How to Create Indexer in C#: Syntax and Example

In C#, create an indexer by defining a property named this with a parameter inside a class. This lets you access objects like arrays using square brackets, e.g., obj[index]. The indexer must have at least a get accessor and optionally a set accessor.

Syntax

An indexer in C# looks like a property named this with one or more parameters inside square brackets. It has get and/or set accessors to read or write values.

• this[int index]: defines the index parameter type and name.
• get: returns the value at the given index.
• set: assigns a value at the given index.

csharp

public class Sample
{
    private int[] data = new int[10];

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

Example

This example shows a class WeekDays with an indexer that stores and retrieves day names by index. It demonstrates how to use the indexer like an array.

csharp

using System;

public class WeekDays
{
    private string[] days = new string[7];

    public string this[int index]
    {
        get
        {
            if (index < 0 || index >= days.Length)
                throw new IndexOutOfRangeException("Index out of range");
            return days[index];
        }
        set
        {
            if (index < 0 || index >= days.Length)
                throw new IndexOutOfRangeException("Index out of range");
            days[index] = value;
        }
    }
}

class Program
{
    static void Main()
    {
        WeekDays week = new WeekDays();
        week[0] = "Sunday";
        week[1] = "Monday";
        week[2] = "Tuesday";

        Console.WriteLine(week[0]);
        Console.WriteLine(week[1]);
        Console.WriteLine(week[2]);
    }
}

Output

Sunday
Monday
Tuesday

Common Pitfalls

Common mistakes when creating indexers include:

• Not defining get accessor (indexer must have at least get).
• Using invalid index types (indexer parameters must be valid types like int).
• Not handling out-of-range indexes, which causes runtime errors.
• Confusing indexers with regular properties or methods.

Always validate index values inside get and set to avoid exceptions.

csharp

public class WrongIndexer
{
    private int[] data = new int[5];

    // Missing get accessor - this will cause a compile error
    public int this[int index]
    {
        set { data[index] = value; }
    }
}

// Correct version
public class CorrectIndexer
{
    private int[] data = new int[5];

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

Quick Reference

Indexer Cheat Sheet:

• this[parameter]: defines the indexer.
• Must have at least a get accessor.
• Can have multiple parameters for complex indexing.
• Use value keyword inside set accessor.
• Validate index parameters to avoid runtime errors.

Key Takeaways

An indexer in C# lets you access class instances like arrays using square brackets.

Define an indexer with the this keyword and at least a get accessor.

Always validate index values inside the indexer to prevent runtime errors.

Indexers can have multiple parameters but usually use an integer index.

Use value inside the set accessor to assign values.