Class and struct differences

Structs differ from classes in several important ways:

· Structs are value types (§11.3.1).

· All struct types implicitly inherit from the class System.ValueType (§11.3.2).

· Assignment to a variable of a struct type creates a copy of the value being assigned (§11.3.3).

· The default value of a struct is the value produced by setting all value type fields to their default value and all reference type fields to null (§11.3.4).

· Boxing and unboxing operations are used to convert between a struct type and object (§11.3.5).

· The meaning of this is different for structs (§7.6.7).

· Instance field declarations for a struct are not permitted to include variable initializers (§11.3.7).

· A struct is not permitted to declare a parameterless instance constructor (§11.3.8).

· A struct is not permitted to declare a destructor (§11.3.9).

Value semantics

Structs are value types (§4.1) and are said to have value semantics. Classes, on the other hand, are reference types (§4.2) and are said to have reference semantics.

A variable of a struct type directly contains the data of the struct, whereas a variable of a class type contains a reference to the data, the latter known as an object. When a struct B contains an instance field of type A and A is a struct type, it is a compile-time error for A to depend on B or a type constructed from B. A struct X directly depends on a struct Y if X contains an instance field of type Y. Given this definition, the complete set of structs upon which a struct depends is the transitive closure of the directly depends on relationship. For example

struct Node
{
int data;

Node next; // error, Node directly depends on itself

}

is an error because Node contains an instance field of its own type. Another example

struct A { B b; }

struct B { C c; }

struct C { A a; }

is an error because each of the types A, B, and C depend on each other.

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 (except in the case of ref and out parameter variables), and it is not possible for operations on one to affect the other. Furthermore, because structs are not reference types, it is not possible for values of a struct type to be null.

Given the declaration

struct Point
{
public int x, y;

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

the code fragment

Point a = new Point(10, 10);
Point b = a;
a.x = 100;
System.Console.WriteLine(b.x);

outputs the value 10. The assignment of a to b creates a copy of the value, and b is thus unaffected by the assignment to a.x. Had Point instead been declared as a class, the output would be 100 because a and b would reference the same object.

Inheritance

All struct types implicitly inherit from the class System.ValueType, which, in turn, inherits from class object. A struct declaration may specify a list of implemented interfaces, but it is not possible for a struct declaration to specify a base class.

Struct types are never abstract and are always implicitly sealed. The abstract and sealed modifiers are therefore not permitted in a struct declaration.

Since inheritance isn’t supported for structs, the declared accessibility of a struct member cannot be protected or protected internal.

Function members in a struct cannot be abstract or virtual, and the override modifier is allowed only to override methods inherited from System.ValueType.

Assignment

Assignment to a variable of a struct type creates a copy of the value being assigned. This differs from assignment to a variable of a class type, which copies the reference but not the object identified by the reference.

Similar to an assignment, when a struct is passed as a value parameter or returned as the result of a function member, a copy of the struct is created. A struct may be passed by reference to a function member using a ref or out parameter.

When a property or indexer of a struct is the target of an assignment, the instance expression associated with the property or indexer access must be classified as a variable. If the instance expression is classified as a value, a compile-time error occurs. This is described in further detail in §7.17.1.

Default values

As described in §5.2, several kinds of variables are automatically initialized to their default value when they are created. For variables of class types and other reference types, this default value is null. However, since structs are value types that cannot be null, the default value of a struct is the value produced by setting all value type fields to their default value and all reference type fields to null.

Referring to the Point struct declared above, the example

Point[] a = new Point[100];

initializes each Point in the array to the value produced by setting the x and y fields to zero.

The default value of a struct corresponds to the value returned by the default constructor of the struct (§4.1.2). Unlike a class, a struct is not permitted to declare a parameterless instance constructor. Instead, every struct implicitly has a parameterless instance constructor which always returns the value that results from setting all value type fields to their default value and all reference type fields to null.

