When should you use C# indexers?

Question

I\'d like to use indexers more, but I\'m not sure when to use them. All I\'ve found online are examples that use classes like MyClass and IndexerClass

Answer 1

The simple answer (as stated above) is when the class represents/contains a collection of items, the indexer will return the elements of the collection.

public Student this[int index] { ..

In a more advanced case you can create a default behavior with a class and make it look a bit like a delegate, especially when the class represents a mapping, or a process. For example a class that calculates the cooling rate of a beer in the refrigerator:

Instead of typing

temperature = coorsLight.CalculateFutureTemperature(time);

you can condence this to

temperature = coorsLight[time];

if the expected behavior (and intent) of the class is to return a value.

Answer 2

Indexers allow instances of a class or struct to be indexed just like arrays. Indexers resemble properties except that their accessors take parameters.

Indexers enable objects to be indexed in a similar manner to arrays.

    // C#: INDEXER 
using System; 
using System.Collections; 

class MyClass 
{ 
    private string []data = new string[5]; 
    public string this [int index] 
    { 
       get 
       { 
           return data[index]; 
       } 
       set 
       { 
           data[index] = value; 
       } 
    } 
}

class MyClient 
{ 
   public static void Main() 
   { 
      MyClass mc = new MyClass(); 
      mc[0] = "Rajesh"; 
      mc[1] = "A3-126"; 
      mc[2] = "Snehadara"; 
      mc[3] = "Irla"; 
      mc[4] = "Mumbai"; 
      Console.WriteLine("{0},{1},{2},{3},{4}",mc[0],mc[1],mc[2],mc[3],mc[4]); 
   } 
} 

Code project

Answer 3

Indexer is a highly specialized property which allows instances of a class (or struct) to be indexed just like an array(properties can be static but indexers cannot).

Why use indexers:
- instead of a new data structure, the class itself is a data structure.
- simplified syntax - syntactic sugar

When to use:
- if your class needs list(/array) of its instances (example 1)
- if your class represents list(/array) of values directly related to your class (example 2)

Example 1:

public class Person{
    public string Name{get; set;}

    private Person[] _backingStore;
    public Person this[int index]
    {
        get{
            return _backingStore[index];
        }
        set{
            _backingStore[index] = value;
        }
    }
}

Person p = new Person();
p[0] = new Person(){Name = "Hassan"};
p[1] = new Person(){Name = "John Skeet"};    

Example 2:

class TempratureRecord{
    private float[] temps = new float[10] { 56.2F, 56.7F, 56.5F, 56.9F, 58.8F, 61.3F, 56.5F, 56.9F, 58.8F, 61.3F};

    public int Length{
        get { return temps.Length; }
    }

    public float this[int index]
    {
        get{
            return temps[index];
        }
        set{
            temps[index] = value;
        }
    }
}

Answer 4

Random order access

You would use an enumerator if your data is normally accessed sequentially.

An indexer on the other hand is useful for directly accessing a specific element, no specific order.

This of course assumes you know the index of the element you want. Comboboxes for example have always supported two values: the string shown to the user, and the id that belongs with it. You could use the id from a selected item in a combobox to directly access the index of your collection, instead of having to search the collection.

The nice thing about indexers in C# is that you can overload them, so you can access items through different kind of keys.