System.NullReferenceException: The object reference does not indicate an object instance

Question

Hello. I write the game "Minesweeper", working from the console window. An error occurs on the code section:

field = new cell[ySize][]; for (int i = 0; i<ySize; i++) field[i] = new cell[xSize]; // инициализация генератора случайных чисел Random rnd = new Random(); int n=0; // расставим мины do { int row= rnd.Next(ySize); //координата мины по Y int col=rnd.Next(xSize); //координата мины по X if (field[row][col].cheсkNearCell()!= (-1)) { field[row][col].setMine(); if (row != 0 && col != 0) field[row - 1][col - 1].newMineNear(); if (row != 0) field[row - 1][col].newMineNear(); if (row != 0 && col != xSize - 1) field[row-1][col + 1].newMineNear(); if (col != 0) field[row][col - 1].newMineNear(); if (col != xSize - 1) field[row][col + 1].newMineNear(); if (row != ySize - 1 && col != 0) field[row + 1][col-1].newMineNear(); if (row != ySize - 1) field[row + 1][col].newMineNear(); if ((row != ySize-1) && (col != xSize-1)) field[row + 1][col + 1].newMineNear(); n++; } } while (n != numberOfMines);

In this section of the code, the array containing the “cells” of which the field consists of must be filled, and mines are placed. The code is compiled, the game starts, but gives an error

System.NullReferenceException: The object reference does not indicate an object instance.

I suspect that when the array array is populated, the class constructor is not called. Here is the Cell class code:

 class cell { private bool opend {get; set;} // - посещали ли мы клеточке private bool hasMine {get; set;} // - установлена ли в клеточке мина private bool hasFlag {get; set;} // - установлен ли в клеточке флаг private int minesNear {get; set;} // - мины рядом private cell() { hasMine=false; opend=false; hasFlag=false; minesNear=0; } //установить флаг public int setFlag() { if (hasMine==true) {hasFlag=true; return 1;} //1 - мина установлена успешно else return 0; } //установка мины public void setMine() { hasMine=true; } //добавление информации о мине поблизости public void newMineNear() { minesNear++; } //проверка этой клетки на наличие мины public int checkThisCell() { if(hasMine==true) return 666; // 666 - означает наличие мины в данной клетке else if (opend==false) {opend=true; return minesNear;} else return -1; //-1 - означает что мы посещали эту клетку и нужно выбрать другую клетку } //проверка соседней клетки на наличие мины public int cheсkNearCell() { if (hasMine==true || opend==true) return -1; // -1 - что нужно игнорировать эту клетку else return minesNear; } //отрисовка клетки public void show() { if (hasFlag==true) Console.Write("F"); // - в клетке установлен флаг else if (opend==false) Console.Write("#"); // - клетка не открывалась ранее else if (minesNear==0) Console.Write(" "); // - рядом нет мин else Console.Write(minesNear); // - есть определённое количество мин } }

Maybe run in debug mode to immediately see exactly which line of error and where exactly is null ?
@SurfinBird debugger indicates an error in the if (field[row][col].cheсkNearCell()!= (-1)) and the entire value of the field [] [] array is null

Accepted Answer · 2016-12-18T08:34:52

@Surfin Bird's answer is correct, let me explain what is happening.

You declared an array of arrays . For an array, you need to allocate memory with new , but the elements of your array are one-dimensional arrays! Under each of them also need to allocate memory. Such picture will turn out:

 field ---> [ элемент 0 ][ элемент 1 ][ элемент 2 ][ элемент 3 ]... | | | | | | | | VVVV [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ]

You have created the top line, but not the rest.

An alternative solution could be to select a rectangular multidimensional array. In this case, you immediately get the necessary elements:

 Cell[,] field; // ... field = new Cell[xSize][ySize];

The rectangular array is even more logical here, as in any games based on a rectangular board in general.
And if you consider that the moves are made by a person and the size of the board is not too large << 100k elements, then you can not think about the efficiency of processing a multidimensional array and improve the readability of the algorithm.

Surfin Bird Surfin Bird 2,182 one four 21 · Answer 2 · 2016-12-18T08:24:02

It seems that the initialization should be replaced by:

 for (int i = 0; i < ySize; i++){ field[i] = new cell[xSize]; for (int j = 0; j < xSize; j++) field[i][j] = new cell(); } }

Surfin Bird

2,182 one four 21

In the string field[i][j] = new cell(); an error occurs> Error CS0122: "Mineswepper.cell.cell ()" is unavailable due to its level of protection. Assembly failed. (00: 00: 00.4056007) - Gidemin
one
@Gidemin: Well, yes, you yourself declared the constructor private. Make it available. - VladD
@VladD yes, thanks for the help. I should have been more attentive. - Gidemin
one
@Gidemin: Please! This is normal, most of the errors are so small. - VladD

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System.NullReferenceException: The object reference does not indicate an object instance

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