What are indexers in C#

An indexer allows an object to be indexed like an array. It is useful to support the creation of specialized arrays respective to one or more constraints. Indexers allows a class to be accessed the same way as an array is access means object of a class be indexed same way as an array. Modifier of an indexer can be of public, private, protected and internal. The return type can be any valid C# types . The indexing is performed using the [ ] operator.

 
Students st = new Students();
st[0] = "ID001";
st[1] = "ID002";
st[2] = "ID002";

An indexer is also similar to a property. As with properties, you use get and set when defining an indexer . Unlike properties, you are not obtaining a specific data member; rather, you are obtaining a value from the object itself.

 
class Students
{
  private string[] studentID = new string[10];
  public string this[int code]
  {
    get
    {
      return studentID[code];
    }
    set
    {
      studentID[code] = value;
    }
  }
}

Regardless of the internal implementation of the class, its data can be obtained consistently through the use of indexers. Indexers in C# must have at least one parameter. Else the compiler will generate a compilation error .

Example

 
using System;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
  class Students
  {
    private string[] studentID = new string[10];
    public string this[int code]
    {
      get
      {
        return studentID[code];
      }
      set
      {
        studentID[code] = value;
      }
    }
  }
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }
    private void button1_Click(object sender, EventArgs e)
    {
      Students st = new Students();
      st[0] = "ID001";
      st[1] = "ID002";
      st[2] = "ID002";
      for (int i = 0; i < 3; i++)
      {
        Console.WriteLine("myInd[{0}]: {1}", i, st[i]);
        MessageBox.Show(st[i]);
      }
    }
  }
}

When should you use C# indexers?

You can typically use an indexer if the class represents a list, collection or array of objects.

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