INDEXERS (INDEXED PROPERTY)
- It is a new concept in .NET and was introduced in c# 2.0 (.NET framework 2.0)
- It is a special type of property which allows an object of class or structure to be indexed like arrays. (An object (instance) act as a virtual array)
- It is also called as smart array or virtual array
- It is possible to create regular arrays like 1D, 2D, etc, using indexer
- It supports only instance fields. It does not support static fields
- Like properties, it is implemented using get and set accessors for the [ ] operator
- Indexer modifier can be: (public / private / protected / internal)
- It is identified using its signatures. It can be overloaded.
Creation
- Indexer is created using the special keyword this (which represents the instance of the current class) and [ ] operator.
Calling (Accessing)
- Indexers are accessed by using index number (indexes)
Types
- Like arrays, it supports two types. They are
1. One Dimensional Indexer
2. Multi-Dimensional Indexer.
SYNTAX FOR 1D INDEXER (1D ARRAY)
Where,
- modifier can be private / protected / public / internal
- type can be any valid type in c# (int, float, char, etc,.)
- this represents the object of the current class
- argument list is formal argument list. Indexer definition should include at least one parameter else the compiler will provide the error message.
NOTE
- The elements of the array are filled through the indexer which is done using the special built-in property value.
- This property should be included in the set accessor of indexer.
EXAMPLE
Here, arr is an user defined array name.
DIFFERENCE BETWEEN ARRAY AND INDEXER
| S.N | Array | Indexer |
| 1. | It is created using new modifier | It is created using the reserved word this |
| 2. | It is an regular array | It is an virtual array (object act as an array) which is a special property of the class |
| 3. | Here elements are inserted and accessed using index position | Here elements are inserted using set {} accessor with the special property value and retrieved using get{} accessor |
| 4. | It supports both instance and static array | It supports only instance indexer |
I. EXAMPLE OF 1D INDEXER
1. SOURCE CODE
using y = System.Console;
namespace TestApp5
{
class Program
{
// instance array variable
int[] num = new int[100];
// 1D indexer definition
public int this[int i]
{
set
{
num[i] = value;
}
get
{
return num[i];
}
}
static void Main()
{
y.WriteLine(“————————–“);
y.WriteLine(“\t 1D Indexer Indexer”);
y.WriteLine(“————————–“);
// object creation for current class
Program obj = new Program();
// calling indexer: setting values
obj[0] = 33;
obj[1] = 21;
obj[2] = 54;
obj[3] = 91;
obj[4] = 72;
// calling indexer: getting values
for(int i=0;i<obj.num.Length;i++)
{
if(obj[i]!=0)
y.WriteLine(” “+obj[i]);
}
}
}
}
2. OUTPUT
II. EXAMPLE OF 1D STRING INDEXER
AIM
- Aim of this code is used to create a 1D regular array using 1D indexer
1. SOURCE CODE
using k = System.Console;
using System;
class City
{
string[] names=new string[5]; // normal 1D array
public string this[int i]
{
// set the value at the specified index
set
{
names[i] = value;
}
// return a value at the specified index
get
{
return names[i];
}
}
static void Main()
{
k.ForegroundColor = ConsoleColor.Yellow;
k.WriteLine(“——————————-“);
k.WriteLine(“\t1D Indexer in C#”);
k.WriteLine(“——————————-“);
k.ForegroundColor = ConsoleColor.White;
// Object creation for current class
City obj = new City();
// Calling Indexer: set accessor
obj[0] = “Chennai”;
obj[1] = “Delhi”;
obj[2] = “Bangalore”;
obj[3] = “Kolkatta”;
obj[4] = “Mumbai”;
k.WriteLine(“City Names:”);
for (int i = 0; i < obj.names.Length; i++)
{
k.WriteLine(” “+(i+1)+”. “+obj[i]+” “);
}
k.WriteLine(“——————————-“);
}
}
2. OUTPUT
SYNTAX FOR 2D INDEXER (2D ARRAY)
NOTE
- The elements of the array are filled through the indexer which is done using the special built-in property value.
- This property should be included in the set accessor of indexer.
EXAMPLE
III. EXAMPLE OF 2D ARRAY CREATION USING 2D-INDEXER
AIM
- Aim of this code used to create a 2D array using 2D indexer
1. SOURCE CODE
using k=System.Console;
using System;
class TwoIndexer
{
int[,] mat=new int[3,4];
// matrix 1
public int this[int i,int j]
{
get
{
return mat[i, j];
}
set
{
mat[i, j] = value;
}
}
static void Main()
{
k.ForegroundColor = ConsoleColor.Yellow;
k.WriteLine(“——————————“);
k.WriteLine(“\t2D Indexer”);
k.WriteLine(“——————————“);
k.ForegroundColor = ConsoleColor.White;
TwoIndexer obj = new TwoIndexer();
// calling the set Indexer
// 1st row
obj[0, 0] = 5;
obj[0, 1] = 15;
obj[0, 2] = 25;
obj[0, 3] = 35;
// 2nd row
obj[1, 0] = 45;
obj[1, 1] = 55;
obj[1, 2] = 65;
obj[1, 3] = 75;
// 3rd row
obj[2, 0] = 10;
obj[2, 1] = 20;
obj[2, 2] = 30;
obj[2, 3] = 40;
k.WriteLine(“Contents of 2D Array\n”);
// calling get indexer
for (int i = 0; i < obj.mat.GetLength(0); i++)
{
k.Write(“Row #”+(i+1)+”: “);
for (int j = 0; j < obj.mat.GetLength(1); j++)
{
k.Write(obj[i, j]+” “);
}
k.WriteLine();
}
k.WriteLine(“——————————“);
}
}
2. OUTPUT
POINTS ABOUT INDEXERS
- Indexer is created using the special keyword this
- The parameterized property is called as indexer
- It is identified using its signatures and implemented using special methods like get() and set() for the [ ] operator
- It is called by using object name with index position
- It is an important to note that, it supports only instance fields (no static fields)
- In the argument list of indexer definition, at least one parameter should be required otherwise the compiler will provide an error message.
- The parameters like ref and out are not permitted in indexer definition.
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