Structs should be designed to consider the default initialization state a valid state. In the example

using System;

struct KeyValuePair
{
string key;
string value;

public KeyValuePair(string key, string value) {
if (key == null || value == null) throw new ArgumentException();
this.key = key;
this.value = value;
}
}

the user-defined instance constructor protects against null values only where it is explicitly called. In cases where a KeyValuePair variable is subject to default value initialization, the key and value fields will be null, and the struct must be prepared to handle this state.

Boxing and unboxing

A value of a class type can be converted to type object or to an interface type that is implemented by the class simply by treating the reference as another type at compile-time. Likewise, a value of type object or a value of an interface type can be converted back to a class type without changing the reference (but of course a run-time type check is required in this case).

Since structs are not reference types, these operations are implemented differently for struct types. When a value of a struct type is converted to type object or to an interface type that is implemented by the struct, a boxing operation takes place. Likewise, when a value of type object or a value of an interface type is converted back to a struct type, an unboxing operation takes place. A key difference from the same operations on class types is that boxing and unboxing copies the struct value either into or out of the boxed instance. Thus, following a boxing or unboxing operation, changes made to the unboxed struct are not reflected in the boxed struct.

When a struct type overrides a virtual method inherited from System.Object (such as Equals, GetHashCode, or ToString), invocation of the virtual method through an instance of the struct type does not cause boxing to occur. This is true even when the struct is used as a type parameter and the invocation occurs through an instance of the type parameter type. For example:

using System;

struct Counter
{
int value;

public override string ToString() {
value++;
return value.ToString();
}
}

class Program
{
static void Test<T>() where T: new() {
T x = new T();
Console.WriteLine(x.ToString());
Console.WriteLine(x.ToString());
Console.WriteLine(x.ToString());
}

static void Main() {
Test<Counter>();
}
}

The output of the program is:

1
2
3

Although it is bad style for ToString to have side effects, the example demonstrates that no boxing occurred for the three invocations of x.ToString().

Similarly, boxing never implicitly occurs when accessing a member on a constrained type parameter. For example, suppose an interface ICounter contains a method Increment which can be used to modify a value. If ICounter is used as a constraint, the implementation of the Increment method is called with a reference to the variable that Increment was called on, never a boxed copy.

using System;

interface ICounter
{
void Increment();
}

struct Counter: ICounter
{
int value;

public override string ToString() {
return value.ToString();
}

void ICounter.Increment() {
value++;
}
}

class Program
{
static void Test<T>() where T: ICounter, new() {
T x = new T();
Console.WriteLine(x);
x.Increment(); // Modify x
Console.WriteLine(x);
((ICounter)x).Increment(); // Modify boxed copy of x
Console.WriteLine(x);
}

static void Main() {
Test<Counter>();
}
}

The first call to Increment modifies the value in the variable x. This is not equivalent to the second call to Increment, which modifies the value in a boxed copy of x. Thus, the output of the program is:

0
1
1

For further details on boxing and unboxing, see §4.3.

Meaning of this

Within an instance constructor or instance function member of a class, this is classified as a value. Thus, while this can be used to refer to the instance for which the function member was invoked, it is not possible to assign to this in a function member of a class.

Within an instance constructor of a struct, this corresponds to an out parameter of the struct type, and within an instance function member of a struct, this corresponds to a ref parameter of the struct type. In both cases, this is classified as a variable, and it is possible to modify the entire struct for which the function member was invoked by assigning to this or by passing this as a ref or out parameter.

Field initializers

As described in §11.3.4, the default value of a struct consists of the value that results from setting all value type fields to their default value and all reference type fields to null. For this reason, a struct does not permit instance field declarations to include variable initializers. This restriction applies only to instance fields. Static fields of a struct are permitted to include variable initializers.

The example

struct Point
{
public int x = 1; // Error, initializer not permitted
public int y = 1; // Error, initializer not permitted
}

is in error because the instance field declarations include variable initializers.

Constructors

Unlike a class, a struct is not permitted to declare a parameterless instance constructor. Instead, every struct implicitly has a parameterless instance constructor which always returns the value that results from setting all value type fields to their default value and all reference type fields to null (§4.1.2). A struct can declare instance constructors having parameters. For example

struct Point
{
int x, y;

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

Given the above declaration, the statements

Point p1 = new Point();

Point p2 = new Point(0, 0);

both create a Point with x and y initialized to zero.

