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8.3 Variables and parameters(cont')

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

pre: x = 1, y = 2

post: x = 2, y = 1

The ref keyword must be used in both the declaration of the formal parameter

and in uses of it. The use of

ref at the call site calls special attention to the parameter, so that a

developer reading the code will

understand that the value of the argument could change as a result of the

call.

An output parameter is similar to a reference parameter, except that the

initial value of the caller-provided

argument is unimportant. An output parameter is declared with an out

modifier. The example

using System;

class Test

{

static void Divide(int a, int b, out int result, out int remainder) {

result = a / b;

remainder = a % b;

}

static void Main() {

for (int i = 1; i < 10; i++)

for (int j = 1; j < 10; j++) {

int ans, r;

Divide(i, j, out ans, out r);

Console.WriteLine("{0} / {1} = {2}r{3}", i, j, ans, r);

}

}

}

shows a Divide method that includes two output parameters.one for the

result of the division and another

for the remainder.

For value, reference, and output parameters, there is a one-to-one

correspondence between caller-provided

arguments and the parameters used to represent them. A parameter array

enables a many-to-one

relationship: many arguments can be represented by a single parameter

array. In other words, parameter

arrays enable variable length argument lists.

A parameter array is declared with a params modifier. There can be only one

parameter array for a given

method, and it must always be the last parameter specified. The type of a

parameter array is always a single

dimensional array type. A caller can either pass a single argument of this

array type, or any number of

arguments of the element type of this array type. For instance, the example

using System;

class Test

{

static void F(params int[] args) {

Console.WriteLine("# of arguments: {0}", args.Length);

for (int i = 0; i < args.Length; i++)

Console.WriteLine("\targs[{0}] = {1}", i, args[i]);

}

static void Main() {

F();

F(1);

F(1, 2);

F(1, 2, 3);

F(new int[] {1, 2, 3, 4});

}

}

shows a method F that takes a variable number of int arguments, and several

invocations of this method.

The output is:

# of arguments:

0

# of arguments:

1

args[0] =

1

# of arguments:

2

args[0] =

1

args[1] =

2

# of arguments:

3

args[0] =

1

args[1] =

2

args[2] =

3

# of arguments:

4

args[0] =

1

args[1] =

2

args[2] =

3

args[3] =

4

Most of the examples presented in this introduction use the WriteLine

method

of the Console class.

The

argument substitution behavior of this method, as exhibited in the

example

int a = 1, b =

2;

Console.WriteLine("a = {0}, b = {1}", a,

b);

is accomplished using a parameter array. The WriteLine method

provides

several overloaded methods

for

the common cases in which a small number of arguments are passed, and

one

method that uses a

parameter

array.

namespace

System

{

public class

Console

{

public static void WriteLine(string s)

{.}

public static void WriteLine(string s, object a)

{.}

public static void WriteLine(string s, object a, object b)

{.}

.

public static void WriteLine(string s, params object[] args)

{.}

}

}

 
 
 
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