My program is throwing exceptions and I can't figure out why

Question

I am trying to create a snake game but my code is throwing exceptions and I can't figure out what it might be.

I am creating this snake game because I want to learn more c# since my teacher said that 4th quarter I get to choose what I want to do with my time.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace snake_game
{
    class Program
    {
        static void Main(string[] args)
        {
            int xposition = 26;
            int yposition = 26;
            int LeftColumn = 1;
            int rightcolumn = 50;
            int topcolumn = 1;
            int bottomcolumn = 50;
            string[,] map = new string[51, 51];

            map = buildWall(LeftColumn, rightcolumn, topcolumn, 
            bottomcolumn, map);

            //places down the player and updates the map to tell where you are
            Console.SetCursorPosition(xposition, yposition);
            Console.Write((char)2);
            map[xposition, yposition] = "player";

            map = generateRandomApple(map, LeftColumn, rightcolumn, 
            topcolumn, bottomcolumn);

            placeApple(map);

            Console.ReadKey();
        }

        private static void placeApple(string[,] map)
        {
            //places down the apple
            for (int x = 0; x < map.Length; x++)
        {
            for (int y = 0; y < map.Length; y++)
            {
                if (map[x, y].Equals("apple"))
                {
                    Console.Write("a");

                    break;
                }
            }
        }
    }

    private static string[,] generateRandomApple(string[,] map, int lc, int 
    rc, int tc, int bc)
    {
        Random rnd = new Random();

        int xposition;
        int yposition;

        while (true)
        {

            //generates random cordinates to place down the apple
            xposition = rnd.Next(lc, rc);
            yposition = rnd.Next(tc, bc);

            //sets the property that the apple wants to be at to the apple 
            if it isnt open in the map
            if ((!map[xposition, yposition].Equals("the wall")) && 
            (!map[xposition, yposition].Equals("player")))
            {
                map[xposition, yposition] = "apple";

                break;
            }
        }

        return map;
    }

    private static string[,] buildWall(int leftcolumn, int rightcolumn, int 
    topcolumn, int bottomcolumn, string[,] map)
    {

        //generates the left and right walls
        for (int i = leftcolumn; i <= rightcolumn ; i++)
        {
            Console.SetCursorPosition(leftcolumn, i);
            Console.BackgroundColor = ConsoleColor.White;
            Console.ForegroundColor = ConsoleColor.Blue;
            Console.Write("#");

            map[leftcolumn, i] = "the wall";

            Console.SetCursorPosition(rightcolumn, i);
            Console.Write("#");
            map[rightcolumn, i] = "the wall";
        }

        //generates the top and bottom walls
        for (int i = topcolumn; i <= bottomcolumn; i++)
        {
            Console.SetCursorPosition(i, topcolumn);
            Console.Write("#");
            map[i, topcolumn] = "the wall";

            Console.SetCursorPosition(i, bottomcolumn);
            Console.Write("#");
            map[i, bottomcolumn] = "the wall";
        }

        return map;
     }
   }
 }

It is supposed to set up the map but the if statement in the function getRandomApple is throwing exceptions specifically the part that checks if the spot has the player in it, the exception it is throwing is saying that a

NullReferenEexception was being unhandled(object reference not set to an 
instance of an object).

Can anyone help me figure out what might be throwing the exceptions? I would appreciate the help.

What exceptions are you seeing? If you edit the question and include the exception messages and line numbers that would help us help you. — Jules Dupont, Apr 19 '18 at 14:54
`if it isnt open in the map` needs to be commented out, as well as the `are` right before `Console.SetCursorPosition(xposition, yposition);` — Brett Holmes, Apr 19 '18 at 14:57
The edit you made helps slightly, but it would much better if you could include the actual stack trace from the exceptions you're seeing. — pushasha, Apr 19 '18 at 15:00
Possible duplicate of [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Jon Skeet, Apr 19 '18 at 15:14
If you look at the stack trace of the exception you will see the line that caused the exception - use the debugger & point a break point on that line & examine the values - you should be able to find which one is null. — PaulF, Apr 19 '18 at 15:56

score 0 · Answer 1 · answered Apr 19 '18 at 15:09

In your generateRandomApple method, you have the following if statement:

if ((!map[xposition, yposition].Equals("the wall")) && (!map[xposition, yposition].Equals("player")))

When you populate your map, you don't add something to every index, so values at many of the map[xposition, yposition]s are null. So when you call .Equals() on those values in your if check, you are getting a null pointer exception. Try performing a null check before calling .Equals(), or using the equivalence operator instead.

My program is throwing exceptions and I can't figure out why

1 Answers1