Tài liệu Lesson 11: Indexers doc

This indexer returns a string, as indicated by the string return value in the Indexer declaration.. myInd[4]: empty myInd[5]: Any Value myInd[6]: empty myInd[7]: empty myInd[8]: empty my

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by Joe Mayo, 04/23/01

Lesson 11: Indexers

This lesson teaches C# Indexers Our objectives are as follows:

l Understand What Indexers Are For

l Implement an Indexer

l Overload Indexers

l Understand How to Implement Multi-Parameter Indexers

Indexers are real easy They allow your class to be used just like an array On the inside of a class, you manage a collection of values any way you want These objects could be a finite set of class members, another array, or some complex data structure Regardless of the internal

implementation of the class, its data can be obtained consistently through the use of indexers Here's an example

Listing 11-1 An Example of An Indexer: IntIndexer.cs

using System;

/// <summary>

/// A simple indexer example

/// </summary>

class IntIndexer {

privatestring [] myData;

public IntIndexer( int size) {

myData = new string [size];

for ( int i=0; i < size; i++) {

myData[i] = "empty";

} }

public string this [ int pos]

{

get

{

return myData[pos];

}

set

{ myData[pos] = value ; }

}

static void Main( string [] args) {

int size = 10;

IntIndexer myInd = new IntIndexer(size);

myInd[9] = "Some Value";

myInd[3] = "Another Value";

myInd[5] = "Any Value";

Console.WriteLine("\nIndexer Output\n");

for ( int i=0; i < size; i++) {

Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]);

} } }

Listing 11-1 shows how to implement an Indexer The IntIndexer class has

a string array named myData This is a private array that external users

can't see this array is initialized in the constructor, which accepts an int

size parameter, instantiates the myData array, and then fills each element

with the word "empty"

The next class member is the Indexer, which is identified by the this keyword and square brackets, this[int pos] It accepts a single position parameter, pos As you may have already guessed, the implementation of

an Indexer is the same as a Property It has get and set accessors that are used exactly like those in a Property This indexer returns a string, as indicated by the string return value in the Indexer declaration

The Main() method simply instantiates a new IntIndexer object, adds some

values, and prints the results Here's the output:

Indexer Output

myInd[0]: empty myInd[1]: empty myInd[2]: empty

myInd[4]: empty myInd[5]: Any Value myInd[6]: empty myInd[7]: empty myInd[8]: empty myInd[9]: Some Value

Using an integer is a common means of accessing arrays in many

languages, but the C# Indexer goes beyond this Indexers can be declared with multiple parameters and each parameter can be a different type Additional parameters are separated by commas, the same as a method

parameter list Valid parameter types for Indexers include integers,

enums, and strings Additionally, Indexers can be overloaded In listing

11-2, we modify the previous program to accept overloaded Indexers that accept different types

Listing 11-2 Overloaded Indexers: OvrIndexer.cs

using System;

/// <summary>

/// Implements overloaded indexers

/// </summary>

class OvrIndexer {

privatestring [] myData;

private int arrSize;

public OvrIndexer( int size) {

arrSize = size;

myData = new string [size];

for ( int i=0; i < size; i++) {

myData[i] = "empty";

} }

public string this [ int pos]

{

get

{

return myData[pos];

}

set

{ myData[pos] = value ; }

}

public string this [ string data]

{

get

{

int count = 0;

for ( int i=0; i < arrSize; i++) {

if (myData[i] == data) {

count++;

} }

return count.ToString();

}

set

{

for ( int i=0; i < arrSize; i++) {

if (myData[i] == data) {

myData[i] = value ; }

} } }

static void Main( string [] args) {

int size = 10;

OvrIndexer myInd = new OvrIndexer(size);

myInd[9] = "Some Value";

myInd[3] = "Another Value";

myInd[5] = "Any Value";

myInd["empty"] = "no value";

Console.WriteLine("\nIndexer Output\n");

for ( int i=0; i < size; i++) {

Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]);

} Console.WriteLine("\nNumber of \"no value\" entries: {0}", myInd["no value"]); }

}

Listing 11-2 shows how to overload Indexers The first Indexer, with the

integer parameter, pos, is the same as in Listing 11-1, but there is a new Indexer that takes a string parameter The get accessor of the new

indexer returns a string representation of the number of items that match

the parameter value, data The set accessor changes each entry in the array that matches the data parameter to the value that is assigned to the

Indexer

The behavior of the overloaded Indexer that takes a string parameter is demonstrated in the Main() method of Listing 11-2 It invokes the set

accessor, which assigns the value of "no value" to every member of the

myInd class that has the value of "empty" It uses the following

command: myInd["empty"] = "no value"; After each entry of the myInd class is

printed, a final entry is printed to the console, indicating the number of

entries with the "no value" string This happens by invoking the get

accessor with the following code: myInd["no value"} Here's the output:

Indexer Output

myInd[0]: no value myInd[1]: no value myInd[2]: no value myInd[3]: Another Value myInd[4]: no value myInd[5]: Any Value myInd[6]: no value myInd[7]: no value myInd[8]: no value myInd[9]: Some Value

Number of "no value" entries: 7

The reason both Indexers in Listing 11-2 can coexist in the same class is because they have different signatures An Indexer signature is specified

by the number and type of parameters in an Indexers parameter list The class will be smart enough to figure out which Indexer to invoke, based on the number and type of arguments in the Indexer call An indexer with multiple parameters would be implemented something like this:

public object this [ int param1, , int paramN]

{

get

{

// process and return some class data }

set

{ // process and assign some class data }

}

In summary, you now know what Indexers are for and how they're used You can create an Indexer to access class members similar to arrays Overloaded and multi-parameter Indexers were also covered

Your feedback is very important and I appreciate any constructive contributions you have Please feel free to contact me for any questions or comments you may have about this lesson

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