Readonly structs in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of readonly struct method call

What is the output of this C# code using a readonly struct?

C Sharp (C#)

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

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

    public int Sum() => X + Y;
}

class Program
{
    static void Main()
    {
        Point p = new Point(3, 4);
        System.Console.WriteLine(p.Sum());
    }
}

A 7

B 0

C Compilation error

D Runtime exception

💡 Hint

Think about what the Sum method returns and how readonly structs behave.

The readonly struct Point has properties X=3 and Y=4. The Sum method returns X + Y, which is 7. There is no error or exception.

❓ Predict Output

intermediate

Effect of modifying field in readonly struct

What happens when you try to modify a field inside a readonly struct method?

C Sharp (C#)

readonly struct Counter
{
    private int count;

    public void Increment()
    {
        count++;
    }

    public int GetCount() => count;
}

class Program
{
    static void Main()
    {
        Counter c = new Counter();
        c.Increment();
        System.Console.WriteLine(c.GetCount());
    }
}

A Output: 0

B Output: 1

C Compilation error: cannot modify field in readonly struct

D Runtime exception

💡 Hint

Readonly structs prevent modification of fields inside instance methods.

In a readonly struct, instance methods cannot modify fields. The Increment method tries to change 'count', causing a compilation error.

🔧 Debug

advanced

Why does this readonly struct code fail to compile?

Identify the reason for the compilation error in this readonly struct code.

C Sharp (C#)

readonly struct Rectangle
{
    public int Width { get; set; }
    public int Height { get; set; }

    public int Area() => Width * Height;
}

class Program
{
    static void Main()
    {
        Rectangle r = new Rectangle { Width = 5, Height = 10 };
        System.Console.WriteLine(r.Area());
    }
}

A Area method cannot access properties in readonly struct

B No compilation error

C Cannot initialize readonly struct with object initializer

D Readonly structs cannot have set accessors in properties

💡 Hint

Think about what readonly means for property setters in structs.

Readonly structs cannot have properties with setters because that allows mutation. The set accessors cause a compilation error.

📝 Syntax

advanced

Which option correctly declares a readonly struct with an immutable field?

Choose the correct syntax to declare a readonly struct with an immutable field 'Id' of type int.

A readonly struct User { public readonly int Id; public User(int id) { Id = id; } }

B readonly struct User { public int Id; public User(int id) { Id = id; } }

C readonly struct User { int Id; public User(int id) { Id = id; } }

D struct User { readonly int Id; public User(int id) { Id = id; } }

💡 Hint

Remember to use 'public readonly' for immutable fields in readonly structs.

Option A correctly declares a readonly struct with a public readonly field and a constructor initializing it. Other options either miss 'readonly' or 'public' or have invalid syntax.

🚀 Application

expert

How many items are in the resulting list after processing readonly structs?

Given this code, how many elements does the list 'areas' contain after execution?

C Sharp (C#)

readonly struct Square
{
    public int Side { get; }
    public Square(int side) => Side = side;
    public int Area() => Side * Side;
}

class Program
{
    static void Main()
    {
        var squares = new Square[] { new(2), new(3), new(4) };
        var areas = new System.Collections.Generic.List<int>();
        foreach (var sq in squares)
        {
            areas.Add(sq.Area());
        }
        System.Console.WriteLine(areas.Count);
    }
}

A 0

B 3

C Compilation error

D Runtime exception

💡 Hint

Count how many squares are in the array and how many times the loop runs.

The array has 3 Square structs. The loop adds the area of each to the list, so areas.Count is 3.