C# LANGUAGE SPECIFICATION

118.1.2 Struct interfaces

2A struct declaration can include a struct-interfaces specification, in which case the struct is said to

3implement the given interface types.

4struct-interfaces:

5: interface-type-list

6The handling of interfaces on multiple parts of a partial struct declaration (§17.1.4) are discussed further

7in §17.1.2.2.

8Interface implementations are discussed further in §20.4.

918.1.3 Struct body

10The struct-body of a struct defines the members of the struct.

11struct-body:

12{ struct-member-declarationsopt }

1318.2 Struct members

14The members of a struct consist of the members introduced by its struct-member-declarations and the

15members inherited from the type System.ValueType.

16struct-member-declarations:

30All kinds of class-member-declarations except destructor-declaration are also struct-member-declarations.

31Except for the differences noted in §18.3, the descriptions of class members provided in §17.1.4 through

32§17.11 apply to struct members as well.

3318.3 Class and struct differences

3418.3.1 Value semantics

35Structs are value types (§11.1) and are said to have value semantics. Classes, on the other hand, are reference

36types (§11.2) and are said to have reference semantics.

37A variable of a struct type directly contains the data of the struct, whereas a variable of a class type contains

38a reference to the data, the latter known as an object.

39With classes, it is possible for two variables to reference the same object, and thus possible for operations on

40one variable to affect the object referenced by the other variable. With structs, the variables each have their

41own copy of the data, and it is not possible for operations on one to affect the other. Furthermore, because

42structs are not reference types, it is not possible for values of a struct type to be null.

43[Example: Given the declaration

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Chapter 18 Structs

1struct Point

2{

7}

8}

9the code fragment

10Point a = new Point(10, 10);

11Point b = a;

12a.x = 100;

13System.Console.WriteLine(b.x);

14outputs the value 10. The assignment of a to b creates a copy of the value, and b is thus unaffected by the

15assignment to a.x. Had Point instead been declared as a class, the output would be 100 because a and b

16would reference the same object. end example]

1718.3.2 Inheritance

18All struct types implicitly inherit from System.ValueType, which, in turn, inherits from class object. A

19struct declaration can specify a list of implemented interfaces, but it is not possible for a struct declaration to

20specify a base class.

21Struct types are never abstract and are always implicitly sealed. The abstract and sealed modifiers are

22therefore not permitted in a struct declaration.

23Since inheritance isn’t supported for structs, the declared accessibility of a struct member cannot be

24protected or protected internal.

25Function members in a struct cannot be abstract or virtual, and the override modifier is allowed

26only to override methods inherited from the type System.ValueType.

2718.3.3 Assignment

28Assignment to a variable of a struct type creates a copy of the value being assigned. This differs from

29assignment to a variable of a class type, which copies the reference but not the object identified by the

30reference.

31Similar to an assignment, when a struct is passed as a value parameter or returned as the result of a function

32member, a copy of the struct is created. A struct can be passed by reference to a function member using a

33ref or out parameter.

34When a property or indexer of a struct is the target of an assignment, the instance expression associated with

35the property or indexer access shall be classified as a variable. If the instance expression is classified as a

36value, a compile-time error occurs. This is described in further detail in §14.13.1.

3718.3.4 Default values

38As described in §12.2, several kinds of variables are automatically initialized to their default value when

39they are created. For variables of class types and other reference types, this default value is null. However,

40since structs are value types that cannot be null, the default value of a struct is the value produced by

41setting all value type fields to their default value and all reference type fields to null.

42[Example: Referring to the Point struct declared above, the example

43Point[] a = new Point[100];

44initializes each Point in the array to the value produced by setting the x and y fields to zero. end example]

45The default value of a struct corresponds to the value returned by the default constructor of the struct

46(§11.1.1). Unlike a class, a struct is not permitted to declare a parameterless instance constructor. Instead,

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Chapter 18 Structs

1This restriction applies only to instance fields. Static fields of a struct are permitted to include variable

2initializers.

318.3.8 Constructors

4Unlike a class, a struct is not permitted to declare a parameterless instance constructor. Instead, every struct

5implicitly has a parameterless instance constructor, which always returns the value that results from setting

6all value type fields to their default value and all reference type fields to null (§11.1.1). A struct can declare

7instance constructors having parameters. [Example:

8struct Point

9{

14}

15}

16Given the above declaration, the statements

17Point p1 = new Point();

18Point p2 = new Point(0, 0);

19both create a Point with x and y initialized to zero. end example]

20A struct instance constructor is not permitted to include a constructor initializer of the form base(…).

21The this variable of a struct instance constructor corresponds to an out parameter of the struct type, and

22similar to an out parameter, this shall be definitely assigned (§12.3) at every location where the

23constructor returns. [Example: Consider the instance constructor implementation below:

24struct Point

25{

36}

37}

38No instance member function (including the set accessors for the properties X and Y) can be called until all

39fields of the struct being constructed have been definitely assigned. Note, however, that if Point were a

40class instead of a struct, the instance constructor implementation would be permitted.

41end example]

4218.3.9 Destructors

43A struct is not permitted to declare a destructor.

4418.3.10 Static constructors

45Static constructors for structs follow most of the same rules as for classes. The execution of a static

46constructor for a struct is triggered by the first of the following events to occur within an application

47domain:

48• An instance member of the struct is referenced.

325