Readonly structs in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a readonly struct named Point.

C Sharp (C#)

public [1] struct Point { public int X; public int Y; }

Drag options to blanks, or click blank then click option'

Areadonly

Bstatic

Csealed

Dabstract

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' or 'sealed' instead of 'readonly'.

The readonly keyword makes the struct immutable after creation.

2fill in blank

medium

Complete the code to create a readonly property in the readonly struct.

C Sharp (C#)

public readonly struct Rectangle { public int Width { [1]; } }

Drag options to blanks, or click blank then click option'

Aset

Binit

Cget

Dprivate set

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a 'set' accessor to a readonly property.

Readonly properties in readonly structs only have a get accessor.

3fill in blank

hard

Fix the error in the readonly struct method that tries to modify a field.

C Sharp (C#)

public readonly struct Circle { public int Radius; public void SetRadius(int r) { [1] = r; } }

Drag options to blanks, or click blank then click option'

ARadius

Bthis.Radius

Creadonly Radius

Dbase.Radius

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign to fields without 'this' or in readonly structs.

In a readonly struct, to modify a field inside a method, you must use this explicitly, but modification is not allowed; the method should be removed or the struct not readonly.

4fill in blank

hard

Fill both blanks to create a readonly struct with a constructor initializing its fields.

C Sharp (C#)

public [1] struct Vector2D { public int X; public int Y; public [2] Vector2D(int x, int y) { X = x; Y = y; } }

Drag options to blanks, or click blank then click option'

Areadonly

Bstatic

CVector2D

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' instead of 'readonly' or wrong constructor name.

The struct must be declared readonly and the constructor has the same name as the struct.

5fill in blank

hard

Fill all three blanks to create a readonly struct with a method that returns the distance from origin.

C Sharp (C#)

public [1] struct Point3D { public int X; public int Y; public int Z; public double [2]() { return Math.[3](X * X + Y * Y + Z * Z); } }

Drag options to blanks, or click blank then click option'

Areadonly

BDistanceFromOrigin

CSqrt

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' for the struct or wrong method name or math function.

The struct is readonly, the method is named DistanceFromOrigin, and it uses Math.Sqrt to calculate distance.

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