bool [,] - What does this syntax mean in c#?

Question

I found in legacy code following line:

protected bool[,] PixelsChecked;

What does [,] mean here?

Answer 1

It's a two-dimensional array.

In .NET you can have two types of arrays that aren't single dimension:

1. Multidimensional arrays:

   int[,] a;    // 2 dimensions
   int[,,] b;   // 3 dimensions, and so on
   

2. Jagged arrays (arrays of arrays):

   int[][] a;   // an array of arrays of ints
   int[][][] a; // an array of arrays of arrays of ints
   

In both cases you need to initialize the variable before using it though.

Usage is also different, in the first case:

int value = a[1, 2]; // note the comma, and single pair of brackets

In the second case, you need to address each array separately:

int value = a[1][2]; // the first [1] will return an array, and then you take
                     // the 3rd element (0-based) of that

Also remember that you can initialize a multidimensional array in just one statement:

int[,] a = new int[10, 20];

whereas a single statement for a jagged array will create a single array full of null-references:

int[][] a = new int[10][];

You will also need to initialize all the elements of that array to their corresponding array references, here's a quick way to do that with LINQ in one statement:

int[][] a = Enumerable.Range(0, 10).Select(new int[20]).ToArray();
// 10 x 20

Also see the MSDN Page on the subject for more information.

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Fun fact: The JITter produces faster code for accessing jagged arrays than it does for multidimensional arrays, see this question for more information.

Answer 2

The [,] is a 2-dimensional array.

You can initialize it like this:

protected bool[,] PixelsChecked = new bool[Width, Height];

This is how you access it:

bool leftTop = PixelsChecked[0, 0];

It is basically a rectangle with values, and you can access them with [x,y].

You could also create 3- and more-dimensional arrays with

protected bool[,,] Cube = new bool[5,5,5];

protected bool[,,,] _4dimensional = new bool[10,10,10,10];