random-bricker, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Drawing;

namespace Rextester {
    public class Program {
        bool[,] hasBrick;
        bool[,] alreadyOpen;
        List<Point> openPositions;
        List<Point> brickPositions;
        int width;
        int height;
        
        public void Init() {
            width = 20;
            height = 10;            
            hasBrick = new bool[width,height];
            alreadyOpen = new bool[width,height];
            openPositions = new List<Point>();
            brickPositions = new List<Point>();
        }
        
        public void AddBrick(int x, int y) {
            if (hasBrick[x,y]) {
                return;
            }
            hasBrick[x,y] = true;
            brickPositions.Add(new Point(x,y));
            if (x+1 < width && !alreadyOpen[x+1,y]) {
                openPositions.Add(new Point(x+1,y));
                alreadyOpen[x+1,y] = true;
            }
            if (x-1 >= 0 && !alreadyOpen[x-1,y]) {
                openPositions.Add(new Point(x-1,y));
                alreadyOpen[x-1,y] = true;
            }
            if (y+1 < height && !alreadyOpen[x,y+1]) {
                openPositions.Add(new Point(x,y+1));
                alreadyOpen[x,y+1] = true;
            }
            if (y-1 >=0 && !alreadyOpen[x,y-1]) {
                openPositions.Add(new Point(x,y-1));
                alreadyOpen[x,y-1] = true;
            }
        }
        public Point GetRandomOpenPosition(Random rng){
            int index = rng.Next(openPositions.Count());
            Point result = openPositions[index];
            openPositions.RemoveAt(index);
            return result;
        }
        public void Display() {
            for(int a=0; a<height; a++) {
                for(int b=0; b<width; b++) {
                    if(hasBrick[b,a]){
                        Console.Write("#");
                    }
                    else if(alreadyOpen[b,a]) {
                        Console.Write("O");
                    }
                    else {
                        Console.Write(".");
                    }                    
                }
                Console.WriteLine();
            }
        }
        public static void Main(string[] args){
        
            Random rng = new Random();
            Program app = new Program();
            app.Init();
            app.AddBrick(4,7);
            
            for(int a=0; a<25; a++){
                Point p = app.GetRandomOpenPosition(rng);
                app.AddBrick(p.X, p.Y);
            }
            
            app.Display();
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,09 sec, cpu time: 0,08 sec, average memory usage: 15 Mb, average nr of threads: 3