Multi-parameter indexers in C Sharp (C#) - Mini Project: Build & Apply

Using Multi-parameter Indexers in C#

📖 Scenario: Imagine you manage a small hotel with rooms on different floors. You want to keep track of which guest is staying in each room using a simple system.

🎯 Goal: You will create a class with a multi-parameter indexer that lets you get and set guest names by specifying the floor and room number.

📋 What You'll Learn

Create a class called Hotel that stores guest names.

Use a multi-parameter indexer with two int parameters: floor and room.

The indexer should allow getting and setting guest names by floor and room.

Demonstrate setting and getting guest names using the indexer.

💡 Why This Matters

🌍 Real World

Multi-parameter indexers are useful when you want to access data stored in a grid or table-like structure, such as seats in a theater, cells in a spreadsheet, or rooms in a hotel.

💼 Career

Understanding multi-parameter indexers helps you write cleaner and more intuitive code when working with complex data structures, a common task in software development jobs.

1

Create the Hotel class with a 2D array

Create a class called Hotel with a private 2D string array called guests sized 3 floors by 5 rooms each. Initialize the array in the constructor.

C Sharp (C#)

public class Hotel
{
    // Your code here
}

Hint

Use a 2D array to store guest names by floor and room.

2

Add a multi-parameter indexer to the Hotel class

Inside the Hotel class, add a public indexer with two int parameters named floor and room. The indexer should get and set values in the guests array.

C Sharp (C#)

public class Hotel
{
    private string[,] guests;

    public Hotel()
    {
        guests = new string[3, 5];
    }

    // Add multi-parameter indexer here
}

Hint

Use this[int floor, int room] to define the indexer with get and set accessors.

3

Create a Hotel object and assign guest names

In the Main method, create a Hotel object called myHotel. Use the indexer to assign "Alice" to floor 0, room 1 and "Bob" to floor 2, room 4.

C Sharp (C#)

public class Hotel
{
    private string[,] guests;

    public Hotel()
    {
        guests = new string[3, 5];
    }

    public string this[int floor, int room]
    {
        get { return guests[floor, room]; }
        set { guests[floor, room] = value; }
    }
}

class Program
{
    static void Main()
    {
        // Create Hotel object and assign guests
        // Your code here
    }
}

Hint

Create the object and use the indexer with two numbers inside square brackets to assign names.

4

Print guest names using the multi-parameter indexer

In the Main method, add two Console.WriteLine statements to print the guests at floor 0, room 1 and floor 2, room 4 using the indexer on myHotel.

C Sharp (C#)

public class Hotel
{
    private string[,] guests;

    public Hotel()
    {
        guests = new string[3, 5];
    }

    public string this[int floor, int room]
    {
        get { return guests[floor, room]; }
        set { guests[floor, room] = value; }
    }
}

class Program
{
    static void Main()
    {
        Hotel myHotel = new Hotel();
        myHotel[0, 1] = "Alice";
        myHotel[2, 4] = "Bob";
        // Print guest names
        // Your code here
    }
}

Hint

Use Console.WriteLine(myHotel[0, 1]) and Console.WriteLine(myHotel[2, 4]) to print the guest names.

Practice

1. What is the main purpose of a multi-parameter indexer in C#?

easy

A. To allow accessing elements using multiple keys like a table or matrix.

B. To create a method that returns multiple values.

C. To define multiple constructors in a class.

D. To overload operators for custom classes.

Solution

1. Step 1: Understand what an indexer does

   An indexer lets you access elements in a class like an array using keys.

2. Step 2: Identify the role of multiple parameters

   Multi-parameter indexers allow using more than one key, like row and column in a table.

3. Final Answer:

   To allow accessing elements using multiple keys like a table or matrix. -> Option A

4. Quick Check:

   Multi-parameter indexer = multiple keys access [OK]

Hint: Think of multi-parameter indexers as multi-key access [OK]

Common Mistakes:

• Confusing indexers with methods returning multiple values
• Thinking indexers define constructors
• Mixing indexers with operator overloading

2. Which of the following is the correct syntax to declare a multi-parameter indexer in C#?

easy

A. public int this[int x; int y] { get { return 0; } set { } }

B. public int this(int x, int y) { get { return 0; } set { } }

C. public int this[int x][int y] { get { return 0; } set { } }

D. public int this[int x, int y] { get { return 0; } set { } }

Solution

1. Step 1: Recall indexer syntax

   Indexers use the keyword this followed by square brackets with parameters separated by commas.

