17 Arrays

17.1 General

An array is a data structure that contains a number of variables that are accessed through computed indices. The variables contained in an array, also called the elements of the array, are all of the same type, and this type is called the element type of the array.

An array has a rank that determines the number of indices associated with each array element. The rank of an array is also referred to as the dimensions of the array. An array with a rank of one is called a single-dimensional array. An array with a rank greater than one is called a multi-dimensional array. Specific sized multi-dimensional arrays are often referred to as two-dimensional arrays, three-dimensional arrays, and so on. Each dimension of an array has an associated length that is an integral number greater than or equal to zero. The dimension lengths are not part of the type of the array, but rather are established when an instance of the array type is created at run-time. The length of a dimension determines the valid range of indices for that dimension: For a dimension of length N, indices can range from 0 to N – 1 inclusive. The total number of elements in an array is the product of the lengths of each dimension in the array. If one or more of the dimensions of an array have a length of zero, the array is said to be empty.

The element type of an array can itself be an array type (§17.2.1). Such arrays of arrays are distinct from multi-dimensional arrays and can be used to represent “jagged arrays”.

Example:

int[][] pascals = 
{
    new int[] {1},
    new int[] {1, 1},
    new int[] {1, 2, 1},
    new int[] {1, 3, 3, 1}
};

end example

Every array type is a reference type (§8.2). The element type of an array can be any type, including value types and array types.

17.2 Array types

17.2.1 General

The grammar productions for array types are provided in §8.2.1.

An array type is written as a non_array_type followed by one or more rank_specifiers.

A non_array_type is any type that is not itself an array_type.

The rank of an array type is given by the leftmost rank_specifier in the array_type: A rank_specifier indicates that the array is an array with a rank of one plus the number of “,” tokens in the rank_specifier.

The element type of an array type is the type that results from deleting the leftmost rank_specifier:

• An array type of the form T[R] is an array with rank R and a non-array element type T.
• An array type of the form T[R][R₁]...[Rₓ] is an array with rank R and an element type T[R₁]...[Rₓ].

In effect, the rank_specifiers are read from left to right before the final non-array element type.

Example: The type in T[][,,][,] is a single-dimensional array of three-dimensional arrays of two-dimensional arrays of int. end example

At run-time, a value of an array type can be null or a reference to an instance of that array type.

Note: Following the rules of §17.6, the value may also be a reference to a covariant array type. end note

17.2.2 The System.Array type

The type System.Array is the abstract base type of all array types. An implicit reference conversion (§10.2.8) exists from any array type to System.Array and to any interface type implemented by System.Array. An explicit reference conversion (§10.3.5) exists from System.Array and any interface type implemented by System.Array to any array type. System.Array is not itself an array_type. Rather, it is a class_type from which all array_types are derived.

At run-time, a value of type System.Array can be null or a reference to an instance of any array type.

17.2.3 Arrays and the generic collection interfaces

A single-dimensional array T[] implements the interface System.Collections.Generic.IList<T> (IList<T> for short) and its base interfaces. Accordingly, there is an implicit conversion from T[] to IList<T> and its base interfaces. In addition, if there is an implicit reference conversion from S to T then S[] implements IList<T> and there is an implicit reference conversion from S[] to IList<T> and its base interfaces (§10.2.8). If there is an explicit reference conversion from S to T then there is an explicit reference conversion from S[] to IList<T> and its base interfaces (§10.3.5).

Similarly, a single-dimensional array T[] also implements the interface System.Collections.Generic.IReadOnlyList<T> (IReadOnlyList<T> for short) and its base interfaces. Accordingly, there is an implicit conversion from T[] to IReadOnlyList<T> and its base interfaces. In addition, if there is an implicit reference conversion from S to T then S[] implements IReadOnlyList<T> and there is an implicit reference conversion from S[] to IReadOnlyList<T> and its base interfaces (§10.2.8). If there is an explicit reference conversion from S to T then there is an explicit reference conversion from S[] to IReadOnlyList<T> and its base interfaces (§10.3.5).

