updated ReadOnlyCollection on Board.cs

Qwirkle/QwirkleClassLibrary/Board.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Linq;
 using System.Security.Cryptography;
 using System.Text;
@@ -9,12 +10,11 @@ namespace QwirkleClassLibrary
 {
     public class Board
     {
-        private List<Cell> cells;
+        public ReadOnlyCollection<Cell> ReadCells => cells.AsReadOnly();
-        public IReadOnlyCollection<Cell> ReadCells { get; private set; }
+        private readonly List<Cell> cells = new();
 
         public Board()
         {
-            cells = new List<Cell>();
 
             for (int i = 0; i < 12; i++)
             {