UTests pass with Pathfinder2D

Gameboard.ShogiUI.BoardState/Array2D.cs

@@ -1,11 +1,12 @@
-using System;
+using PathFinding;
+using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Numerics;
 
 namespace Gameboard.ShogiUI.BoardState
 {
-  public class Array2D<T> : IEnumerable<T>
+  public class Array2D<T> : IPlanarCollection<T>, IEnumerable<T>
   {
     /// <returns>False to stop iterating.</returns>
     public delegate void ForEachDelegate(T element, int x, int y);
@@ -31,6 +32,13 @@ namespace Gameboard.ShogiUI.BoardState
       set => array[(int)y * width + (int)x] = value;
     }
 
+    public int GetLength(int dimension) => dimension switch
+    {
+      0 => width,
+      1 => height,
+      _ => throw new IndexOutOfRangeException()
+    };
+
     public void ForEach(ForEachDelegate callback)
     {
       for (var x = 0; x < width; x++)