instance.Nodes.Count + 1).ToString("D3"),
                 RoadNetwork.Instance.transform.FindChild("Nodes"));

    rn.Position = new Vector3(x, y, z);

    rn.gameObject.active = true;

    return rn;
}

Where the appropriate method in RoadNetwork is:

public RoadNode2 GetNodeAtPosition(Vector3 position)
{
    var tempList = new List<RoadNode2>();

    return tempList.Single(x => x.Position == position);
}

tempList was an attempt at narrowing down the problem but I get precisely the same error. It should be "Nodes.Single(...", but I doubt it matters. I get the same error if I call the Linq query directly in the New() method.

So yes, this Exception throws and points me to that tempList.Single() line. What would the reason be?

Answer 1

someEnumerable.Single(...) throws an exception if there is not exactly one element in someEnumerable. Given that you just declared tempList to be an empty list, it will always throw an exception.

If you want to retrieve null if there are no elements, use SingleOrDefault. (This will still throw an exception if the enumerable contains more than one element.) If you want the first element, so that your enumerable is allowed to contain any number of elements, use First (throws an exception if the enumerable contains no elements) or FirstOrDefault (returns null in the case).

Finally, if you want to simply check if there are any elements of a list matching a given predicate, use Any.