1517.10 Instance constructors

C# LANGUAGE SPECIFICATION

1When a constructor declaration includes an extern modifier, the constructor is said to be an external

2constructor.

3Because an external constructor declaration provides no actual implementation, its constructor-body consists

4of a semicolon. For all other constructors, the constructor-body consists of a block, which specifies the

5statements to initialize a new instance of the class. This corresponds exactly to the block of an instance

6method with a void return type (§17.5.8).

7Instance constructors are not inherited. Thus, a class has no instance constructors other than those actually

8declared in the class. If a class contains no instance constructor declarations, a default instance constructor is

9automatically provided (§17.10.4).

10Instance constructors are invoked by object-creation-expressions (§14.5.10.1) and through constructor-

11initializers.

1217.10.1 Constructor initializers

13All instance constructors (except those for class object) implicitly include an invocation of another

14instance constructor immediately before the constructor-body. The constructor to implicitly invoke is

15determined by the constructor-initializer:

16• An instance constructor initializer of the form base(argument-listopt) causes an instance constructor

17from the direct base class to be invoked. That constructor is selected using argument-list and the

18overload resolution rules of §14.4.2. The set of candidate instance constructors consists of all accessible

19instance constructors declared in the direct base class, or the default constructor (§17.10.4), if no

20instance constructors are declared in the direct base class. If this set is empty, or if a single best instance

21constructor cannot be identified, a compile-time error occurs.

22• An instance constructor initializer of the form this(argument-listopt) causes an instance constructor

23from the class itself to be invoked. The constructor is selected using argument-list and the overload

24resolution rules of §14.4.2. The set of candidate instance constructors consists of all accessible instance

25constructors declared in the class itself. If that set is empty, or if a single best instance constructor cannot

26be identified, a compile-time error occurs. If an instance constructor declaration includes a constructor

27initializer that invokes the constructor itself, a compile-time error occurs.

28If an instance constructor has no constructor initializer, a constructor initializer of the form base() is

29implicitly provided. [Note: Thus, an instance constructor declaration of the form

30C(…) {…}

31is exactly equivalent to

32C(…): base() {…}

33end note]

34The scope of the parameters given by the formal-parameter-list of an instance constructor declaration

35includes the constructor initializer of that declaration. Thus, a constructor initializer is permitted to access

36the parameters of the constructor. [Example:

37class A

38{

39public A(int x, int y) {}

40}

41class B: A

42{

43public B(int x, int y): base(x + y, x - y) {}

44}

45end example]

46An instance constructor initializer cannot access the instance being created. Therefore it is a compile-time

47error to reference this in an argument expression of the constructor initializer, as it is a compile-time error

48for an argument expression to reference any instance member through a simple-name.

312

C# LANGUAGE SPECIFICATION

1class B: A

2{

11}

12}

13contains several variable initializers; it also contains constructor initializers of both forms (base and this).

14The example corresponds to the code shown below, where each comment indicates an automatically inserted

15statement (the syntax used for the automatically inserted constructor invocations isn’t valid, but merely

16serves to illustrate the mechanism).

17using System.Collections;

18class A

19{

32}

33}

34class B: A

35{

48}

49}

50end example]

5117.10.4 Default constructors

52If a class contains no instance constructor declarations, a default instance constructor is automatically

53provided. That default constructor simply invokes the parameterless constructor of the direct base class. If

54the direct base class does not have an accessible parameterless instance constructor, a compile-time error

55occurs. If the class is abstract then the declared accessibility for the default constructor is protected.

56Otherwise, the declared accessibility for the default constructor is public. [Note: Thus, the default

57constructor is always of the form

314