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C#

Array types


An array type is written as a non-array-type followed by one or more rank-specifiers:

array-type:
non-array-type rank-specifiers

non-array-type:
type

rank-specifiers:
rank-specifier
rank-specifiers rank-specifier

rank-specifier:
[ dim-separatorsopt ]

dim-separators:
,
dim-separators ,

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]...[RN] is an array with rank R and an element type T[R1]...[RN].

In effect, the rank-specifiers are read from left to right before the final non-array element type. The type int[][,,][,] is a single-dimensional array of three-dimensional arrays of two-dimensional arrays of int.

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

      1. The System.Array type

The type System.Array is the abstract base type of all array types. An implicit reference conversion exists from any array type to System.Array, and an explicit reference conversion exists from System.Array to any array type. Note that 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.

      1. Arrays and the generic IList interface

A one-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 . 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 . For example:

using System.Collections.Generic;

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
}
}

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 runtime 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[].

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[] .

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.



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