Indexer declaration in C Sharp (C#)

Introduction

An indexer lets you access objects like arrays using square brackets. It makes your class easier to use and feels natural.

You want to access elements inside a class using an index, like a list or array.

You have a collection inside your class and want to provide easy access to its items.

You want to hide the internal data structure but still allow users to get or set values by position.

You want to make your class behave like a container with simple syntax.

Syntax

C Sharp (C#)

public return_type this[type index] {
    get {
        // return value at index
    }
    set {
        // set value at index
    }
}

The keyword this is used to declare an indexer.

You can define get and set accessors to read and write values.

Examples

Simple indexer for a string array inside a class.

C Sharp (C#)

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

Indexer using a string key, like a dictionary.

C Sharp (C#)

public int this[string key] {
    get { return dictionary[key]; }
    set { dictionary[key] = value; }
}

Sample Program

This program shows how to use an indexer to get and set days of the week by index. It also handles invalid indexes gracefully.

C Sharp (C#)

using System;

class WeekDays {
    private string[] days = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

    public string this[int index] {
        get {
            if (index < 0 || index >= days.Length) {
                return "Invalid index";
            }
            return days[index];
        }
        set {
            if (index >= 0 && index < days.Length) {
                days[index] = value;
            }
        }
    }
}

class Program {
    static void Main() {
        WeekDays week = new WeekDays();
        Console.WriteLine(week[1]);  // Access using indexer
        week[1] = "Monday";         // Set using indexer
        Console.WriteLine(week[1]);
        Console.WriteLine(week[10]); // Invalid index
    }
}

Important Notes

Indexers can have multiple parameters but usually have one.

You can make indexers read-only by only providing a get accessor.

Indexers improve code readability by allowing array-like access to objects.

Summary

Indexers let you use square brackets to access class data like arrays.

Use this keyword with parameters to declare an indexer.

Define get and set to control reading and writing.

Practice

1. What is the purpose of an indexer in a C# class?

easy

A. To declare a constant value inside the class

B. To allow objects to be accessed like arrays using square brackets

C. To create a new class instance

D. To define a method that returns the class name

Solution

1. Step 1: Understand what an indexer does

   An indexer lets you access elements in an object using square brackets, similar to arrays.

2. Step 2: Compare options with this definition

   Only To allow objects to be accessed like arrays using square brackets describes this behavior correctly.

3. Final Answer:

   To allow objects to be accessed like arrays using square brackets -> Option B

4. Quick Check:

   Indexer = access with [] [OK]

Hint: Indexers let objects act like arrays with [] access [OK]

Common Mistakes:

• Confusing indexers with methods
• Thinking indexers create new objects
• Mixing indexers with constants

2. Which of the following is the correct syntax to declare an indexer in C#?

easy

A. public int this[int index] get set;

B. public int indexer(int index) { get; set; }

C. public int this[index] { get; set; }

D. public int this[int index] { get; set; }

Solution

1. Step 1: Recall indexer syntax

   An indexer uses the keyword this with a parameter list in square brackets and defines get and/or set accessors inside braces.

2. Step 2: Check each option

   public int this[int index] { get; set; } matches the correct syntax exactly. public int indexer(int index) { get; set; } uses a method-like name, which is incorrect. public int this[index] { get; set; } misses the type in the parameter. public int this[int index] get set; misses braces and semicolons.

3. Final Answer:

   public int this[int index] { get; set; } -> Option D

4. Quick Check:

   Indexer syntax = this[type param] { get; set; } [OK]

Hint: Indexer syntax always uses 'this' with parameter in brackets [OK]

Common Mistakes:

• Using method name instead of 'this'
• Omitting parameter type
• Missing braces around get/set

3. What will be the output of the following code?

class Sample {
  private int[] data = {10, 20, 30};
  public int this[int i] {
    get { return data[i]; }
    set { data[i] = value; }
  }
}

var s = new Sample();
s[1] = 50;
Console.WriteLine(s[1]);

medium

A. 50

B. 20

C. 30

D. Compilation error

Solution

1. Step 1: Understand the indexer behavior

   The indexer accesses the private array data. Setting s[1] = 50; changes data[1] from 20 to 50.

2. Step 2: Check the output of Console.WriteLine(s[1])

   This prints the updated value at index 1, which is 50.

3. Final Answer:

   50 -> Option A

4. Quick Check:

   Indexer set changes value, output = 50 [OK]

Hint: Indexer set changes array value, get returns updated value [OK]

Common Mistakes:

• Assuming original value prints
• Confusing indexer with method call
• Expecting compilation error

4. Identify the error in this indexer declaration:

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

medium

A. Parameter name cannot be 'index'

B. Indexer must return int, not string

C. Missing braces around set accessor body

D. Missing semicolon after get accessor

Solution

1. Step 1: Review accessor syntax

   Both get and set accessors must have braces enclosing their bodies.

2. Step 2: Locate the error

   The set accessor lacks braces around data[index] = value;, causing a syntax error.

3. Final Answer:

   Missing braces around set accessor body -> Option C

4. Quick Check:

   Accessor bodies need braces { } [OK]

Hint: Always use braces { } for get and set bodies [OK]

Common Mistakes:

• Omitting braces in set accessor
• Wrong return type assumption
• Thinking parameter name is restricted

5. You want to create a class that stores names and allows access by index using an indexer. Which of the following indexer declarations correctly supports both reading and writing names by index?

hard

A. public string this[int i] { get { return names[i]; } set { names[i] = value; } }

B. public string this[int i] { set { names[i] = value; } }

C. public string this[int i] { get { return names[i]; } }

D. public string this[string name] { get { return name; } set { name = value; } }

Solution

1. Step 1: Understand requirements for read/write access

   The indexer must have both get and set accessors to allow reading and writing by index.

2. Step 2: Evaluate each option

   public string this[int i] { get { return names[i]; } } only has get, so no writing. public string this[int i] { set { names[i] = value; } } only has set, so no reading. public string this[int i] { get { return names[i]; } set { names[i] = value; } } has both get and set with int index parameter, which is correct. public string this[string name] { get { return name; } set { name = value; } } uses a string parameter, which does not match the requirement to access by index.

3. Final Answer:

   public string this[int i] { get { return names[i]; } set { names[i] = value; } } -> Option A

4. Quick Check:

   Read/write indexer needs both get and set [OK]

Hint: Read/write indexers need both get and set accessors [OK]

Common Mistakes:

• Defining only get or only set
• Using wrong parameter type
• Confusing indexer parameter with property