C# Jagged array: outside the bounds of the array issue

Question

I have a jagged array declared as:

char[][] m = new char[10][];

Later it is populated.

System.Text.StringBuilder s = new System.Text.StringBuilder(c);
for (int x = 0; x < 10; x++)
    s.Append('=');

    for (int x = 0; x < 10; x++) m[x] = s.ToString().ToCharArray();

If I perform the below operations, I get an error on the second dimension:

Console.WriteLine(String.Format("width={0}", m.GetLength(0)));
Console.WriteLine(String.Format("height={0}", m.GetLength(1))); <---- ERROR HERE

Any ideas?

Answer 1

You are confusing square arrays and jagged arrays. A jagged array is also known as an array of arrays, and it looks like this:

char[][] a = new char[10][];

Each element in a jagged array is an array in its own right, and it can have a completely different length from any of the others:

a[0] = new char[1];
a[1] = new char[100];
a[2] = new char[5];
...
a[9] = new char[999999];

As you can see, GetLength(1) makes no sense on a jagged array. What would it even return? To get the lengths of the second level of arrays, you would have to iterate through them and call their various Length properties:

for (int i = 0; i < a.Length; i++)
{
    int length = a[i].Length;
}

For your purposes, you probably want to use a square array (also known as a 2D array) instead:

char[,] a = new char[10,10];

Unlike a jagged array, the second dimension of a square array is guaranteed to be the same length. That means GetLength(1) can return a value with confidence.