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8.3 Variables and parameters

王朝other·作者佚名  2006-01-10
窄屏简体版  字體: |||超大  

8.3 Variables and parameters

Variables represent storage locations. Every variable has a type that

determines what values can be stored in

the variable. Local variables are variables that are declared in

methods,

properties, or indexers. A local

variable is defined by specifying a type name and a declarator that

specifies the variable name and an

optional initial value, as in:

int a;

int b = 1;

but it is also possible for a local variable declaration to include

multiple declarators. The declarations of a

and b can be rewritten as:

int a, b = 1;

A variable must be assigned before its value can be obtained. The

example

class Test

{

static void Main() {

int a;

int b = 1;

int c = a + b; // error, a not yet assigned

.

}

}

results in a compile-time error because it attempts to use the variable

a

before it is assigned a value. The

rules governing definite assignment are defined in §12.3.

A field (§17.4) is a variable that is associated with a class or

struct,

or an instance of a class or struct. A field

declared with the static modifier defines a static variable, and a

field

declared without this modifier

defines an instance variable. A static field is associated with a type,

whereas an instance variable is

associated with an instance. The example

using Personnel.Data;

class Employee { private static DataSet ds; public string Name;

public decimal Salary;

.

}

shows an Employee class that has a private static variable and two public

instance variables.

Formal parameter declarations also define variables. There are four kinds

of parameters: value parameters,

reference parameters, output parameters, and parameter arrays.

A value parameter is used for .in. parameter passing, in which the value of

an argument is passed into a

method, and modifications of the parameter do not impact the original

argument. A value parameter refers to

its own variable, one that is distinct from the corresponding argument.

This variable is initialized by copying

the value of the corresponding argument. The example

using System;

class Test

{

static void F(int p) {

Console.WriteLine("p = {0}", p);

p++;

}

static void Main() {

int a = 1;

Console.WriteLine("pre: a = {0}", a);

F(a);

Console.WriteLine("post: a = {0}", a);

}

}

shows a method F that has a value parameter named p. The example produces

the output:

pre: a = 1

p = 1

post: a = 1

even though the value parameter p is modified.

A reference parameter is used for .by reference. parameter passing, in

which the parameter acts as an alias

for a caller-provided argument. A reference parameter does not itself

define a variable, but rather refers to

the variable of the corresponding argument. Modifications of a reference

parameter impact the

corresponding argument. A reference parameter is declared with a ref

modifier. The example

using System;

class Test

{

static void Swap(ref int a, ref int b) {

int t = a;

a = b;

b = t;

}

static void Main() {

int x = 1;

int y = 2;

Console.WriteLine("pre: x = {0}, y = {1}", x, y);

Swap(ref x, ref y);

Console.WriteLine("post: x = {0}, y = {1}", x, y);

}

}

shows a Swap method that has two reference parameters. The output produced

is:

 
 
 
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