Indexer declaration in C Sharp (C#) - Mini Project: Build & Apply

Indexer Declaration in C#

📖 Scenario: Imagine you are creating a simple class to store and access the names of days in a week by their number. You want to use an indexer so that you can get or set the day name using the day number like an array.

🎯 Goal: Build a class called WeekDays that uses an indexer to get and set day names by their index (0 to 6).

📋 What You'll Learn

Create a class called WeekDays with a private string array of length 7.

Add an indexer to WeekDays that takes an int index and returns or sets the day name at that index.

Use the indexer to set the names of the days from Sunday to Saturday.

Print the day name at index 3 using the indexer.

💡 Why This Matters

🌍 Real World

Indexers let you create classes that behave like arrays or lists, making your code easier to read and use.

💼 Career

Many software jobs require creating custom data structures with indexers to simplify data access and improve code clarity.

1

Create the WeekDays class with a private string array

Create a class called WeekDays with a private string array called days of length 7.

C Sharp (C#)

public class WeekDays
{
    // Your code here
}

Hint

Use private string[] days = new string[7]; inside the class.

2

Add an indexer to get and set day names by index

Inside the WeekDays class, add an indexer with public string this[int index] that gets and sets the value in the days array at index.

C Sharp (C#)

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

    // Add the indexer below
    // Your code here
}

Hint

The indexer looks like a property but uses this[int index].

3

Set the names of the days using the indexer

Create a WeekDays object called week and use the indexer to set the days: Sunday at index 0, Monday at 1, Tuesday at 2, Wednesday at 3, Thursday at 4, Friday at 5, and Saturday at 6.

C Sharp (C#)

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

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

WeekDays week = new WeekDays();
// Set the days using the indexer below
// Your code here

Hint

Use week[index] = "DayName"; to set each day.

4

Print the day name at index 3 using the indexer

Write a Console.WriteLine statement to print the day at index 3 from the week object using the indexer.

C Sharp (C#)

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

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

WeekDays week = new WeekDays();
week[0] = "Sunday";
week[1] = "Monday";
week[2] = "Tuesday";
week[3] = "Wednesday";
week[4] = "Thursday";
week[5] = "Friday";
week[6] = "Saturday";

Console.WriteLine(week[3]); // Print the day at index 3

Hint

Use Console.WriteLine(week[3]); to print the day at index 3.

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