C# LANGUAGE SPECIFICATION

1class Text

2{

7}

8}

9the first two instance constructors merely provide the default values for the missing arguments. Both use a

10this(…) constructor initializer to invoke the third instance constructor, which actually does the work of

11initializing the new instance. The effect is that of optional constructor parameters:

14Text t3 = new Text(5, 20, "Hello");

15end note]

1617.11 Static constructors

17A static constructor is a member that implements the actions required to initialize a class. Static

18constructors are declared using static-constructor-declarations:

19static-constructor-declaration:

23static externopt

24static-constructor-body:

26;

27A static-constructor-declaration can include a set of attributes (§24) and an extern modifier (§17.5.7).

28The identifier of a static-constructor-declaration shall name the class in which the static constructor is

29declared. If any other name is specified, a compile-time error occurs.

30When a static constructor declaration includes an extern modifier, the static constructor is said to be an

31external static constructor. Because an external static constructor declaration provides no actual

32implementation, its static-constructor-body consists of a semicolon. For all other static constructor

33declarations, the static-constructor-body consists of a block, which specifies the statements to execute in

34order to initialize the class. This corresponds exactly to the method-body of a static method with a void

35return type (§17.5.8).

36Static constructors are not inherited, and cannot be called directly.

37The static constructor for a non-generic class executes at most once in a given application domain. The static

38constructor for a generic class declaration executes at most once for each closed constructed type

39constructed from the class declaration (§26.1.5). The execution of a static constructor is triggered by the first

40of the following events to occur within an application domain:

41• An instance of the class is created.

42• Any of the static members of the class are referenced.

43If a class contains the Main method (§10.1) in which execution begins, the static constructor for that class

44executes before the Main method is called. If a class contains any static fields with initializers, those

45initializers are executed in textual order immediately prior to executing the static constructor (§17.4.5).

46[Example: The example

47using System;

316

Chapter 17 Classes

1class Test

2{

6}

7}

8class A

9{

15}

16}

17class B

18{

24}

25}

26shall produce the output:

27Init A

28A.F

29Init B

30B.F

31because the execution of A's static constructor is triggered by the call to A.F, and the execution of B's static

32constructor is triggered by the call to B.F. end example]

33It is possible to construct circular dependencies that allow static fields with variable initializers to be

34observed in their default value state.

35[Example: The example

36using System;

37class A

38{

40static A() { X = B.Y + 1;}

41}

42class B

43{

48}

49}

50produces the output

51X = 1, Y = 2

52To execute the Main method, the system first runs the initializer for B.Y, prior to class B's static constructor.

53Y's initializer causes A's static constructor to be run because the value of A.X is referenced. The static

54constructor of A in turn proceeds to compute the value of X, and in doing so fetches the default value of Y,

55which is zero. A.X is thus initialized to 1. The process of running A's static field initializers and static

56constructor then completes, returning to the calculation of the initial value of Y, the result of which

57becomes 2. end example]

317

Chapter 17 Classes

1Destructors can be implemented by overriding the virtual method Finalize on System.Object. In any

2event, C# programs are not permitted to override this method or call it (or overrides of it) directly.

3[Example: For instance, the program

4class A

5{

9}

10}

11contains two errors. end example]

12The compiler behaves as if System.Object.Finalize, and overrides of it, does not exist at all.

13[Example: Thus, this program:

14class A

15{

17}

18is valid and the method shown hides System.Object's Finalize method, without requiring the new

19modifier. end example]

20For a discussion of the behavior when an exception is thrown from a destructor, see §23.3.

319