Anyone help with an infinite loop problem on a while list.count >0 condition

Ok, this is a slightly messy, massively unoptimized bit of code. My problem is that whenever I run it, it goes infinite and I get nothing output in the console log at all. Have to quit unity to get back in.. The code is part of a pathfinding testing process based on grid movement.

The core loop is an empty list with a single start location added to process from. The loop is a while loop (while list.count>0), and if movement is possible (up,down,left,right only) then those cells are added to the open list for further processing. Except it doesn't work and I just can't figure out why. The codes not finished yet, but I need to figure out why this isn't working, so this isn't the endgoal (need a closed list and a bunch of other stuff, but this ought to be working in its current state)

Gamemanager is a separate gameobject and script with an array called GridContents which is how in this test project I'm tracking the state of the grid and whether each square is empty or not (non walkable).

WorldTile is a custom class representing each game square, which I'll be using later on, if I can get this infinite loop thing sorted. Ok here's the code...

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class PathFinding : MonoBehaviour
 {
     public GameObject GameController;
 
     public void Start()
     {
         GameController gm = GameController.GetComponent<GameController>();
     }
     public void PathfindingTest()
 
     {
         Debug.Log("This is the START of the pathfinding test PATHFINDING function");
         GameController gm = GameController.GetComponent<GameController>();
         
         List<WorldTile> openSet = new List<WorldTile>();
         HashSet<WorldTile> closedSet = new HashSet<WorldTile>();
         
         // TESTING ONLY Add the 0,0 location to the open set
         WorldTile StartingSearchTile = new WorldTile(0, 0);
         openSet.Add(StartingSearchTile);
         Debug.Log("The starting search Tile is " + openSet[0].gridX + "," + openSet[0].gridY);
   
        while (openSet.Count>0)
        {
             WorldTile currentnode = openSet[0];
             int tempx = currentnode.gridX;
             int tempy = currentnode.gridY;
             Debug.Log("The current active node is " +tempx + "," + tempy);
             Debug.Log("New open set count:"+openSet.Count);
 
             // TESTING ONLY - just manual checks based on a 20x7 grid (0,0 BottomLeft > 19,6 UpperRight)
 
             // Check to the left
             if (tempx > 0)
             {
                 if (gm.GridContents[(tempx - 1), tempy] == "empty")
                 {
                   WorldTile wtleft = new WorldTile(tempx - 1, tempy);
                openSet.Add(wtleft);
                 }
             }
 
             // Check to the right, invalid if x>=19 (last column in grid)
             if (tempx < 19)
             {
                 if (gm.GridContents[(tempx + 1), tempy] == "empty")
                 {
                   WorldTile wtright = new WorldTile(tempx + 1, tempy);
                 openSet.Add(wtright);
                 }
             }
 
             // Check above, invalid if y=6 (top row)
             if (tempy < 6)
             {
                 if (gm.GridContents[tempx, (tempy + 1)] == "empty")
                 {
                  WorldTile wtup = new WorldTile(tempx, tempy + 1);
                 openSet.Add(wtup);
                 }
             }
 
             // Check below, invalid if y=0 (bottom row)
             if (tempy>0)
             {
                 if (gm.GridContents[tempx, (tempy - 1)] == "empty")
                 {
                   WorldTile wtdown = new WorldTile(tempx, tempy - 1);
                  openSet.Add(wtdown);
                 }
             }
 
             // We should now have checked all four possible directions, including checking for invalid values, and new items have been added to the open set.
             Debug.Log("All routes checked from: " + tempx + "," + tempy);
             Debug.Log("Removing current item from open list");
                 
             openSet.Remove(currentnode);
             Debug.Log("The open list now has " + openSet.Count + " items");
       }
         Debug.Log("The While loop has exited");
     }
 }