Dijkstra Algorithm, ArgumentNullException: Argument cannot be null.

Questions & Answers
1

Hi
I am trying to implement Dijkstras Algorithm as a basic pathfinding mathod on an chessboard like grid. I want place a block at every tile I used for a path between two tiles given by the user.
I got it working so far, but when I try to use every tile just once, so it is blocked for every future path, I get this Error:

ArgumentNullException: Argument cannot be null.
Parameter name: key
System.Collections.Generic.Dictionary`2[UnityEngine.GameObject,System.Single].get_Item (UnityEngine.GameObject key) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:136)
DijkstraAlgorithm.Dijkstra (UnityEngine.GameObject[,] grid, UnityEngine.GameObject start, UnityEngine.GameObject end) (at Assets/Scripts/DijkstraAlgorithm.cs:50)
MouseScript.Update () (at Assets/Scripts/MouseScript.cs:41)

This would be my code:

static public Stack<GameObject> Dijkstra(GameObject[,]grid, GameObject start, GameObject end)
    {
        Dictionary<GameObject, float> dist = new Dictionary<GameObject, float>();
        Dictionary<GameObject, GameObject> previous = new Dictionary<GameObject, GameObject>();
        List<GameObject> Q = new List<GameObject>();
        
        foreach(GameObject v in grid)
        {
            dist[v] = Mathf.Infinity;
            previous[v] = null;
            Q.Add(v);
        }

        dist[start] = 0;
        while(Q.Count > 0)
        {
            float shortestDistance = Mathf.Infinity;
            GameObject shortestDistanceNode = null;
            foreach(GameObject obj in Q)
            {
                if (dist[obj] < shortestDistance )
                {
                    shortestDistance = dist[obj];
                    shortestDistanceNode = obj;
                }
                
            }

            GameObject u = shortestDistanceNode;

            Q.Remove(u);
            if(u==end)
            {
                Stack<GameObject> stack = new Stack<GameObject>();
                while(previous != null)

{
stack.Push(u);
u = previous;
}
return stack;
}

if(dist == Mathf.Infinity)
{
break;
}

foreach(GameObject v in u.GetComponent().neighbors)
{
if(v.GetComponent().getBlocked()==false)
{
break;
}
float alt = dist + (u.transform.position - v.transform.position).magnitude;
if(alt< dist[v])
{
dist[v] = alt;
previous[v] = u;
}
}

}
return null;

}
}
I hope someone here can help me, googleing the error has not produced anything helpfull.
Thank you in advance
Greets
Harker

2

if(v.GetComponent() != null && v.GetComponent().getBlocked()==false)