Example: For example:

class Test
{
    static void Main()
    {
        string[] sa = new string[5];
        object[] oa1 = new object[5];
        object[] oa2 = sa;

        IList<string> lst1 = sa;  // Ok
        IList<string> lst2 = oa1; // Error, cast needed
        IList<object> lst3 = sa;  // Ok
        IList<object> lst4 = oa1; // Ok

        IList<string> lst5 = (IList<string>)oa1; // Exception
        IList<string> lst6 = (IList<string>)oa2; // Ok

        IReadOnlyList<string> lst7 = sa;        // Ok
        IReadOnlyList<string> lst8 = oa1;       // Error, cast needed
        IReadOnlyList<object> lst9 = sa;        // Ok
        IReadOnlyList<object> lst10 = oa1;      // Ok
        IReadOnlyList<string> lst11 = (IReadOnlyList<string>)oa1; // Exception
        IReadOnlyList<string> lst12 = (IReadOnlyList<string>)oa2; // Ok
    }
}

The assignment lst2 = oa1 generates a compile-time error since the conversion from object[] to IList<string> is an explicit conversion, not implicit. The cast (IList<string>)oa1 will cause an exception to be thrown at run-time since oa1 references an object[] and not a string[]. However the cast (IList<string>)oa2 will not cause an exception to be thrown since oa2 references a string[].

end example

Whenever there is an implicit or explicit reference conversion from S[] to IList<T>, there is also an explicit reference conversion from IList<T> and its base interfaces to S[] (§10.3.5).

When an array type S[] implements IList<T>, some of the members of the implemented interface may throw exceptions. The precise behavior of the implementation of the interface is beyond the scope of this specification.

17.3 Array creation

Array instances are created by array_creation_expressions (§12.8.17.5) or by field or local variable declarations that include an array_initializer (§17.7). Array instances can also be created implicitly as part of evaluating an argument list involving a parameter array (§15.6.2.4).

When an array instance is created, the rank and length of each dimension are established and then remain constant for the entire lifetime of the instance. In other words, it is not possible to change the rank of an existing array instance, nor is it possible to resize its dimensions.

An array instance is always of an array type. The System.Array type is an abstract type that cannot be instantiated.

Elements of arrays created by array_creation_expressions are always initialized to their default value (§9.3).

17.4 Array element access

Array elements are accessed using element_access expressions (§12.8.12.2) of the form A[I₁, I₂, ..., Iₓ], where A is an expression of an array type and each Iₑ is an expression of type int, uint, long, ulong, or can be implicitly converted to one or more of these types. The result of an array element access is a variable, namely the array element selected by the indices.

The elements of an array can be enumerated using a foreach statement (§13.9.5).

17.5 Array members

Every array type inherits the members declared by the System.Array type.

17.6 Array covariance

For any two reference_types A and B, if an implicit reference conversion (§10.2.8) or explicit reference conversion (§10.3.5) exists from A to B, then the same reference conversion also exists from the array type A[R] to the array type B[R], where R is any given rank_specifier (but the same for both array types). This relationship is known as array covariance. Array covariance, in particular, means that a value of an array type A[R] might actually be a reference to an instance of an array type B[R], provided an implicit reference conversion exists from B to A.

Because of array covariance, assignments to elements of reference type arrays include a run-time check which ensures that the value being assigned to the array element is actually of a permitted type (§12.21.2).

Example:

class Test
{
    static void Fill(object[] array, int index, int count, object value) 
    {
        for (int i = index; i < index + count; i++)
        {
            array[i] = value;
        }
    }

    static void Main() 
    {
        string[] strings = new string[100];
        Fill(strings, 0, 100, "Undefined");
        Fill(strings, 0, 10, null);
        Fill(strings, 90, 10, 0);
    }
}

The assignment to array[i] in the Fill method implicitly includes a run-time check, which ensures that value is either a null reference or a reference to an object of a type that is compatible with the actual element type of array. In Main, the first two invocations of Fill succeed, but the third invocation causes a System.ArrayTypeMismatchException to be thrown upon executing the first assignment to array[i]. The exception occurs because a boxed int cannot be stored in a string array.

end example

Array covariance specifically does not extend to arrays of value_types. For example, no conversion exists that permits an int[] to be treated as an object[].