2. Step 2: Check each option

   public int this[int x, int y] { get { return 0; } set { } } uses square brackets with comma-separated parameters, which is correct syntax.

3. Final Answer:

   public int this[int x, int y] { get { return 0; } set { } } -> Option D

4. Quick Check:

   Indexer syntax uses square brackets with commas [OK]

Hint: Use square brackets with commas for multiple parameters [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Separating parameters with semicolons
• Trying to nest square brackets for parameters

3. What will be the output of this code snippet?

class Matrix {
  private int[,] data = new int[2,2] {{1,2},{3,4}};
  public int this[int x, int y] {
    get { return data[x, y]; }
    set { data[x, y] = value; }
  }
}

var m = new Matrix();
Console.WriteLine(m[1, 0]);

medium

A. 4

B. 2

C. 3

D. 1

Solution

1. Step 1: Understand the data array

   The 2D array data is {{1,2},{3,4}}. The first index is row, second is column.

2. Step 2: Access element at m[1, 0]

   Row 1, column 0 is the second row, first column, which is 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   m[1,0] = 3 [OK]

Hint: Remember: first index = row, second = column [OK]

Common Mistakes:

• Mixing row and column indices
• Confusing array initialization syntax
• Assuming zero-based indexing is off

4. Identify the error in this multi-parameter indexer declaration:

public class Table {
  private string[,] data = new string[3,3];
  public string this[int x; int y] {
    get { return data[x, y]; }
    set { data[x, y] = value; }
  }
}

medium

A. Missing return type for the indexer.

B. Using semicolon instead of comma between parameters in indexer.

C. Data array should be one-dimensional for indexers.

D. Indexer must be static.

Solution

1. Step 1: Check parameter separator in indexer

   Parameters in indexers must be separated by commas, not semicolons.

2. Step 2: Verify other parts

   Return type is present, data array can be multi-dimensional, indexers are instance members by default.

3. Final Answer:

   Using semicolon instead of comma between parameters in indexer. -> Option B

4. Quick Check:

   Parameters separated by commas, not semicolons [OK]

Hint: Separate indexer parameters with commas, not semicolons [OK]

Common Mistakes:

• Using semicolons instead of commas
• Thinking indexers must be static
• Confusing array dimensions with indexer parameters

5. You want to create a class that stores grades for students in different subjects using a multi-parameter indexer. Which implementation correctly allows setting and getting grades by student name and subject?

hard

A. public class Grades {
private Dictionary<(string, string), int> data = new();
public int this[string student, string subject] {
get => data[(student, subject)];
set => data[(student, subject)] = value;
}
}

B. public class Grades {
private Dictionary data = new();
public int this[string student, string subject] {
get => data[student + subject];
set => data[student + subject] = value;
}
}

C. public class Grades {
private Dictionary data = new();
public int this[string student] {
get => data[student];
set => data[student] = value;
}
}

D. public class Grades {
private Dictionary<(string, string), int> data = new();
public int this[string student; string subject] {
get => data[(student, subject)];
set => data[(student, subject)] = value;
}
}

Solution

1. Step 1: Understand the data structure needed

   We need a dictionary with a tuple key (student, subject) to store grades.

2. Step 2: Check indexer syntax and dictionary usage

   public class Grades { private Dictionary<(string, string), int> data = new(); public int this[string student, string subject] { get => data[(student, subject)]; set => data[(student, subject)] = value; } } uses a tuple key dictionary and correct indexer syntax with comma-separated parameters.

3. Step 3: Identify errors in other options

   public class Grades { private Dictionary data = new(); public int this[string student, string subject] { get => data[student + subject]; set => data[student + subject] = value; } } concatenates keys as string which can cause collisions; C uses only one parameter; D uses semicolon instead of comma in indexer.

4. Final Answer:

   public class Grades { private Dictionary<(string, string), int> data = new(); public int this[string student, string subject] { get => data[(student, subject)]; set => data[(student, subject)] = value; } } -> Option A

5. Quick Check:

   Tuple keys + comma-separated indexer = correct [OK]

Hint: Use tuple keys and commas in indexer parameters [OK]

Common Mistakes:

• Concatenating keys as strings causing collisions
• Using semicolons instead of commas in indexer
• Using single-parameter indexer for multi-key data