Structs

Like classes, structs are data structures that can contain data members and function members, but unlike classes, structs are value types and do not require heap allocation. A variable of a struct type directly stores the data of the struct, whereas a variable of a class type stores a reference to a dynamically allocated object. Struct types do not support user-specified inheritance, and all struct types implicitly inherit from type object.

Structs are particularly useful for small data structures that have value semantics. Complex numbers, points in a coordinate system, or key-value pairs in a dictionary are all good examples of structs. The use of structs rather than classes for small data structures can make a large difference in the number of memory allocations an application performs. For example, the following program creates and initializes an array of 100 points. With Point implemented as a class, 101 separate objects are instantiated—one for the array and one each for the 100 elements.

class Point
{
public int x, y;

public Point(int x, int y) {
this.x = x;
this.y = y;
}
}

class Test
{
static void Main() {
Point[] points = new Point[100];
for (int i = 0; i < 100; i++) points[i] = new Point(i, i);
}
}

An alternative is to make Point a struct.

struct Point
{
public int x, y;

public Point(int x, int y) {
this.x = x;
this.y = y;
}
}

Now, only one object is instantiated—the one for the array—and the Point instances are stored in-line in the array.

Struct constructors are invoked with the new operator, but that does not imply that memory is being allocated. Instead of dynamically allocating an object and returning a reference to it, a struct constructor simply returns the struct value itself (typically in a temporary location on the stack), and this value is then copied as necessary.

With classes, it is possible for two variables to reference the same object and thus possible for operations on one variable to affect the object referenced by the other variable. With structs, the variables each have their own copy of the data, and it is not possible for operations on one to affect the other. For example, the output produced by the following code fragment depends on whether Point is a class or a struct.

Point a = new Point(10, 10);
Point b = a;
a.x = 20;
Console.WriteLine(b.x);

If Point is a class, the output is 20 because a and b reference the same object. If Point is a struct, the output is 10 because the assignment of a to b creates a copy of the value, and this copy is unaffected by the subsequent assignment to a.x.

The previous example highlights two of the limitations of structs. First, copying an entire struct is typically less efficient than copying an object reference, so assignment and value parameter passing can be more expensive with structs than with reference types. Second, except for ref and out parameters, it is not possible to create references to structs, which rules out their usage in a number of situations.

Arrays

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.

Array types are reference types, and the declaration of an array variable simply sets aside space for a reference to an array instance. Actual array instances are created dynamically at run-time using the new operator. The new operation specifies the length of the new array instance, which is then fixed for the lifetime of the instance. The indices of the elements of an array range from 0 to Length - 1. The new operator automatically initializes the elements of an array to their default value, which, for example, is zero for all numeric types and null for all reference types.

The following example creates an array of int elements, initializes the array, and prints out the contents of the array.

using System;

class Test
{
static void Main() {
int[] a = new int[10];
for (int i = 0; i < a.Length; i++) {
a[i] = i * i;
}
for (int i = 0; i < a.Length; i++) {
Console.WriteLine("a[{0}] = {1}", i, a[i]);
}
}
}

This example creates and operates on a single-dimensional array. C# also supports multi-dimensional arrays. The number of dimensions of an array type, also known as the rank of the array type, is one plus the number of commas written between the square brackets of the array type. The following example allocates a one-dimensional, a two-dimensional, and a three-dimensional array.

int[] a1 = new int[10];
int[,] a2 = new int[10, 5];
int[,,] a3 = new int[10, 5, 2];

The a1 array contains 10 elements, the a2 array contains 50 (10 × 5) elements, and the a3 array contains 100 (10 × 5 × 2) elements.

The element type of an array can be any type, including an array type. An array with elements of an array type is sometimes called a jagged array because the lengths of the element arrays do not all have to be the same. The following example allocates an array of arrays of int:

int[][] a = new int[3][];
a[0] = new int[10];
a[1] = new int[5];
a[2] = new int[20];

The first line creates an array with three elements, each of type int[] and each with an initial value of null. The subsequent lines then initialize the three elements with references to individual array instances of varying lengths.

The new operator permits the initial values of the array elements to be specified using an array initializer, which is a list of expressions written between the delimiters { and }. The following example allocates and initializes an int[] with three elements.

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

Note that the length of the array is inferred from the number of expressions between { and }. Local variable and field declarations can be shortened further such that the array type does not have to be restated.

int[] a = {1, 2, 3};

Both of the previous examples are equivalent to the following:

int[] t = new int[3];
t[0] = 1;
t[1] = 2;
t[2] = 3;
int[] a = t;