A struct instance constructor is not permitted to include a constructor initializer of the form base(...).

If the struct instance constructor doesn’t specify a constructor initializer, the this variable corresponds to an out parameter of the struct type, and similar to an out parameter, this must be definitely assigned (§5.3) at every location where the constructor returns. If the struct instance constructor specifies a constructor initializer, the this variable corresponds to a ref parameter of the struct type, and similar to a ref parameter, this is considered definitely assigned on entry to the constructor body. Consider the instance constructor implementation below:

struct Point
{
int x, y;

public int X {
set { x = value; }
}

public int Y {
set { y = value; }
}

public Point(int x, int y) {
X = x; // error, this is not yet definitely assigned
Y = y; // error, this is not yet definitely assigned
}
}

No instance member function (including the set accessors for the properties X and Y) can be called until all fields of the struct being constructed have been definitely assigned. Note, however, that if Point were a class instead of a struct, the instance constructor implementation would be permitted.

Destructors

A struct is not permitted to declare a destructor.

Static constructors

Static constructors for structs follow most of the same rules as for classes. The execution of a static constructor for a struct type is triggered by the first of the following events to occur within an application domain:

· A static member of the struct type is referenced.

· An explicitly declared constructor of the struct type is called.

The creation of default values (§11.3.4) of struct types does not trigger the static constructor. (An example of this is the initial value of elements in an array.)

Struct examples

The following shows two significant examples of using struct types to create types that can be used similarly to the predefined types of the language, but with modified semantics.

Database integer type

The DBInt struct below implements an integer type that can represent the complete set of values of the int type, plus an additional state that indicates an unknown value. A type with these characteristics is commonly used in databases.

using System;

public struct DBInt
{
// The Null member represents an unknown DBInt value.

public static readonly DBInt Null = new DBInt();

// When the defined field is true, this DBInt represents a known value
// which is stored in the value field. When the defined field is false,
// this DBInt represents an unknown value, and the value field is 0.

int value;
bool defined;

// Private instance constructor. Creates a DBInt with a known value.

DBInt(int value) {
this.value = value;
this.defined = true;
}

// The IsNull property is true if this DBInt represents an unknown value.

public bool IsNull { get { return!defined; } }

// The Value property is the known value of this DBInt, or 0 if this
// DBInt represents an unknown value.

public int Value { get { return value; } }

// Implicit conversion from int to DBInt.

public static implicit operator DBInt(int x) {
return new DBInt(x);
}

// Explicit conversion from DBInt to int. Throws an exception if the
// given DBInt represents an unknown value.

public static explicit operator int(DBInt x) {
if (!x.defined) throw new InvalidOperationException();
return x.value;
}

public static DBInt operator +(DBInt x) {
return x;
}

public static DBInt operator -(DBInt x) {
return x.defined? -x.value: Null;
}

public static DBInt operator +(DBInt x, DBInt y) {
return x.defined && y.defined? x.value + y.value: Null;
}

public static DBInt operator -(DBInt x, DBInt y) {
return x.defined && y.defined? x.value - y.value: Null;
}

public static DBInt operator *(DBInt x, DBInt y) {
return x.defined && y.defined? x.value * y.value: Null;
}

public static DBInt operator /(DBInt x, DBInt y) {
return x.defined && y.defined? x.value / y.value: Null;
}

public static DBInt operator %(DBInt x, DBInt y) {
return x.defined && y.defined? x.value % y.value: Null;
}

public static DBBool operator ==(DBInt x, DBInt y) {
return x.defined && y.defined? x.value == y.value: DBBool.Null;
}

public static DBBool operator!=(DBInt x, DBInt y) {
return x.defined && y.defined? x.value!= y.value: DBBool.Null;
}

public static DBBool operator >(DBInt x, DBInt y) {
return x.defined && y.defined? x.value > y.value: DBBool.Null;
}

