Length, array

A C# array has a length—this is its size (its element count). We access the Length property. An int of 0 or greater is returned—no iteration is done (a cache is used).

We see what happens when you get the Length of a one-dimensional array, an empty array, and a null array reference.

Initial Length example

Here we access Length on several instances of arrays. We see related properties, such as LongLength and the GetLength method.

using System;

// Step 1: basic array length example.
string[] arrayA = new string[] { "cat", "apple", "frog" };
int lengthA = arrayA.Length;
Console.WriteLine("LENGTH: " + lengthA);

// Step 2: long array length example.
long longLength = arrayA.LongLength;
Console.WriteLine("LONGLENGTH: " + longLength);

// Step 3: zero length array example.
int[] zero = new int[0];
int lengthZero = zero.Length;
Console.WriteLine("LENGTH: " + lengthZero);

// Step 4: GetLength 0 example.
int lengthE = arrayA.GetLength(0);
Console.WriteLine("GETLENGTH: " + lengthE);
LENGTH: 3
LONGLENGTH: 3
LENGTH: 0
GETLENGTH: 3

Two-dimensional length

Here we see a simple example of using GetLength on a two-dimensional array. GetLength receives a rank (0 or 1) and prints the result size.

using System;

// Two-dimensional GetLength example.
int[,] two = new int[5, 10];
Console.WriteLine("GETLENGTH: " + two.GetLength(0));
Console.WriteLine("GETLENGTH: " + two.GetLength(1));

// Two-dimensional Length example.
Console.WriteLine("LENGTH: " + two.Length);
GETLENGTH: 5
GETLENGTH: 10
LENGTH: 50

Null array length

This program will raise an exception when you run it. Because the array reference is null, you will get an exception.

using System;

class Program
{
    static void Main()
    {
        int[] test = null;
        Console.WriteLine(test.Length);
    }
}
Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at Program.Main()...

Index error

GetLength can cause an exception when we access a dimension that is not part of the array. Here the array has one dimension, but we ask for the second dimension.

using System;

class Program
{
    static void Main()
    {
        int[] test = new int[500];
        Console.WriteLine(test.GetLength(1));
    }
}
Unhandled Exception: System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.Array.GetLength(Int32 dimension)
   at Program.Main()...

Discussion

Jagged arrays are essentially single-dimensional arrays of single-dimensional arrays. You can access their lengths the same as with one-dimensional arrays.

Initialization

It does not matter if the array elements are initialized. You can get the length of any allocated array. Int array elements are initialized to 0.

Performance

In a tight loop where hoisting the Length check will not affect JIT, you can cache it in a variable for a performance boost.

There are many reasons in C# programs to access array lengths. Sometimes exceptions are raised by Length and GetLength.