17.7 Array initializers

Array initializers may be specified in field declarations (§15.5), local variable declarations (§13.6.2), and array creation expressions (§12.8.17.5):

array_initializer
    : '{' variable_initializer_list? '}'
    | '{' variable_initializer_list ',' '}'
    ;

variable_initializer_list
    : variable_initializer (',' variable_initializer)*
    ;
    
variable_initializer
    : expression
    | array_initializer
    ;

An array initializer consists of a sequence of variable initializers, enclosed by “{” and “}” tokens and separated by “,” tokens. Each variable initializer is an expression or, in the case of a multi-dimensional array, a nested array initializer.

The context in which an array initializer is used determines the type of the array being initialized. In an array creation expression, the array type immediately precedes the initializer, or is inferred from the expressions in the array initializer. In a field or variable declaration, the array type is the type of the field or variable being declared. When an array initializer is used in a field or variable declaration,

int[] a = {0, 2, 4, 6, 8};

it is simply shorthand for an equivalent array creation expression:

int[] a = new int[] {0, 2, 4, 6, 8};

For a single-dimensional array, the array initializer shall consist of a sequence of expressions, each having an implicit conversion to the element type of the array (§10.2). The expressions initialize array elements in increasing order, starting with the element at index zero. The number of expressions in the array initializer determines the length of the array instance being created.

Example: The array initializer above creates an int[] instance of length 5 and then initializes the instance with the following values:

a[0] = 0; a[1] = 2; a[2] = 4; a[3] = 6; a[4] = 8;

end example

For a multi-dimensional array, the array initializer shall have as many levels of nesting as there are dimensions in the array. The outermost nesting level corresponds to the leftmost dimension and the innermost nesting level corresponds to the rightmost dimension. The length of each dimension of the array is determined by the number of elements at the corresponding nesting level in the array initializer. For each nested array initializer, the number of elements shall be the same as the other array initializers at the same level.

Example: The example:

int[,] b = {{0, 1}, {2, 3}, {4, 5}, {6, 7}, {8, 9}};

creates a two-dimensional array with a length of five for the leftmost dimension and a length of two for the rightmost dimension:

int[,] b = new int[5, 2];

and then initializes the array instance with the following values:

b[0, 0] = 0; b[0, 1] = 1;
b[1, 0] = 2; b[1, 1] = 3;
b[2, 0] = 4; b[2, 1] = 5;
b[3, 0] = 6; b[3, 1] = 7;
b[4, 0] = 8; b[4, 1] = 9;

end example

If a dimension other than the rightmost is given with length zero, the subsequent dimensions are assumed to also have length zero.

Example:

int[,] c = {};

creates a two-dimensional array with a length of zero for both the leftmost and the rightmost dimension:

int[,] c = new int[0, 0];

end example

When an array creation expression includes both explicit dimension lengths and an array initializer, the lengths shall be constant expressions and the number of elements at each nesting level shall match the corresponding dimension length.

Example: Here are some examples:

int i = 3;
int[] x = new int[3] {0, 1, 2}; // OK
int[] y = new int[i] {0, 1, 2}; // Error, i not a constant
int[] z = new int[3] {0, 1, 2, 3}; // Error, length/initializer mismatch

Here, the initializer for y results in a compile-time error because the dimension length expression is not a constant, and the initializer for z results in a compile-time error because the length and the number of elements in the initializer do not agree.

end example

Note: C# allows a trailing comma at the end of an array_initializer. This syntax provides flexibility in adding or deleting members from such a list, and simplifies machine generation of such lists. end note