public static DBBool operator <(DBInt x, DBInt y) {
return x.defined && y.defined? x.value < y.value: DBBool.Null;
}

public static DBBool operator >=(DBInt x, DBInt y) {
return x.defined && y.defined? x.value >= y.value: DBBool.Null;
}

public static DBBool operator <=(DBInt x, DBInt y) {
return x.defined && y.defined? x.value <= y.value: DBBool.Null;
}

public override bool Equals(object obj) {
if (!(obj is DBInt)) return false;
DBInt x = (DBInt)obj;
return value == x.value && defined == x.defined;
}

public override int GetHashCode() {
return value;
}

public override string ToString() {
return defined? value.ToString(): “DBInt.Null”;
}
}

Database boolean type

The DBBool struct below implements a three-valued logical type. The possible values of this type are DBBool.True, DBBool.False, and DBBool.Null, where the Null member indicates an unknown value. Such three-valued logical types are commonly used in databases.

using System;

public struct DBBool
{
// The three possible DBBool values.

public static readonly DBBool Null = new DBBool(0);
public static readonly DBBool False = new DBBool(-1);
public static readonly DBBool True = new DBBool(1);

// Private field that stores –1, 0, 1 for False, Null, True.

sbyte value;

// Private instance constructor. The value parameter must be –1, 0, or 1.

DBBool(int value) {
this.value = (sbyte)value;
}

// Properties to examine the value of a DBBool. Return true if this
// DBBool has the given value, false otherwise.

public bool IsNull { get { return value == 0; } }

public bool IsFalse { get { return value < 0; } }

public bool IsTrue { get { return value > 0; } }

// Implicit conversion from bool to DBBool. Maps true to DBBool.True and
// false to DBBool.False.

public static implicit operator DBBool(bool x) {
return x? True: False;
}

// Explicit conversion from DBBool to bool. Throws an exception if the
// given DBBool is Null, otherwise returns true or false.

public static explicit operator bool(DBBool x) {
if (x.value == 0) throw new InvalidOperationException();
return x.value > 0;
}

// Equality operator. Returns Null if either operand is Null, otherwise
// returns True or False.

public static DBBool operator ==(DBBool x, DBBool y) {
if (x.value == 0 || y.value == 0) return Null;
return x.value == y.value? True: False;
}

// Inequality operator. Returns Null if either operand is Null, otherwise
// returns True or False.

public static DBBool operator!=(DBBool x, DBBool y) {
if (x.value == 0 || y.value == 0) return Null;
return x.value!= y.value? True: False;
}

// Logical negation operator. Returns True if the operand is False, Null
// if the operand is Null, or False if the operand is True.

public static DBBool operator!(DBBool x) {
return new DBBool(-x.value);
}

// Logical AND operator. Returns False if either operand is False,
// otherwise Null if either operand is Null, otherwise True.

public static DBBool operator &(DBBool x, DBBool y) {
return new DBBool(x.value < y.value? x.value: y.value);
}

// Logical OR operator. Returns True if either operand is True, otherwise
// Null if either operand is Null, otherwise False.

public static DBBool operator |(DBBool x, DBBool y) {
return new DBBool(x.value > y.value? x.value: y.value);
}

// Definitely true operator. Returns true if the operand is True, false
// otherwise.

public static bool operator true(DBBool x) {
return x.value > 0;
}

// Definitely false operator. Returns true if the operand is False, false
// otherwise.

public static bool operator false(DBBool x) {
return x.value < 0;
}

public override bool Equals(object obj) {
if (!(obj is DBBool)) return false;
return value == ((DBBool)obj).value;
}

public override int GetHashCode() {
return value;
}

public override string ToString() {
if (value > 0) return "DBBool.True";
if (value < 0) return "DBBool.False";
return "DBBool.Null";
}
}

Arrays

An array is a data structure that contains a number of variables which 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 which 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 which 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 be any type, including an array type.

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-separators_opt ]

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

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.

The System.Array type

The type System.Array is the abstract base type of all array types. An implicit reference conversion (§6.1.6) exists from any array type to System.Array, and an explicit reference conversion (§6.2.4) 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.