2.11 Arrays

Syntax:
type[*]+ array-name = new type [ dimension+ ][*]*;

Note that [*] is the set: [] [,] [,,] ...

Arrays allow a group of elements of a particular type to be stored in a contiguous block of memory. Array types derive from System.Array and are declared in C# using brackets ([]). For instance:

char[] vowels = new char[] {'a','e','i','o','u'};
Console.WriteLine(vowels [1]); // Prints "e"

The preceding function call prints "e" because array indexes start at 0. To support other languages, .NET can create arrays based on arbitrary start indexes, but the FCL libraries always use zero-based indexing. Once an array has been created, its length can't be changed. However, the System.Collection classes provide dynamically sized arrays, as well as other data structures, such as associative (key/value) arrays (see Section 3.4 in Chapter 3).

2.11.1 Multidimensional Arrays

Multidimensional arrays come in two varieties, rectangular and jagged. Rectangular arrays represent an n-dimensional block; jagged arrays are arrays of arrays:

// rectangular
int [,,] matrixR = new int [3, 4, 5]; // creates one big cube
// jagged
int [][][] matrixJ = new int [3][][];
for (int i = 0; i < 3; i++) {
   matrixJ[i] = new int [4][];
   for (int j = 0; j < 4; j++)
      matrixJ[i][j] = new int [5];
} 
// assign an element
matrixR [1,1,1] = matrixJ [1][1][1] = 7;

2.11.2 Local and Field Array Declarations

For convenience, local and field declarations can omit the array type when assigning a known value, because the type is specified in the declaration:

int[,] array = {{1,2},{3,4}};

2.11.3 Array Length and Rank

Arrays know their own length. For multidimensional arrays, the GetLength method returns the number of elements for a given dimension, which is counted from (the outermost) to the array's Rank-1 (the innermost):

// one-dimensional
for(int i = 0; i < vowels.Length; i++);
// multidimensional
for(int i = 0; i < matrixR.GetLength(2); i++);

2.11.4 Bounds Checking

All array indexing is bounds-checked by the CLR, with IndexOutOf-RangeException thrown for invalid indexes. As in Java, bounds checking prevents program faults and debugging difficulties while enabling code to be executed with security restrictions.

Generally the performance hit from bounds checking is minor, and the JIT can perform optimizations such as determining each array index is safe before entering a loop, thus avoiding a check made for each iteration. In addition, C# provides unsafe code to explicitly bypass bounds checking (see Section 2.17 later in this chapter).

2.11.5 Array Conversions

Arrays of reference types can be converted to other arrays using the same logic you apply to its element type (this is called array covariance). All arrays implement System.Array, which provides methods to generic get and set elements regardless of array type.