Readonly structs in C Sharp (C#) - Mini Project: Build & Apply

Working with Readonly Structs in C#

📖 Scenario: You are building a simple program to represent a point in 2D space. The point's coordinates should never change once created, to avoid accidental changes in your program.

🎯 Goal: Create a readonly struct called Point with two properties X and Y. Initialize these properties through a constructor. Then, create an instance of Point and print its coordinates.

📋 What You'll Learn

Create a readonly struct named Point

Add two readonly properties: X and Y of type int

Create a constructor that sets X and Y

Create an instance of Point with X = 3 and Y = 4

Print the coordinates in the format: (X, Y)

💡 Why This Matters

🌍 Real World

Readonly structs are useful when you want to create small data containers that should not change after creation, like points, colors, or measurements.

💼 Career

Understanding readonly structs helps you write safer and more efficient C# code, which is valuable in software development jobs involving performance and data integrity.

1

Create the readonly struct with properties

Create a readonly struct named Point with two readonly properties: int X and int Y.

C Sharp (C#)

# Your code here to create readonly struct Point with int properties X and Y

Hint

Use readonly struct keyword and create properties with only getters.

2

Add a constructor to initialize properties

Add a constructor to the Point struct that takes two int parameters named x and y, and sets the properties X and Y.

C Sharp (C#)

public readonly struct Point
{
    public int X { get; }
    public int Y { get; }

    // Add constructor here
    # Your code here
}

Hint

Constructor parameters should be int x and int y. Assign them to properties inside the constructor.

3

Create an instance of Point

Create a variable named point of type Point and initialize it with X = 3 and Y = 4 using the constructor.

C Sharp (C#)

public readonly struct Point
{
    public int X { get; }
    public int Y { get; }

    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }
}

// Create an instance of Point
# Your code here

Hint

Use the constructor with 3 and 4 as arguments.

4

Print the coordinates of the Point

Write a Console.WriteLine statement to print the coordinates of point in the format (X, Y) using string interpolation.

C Sharp (C#)

public readonly struct Point
{
    public int X { get; }
    public int Y { get; }

    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }
}

Point point = new Point(3, 4);

// Print the point coordinates
# Your code here

Hint

Use Console.WriteLine($"({point.X}, {point.Y})") to print the coordinates.

Practice

1.

What is the main purpose of declaring a struct as readonly in C#?

easy

A. To allow inheritance from the struct

B. To make the struct mutable

C. To enable dynamic resizing of the struct

D. To prevent modification of its fields after creation

Solution

1. Step 1: Understand what readonly means for structs

   A readonly struct means its fields cannot be changed after the struct is created.

2. Step 2: Compare options with this meaning

   Only To prevent modification of its fields after creation matches this behavior by preventing modification after creation.

3. Final Answer:

   To prevent modification of its fields after creation -> Option D

4. Quick Check:

   Readonly struct = Immutable fields [OK]

Hint: Readonly structs make data unchangeable after creation [OK]

Common Mistakes:

• Thinking readonly allows inheritance
• Confusing readonly with mutable structs
• Assuming readonly structs can resize dynamically

2.

Which of the following is the correct way to declare a readonly struct in C#?

?

easy

A. readonly struct Point { public int X; public int Y; }

B. struct readonly Point { public int X; public int Y; }

C. struct Point readonly { public int X; public int Y; }

D. readonly Point struct { public int X; public int Y; }

Solution

1. Step 1: Recall correct syntax for readonly structs

   The keyword readonly comes before struct in the declaration.

2. Step 2: Check each option's keyword order

   Only readonly struct Point { public int X; public int Y; } places readonly before struct correctly.

3. Final Answer:

   readonly struct Point { public int X; public int Y; } -> Option A

4. Quick Check:

   Keyword order: readonly struct [OK]

Hint: Put 'readonly' before 'struct' keyword [OK]

Common Mistakes:

• Placing readonly after struct
• Mixing order of keywords
• Using readonly as a suffix

3.

What will be the output of the following C# code?

readonly struct Point
{
    public int X { get; }
    public int Y { get; }
    public Point(int x, int y) { X = x; Y = y; }
}

var p = new Point(3, 4);
Console.WriteLine($"X: {p.X}, Y: {p.Y}");

medium

A. X: 3, Y: 4

B. X: 0, Y: 0

C. Compilation error due to readonly struct

D. Runtime error when accessing properties

Solution

1. Step 1: Understand readonly struct with get-only properties

   The struct has get-only properties set in the constructor, which is allowed.

2. Step 2: Check the constructor and output

   The constructor sets X=3 and Y=4, so printing them outputs "X: 3, Y: 4".

3. Final Answer:

   X: 3, Y: 4 -> Option A

4. Quick Check:

   Constructor sets values correctly [OK]

Hint: Readonly structs allow get-only properties set in constructor [OK]

Common Mistakes:

• Assuming readonly structs cannot have properties
• Thinking constructor cannot set properties
• Expecting runtime errors accessing properties

4.

Identify the error in this code snippet using a readonly struct:

readonly struct Rectangle
{
    public int Width;
    public int Height;

    public void Resize(int w, int h)
    {
        Width = w;
        Height = h;
    }
}

medium

A. Fields must be properties in readonly structs

B. Missing constructor for readonly struct

C. Cannot assign to fields inside a method of a readonly struct

D. Readonly structs cannot have methods

Solution

1. Step 1: Understand mutability rules in readonly structs

   Readonly structs cannot modify their fields inside instance methods.

2. Step 2: Analyze the Resize method

   Resize tries to assign new values to Width and Height fields, which is not allowed.

3. Final Answer:

   Cannot assign to fields inside a method of a readonly struct -> Option C

4. Quick Check:

   Readonly struct fields are immutable in methods [OK]

Hint: Readonly struct methods cannot change fields [OK]

Common Mistakes:

• Thinking methods can modify fields in readonly structs
• Assuming missing constructor causes error
• Believing readonly structs cannot have any methods

5.

You want to create a readonly struct Circle that stores a radius and calculates area. Which of these implementations is correct and efficient?

?

hard

A. readonly struct Circle { public double Radius; public void SetRadius(double r) { Radius = r; } }

B. readonly struct Circle { public double Radius { get; } public Circle(double r) => Radius = r; public double Area => Math.PI * Radius * Radius; }

C. struct Circle { public double Radius; public double Area => Math.PI * Radius * Radius; }

D. readonly struct Circle { public double Radius { get; set; } public double Area() => Math.PI * Radius * Radius; }

Solution

1. Step 1: Check readonly struct with immutable radius

   readonly struct Circle { public double Radius { get; } public Circle(double r) => Radius = r; public double Area => Math.PI * Radius * Radius; } uses a readonly struct with a get-only property set in constructor, which is correct.

2. Step 2: Verify area calculation and mutability

   Area is a property calculated from Radius, no setters modify Radius, ensuring immutability and efficiency.

3. Final Answer:

   readonly struct Circle { public double Radius { get; } public Circle(double r) => Radius = r; public double Area => Math.PI * Radius * Radius; } -> Option B

4. Quick Check:

   Immutable radius with calculated area [OK]

Hint: Use get-only property and constructor for readonly struct fields [OK]

Common Mistakes:

• Trying to set fields in readonly struct after creation
• Using mutable fields in readonly structs
• Adding setters that break immutability