17.11 Static constructors
A static constructor is a member that implements the actions required to
initialize a class. Static constructors are
declared using static-constructor-declarations:
static-constructor-declaration:
attributesopt static-constructor-modifiers identifier ( )
static-constructor-body
static-constructor-modifiers:
externopt static
static externopt
static-constructor-body:
block
;
A static-constructor-declaration may include a set of attributes (§24) and
an extern modifier (§17.5.7).
Chapter 17 Classes
263
The identifier of a static-constructor-declaration must name the class in
which the static constructor is declared.
If any other name is specified, a compile-time error occurs.
When a static constructor declaration includes an extern modifier, the
static constructor is said to be an external
static constructor. Because an external static constructor declaration
provides no actual implementation, its
static-constructor-body consists of a semicolon. For all other static
constructor declarations, the staticconstructor-
body consists of a block, which specifies the statements to execute in
order to initialize the class. This
corresponds exactly to the method-body of a static method with a void
return type (§17.5.8).
Static constructors are not inherited, and cannot be called directly.
The static constructor for a class executes at most once in a given
application domain. The execution of a static
constructor is triggered by the first of the following events to occur
within an application domain:
? An instance of the class is created.
? Any of the static members of the class are referenced.
If a class contains the Main method (§10.1) in which execution begins, the
static constructor for that class
executes before the Main method is called. If a class contains any static
fields with initializers, those initializers
are executed in textual order immediately prior to executing the static
constructor.
[Example: The example
using System;
class Test
{
static void Main() {
A.F();
B.F();
}
}
class A
{
static A() {
Console.WriteLine("Init A");
}
public static void F() {
Console.WriteLine("A.F");
}
}
class B
{
static B() {
Console.WriteLine("Init B");
}
public static void F() {
Console.WriteLine("B.F");
}
}
must produce the output:
Init A
A.F
Init B
B.F
because the execution of A’s static constructor is triggered by the call
to A.F, and the execution of B’s static
constructor is triggered by the call to B.F. end example]
It is possible to construct circular dependencies that allow static fields
with variable initializers to be observed in
their default value state.
[Example: The example
C# LANGUAGE SPECIFICATION
264
using System;
class A
{
public static int X;
static A() { X = B.Y + 1;}
}
class B
{
public static int Y = A.X + 1;
static B() {}
static void Main() {
Console.WriteLine("X = {0}, Y = {1}", A.X, B.Y);
}
}
produces the output
X = 1, Y = 2
To execute the Main method, the system first runs the initializer for B.Y,
prior to class B’s static constructor.
Y’s initializer causes A’s static constructor to be run because the value
of A.X is referenced. The static constructor
of A in turn proceeds to compute the value of X, and in doing so fetches
the default value of Y, which is zero. A.X
is thus initialized to 1. The process of running A’s static field
initializers and static constructor then completes,
returning to the calculation of the initial value of Y, the result of which
becomes 2. end example]
