Constructors and initialization in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constructor chaining in C#

What is the output of this C# program when creating an instance of Person with new Person()?

C Sharp (C#)

using System;
class Person {
    public string Name;
    public int Age;

    public Person() : this("Unknown") {
        Console.WriteLine("Default constructor called");
    }

    public Person(string name) {
        Name = name;
        Age = 0;
        Console.WriteLine($"Constructor with name: {Name}");
    }
}

class Program {
    static void Main() {
        Person p = new Person();
    }
}

A

Constructor with name: Unknown
Default constructor called

B

Default constructor called
Constructor with name: Unknown

C Default constructor called

D

Constructor with name: 
Default constructor called

💡 Hint

Remember that constructor chaining calls the chained constructor first before executing the body of the calling constructor.

The parameterless constructor calls the constructor with a string parameter using ': this("Unknown")'. So first, the constructor with the name parameter runs and prints 'Constructor with name: Unknown'. Then control returns to the parameterless constructor, which prints 'Default constructor called'.

❓ Predict Output

intermediate

Field initialization order in C# class

What is the output of this C# program?

C Sharp (C#)

using System;
class Sample {
    private int x = 5;
    private int y;

    public Sample() {
        y = x * 2;
        Console.WriteLine($"x = {x}, y = {y}");
    }
}

class Program {
    static void Main() {
        Sample s = new Sample();
    }
}

A x = 0, y = 10

B x = 5, y = 10

C x = 0, y = 0

D x = 5, y = 0

💡 Hint

Fields initialized inline are set before the constructor body runs.

The field 'x' is initialized to 5 before the constructor runs. Then in the constructor, 'y' is set to 'x * 2', which is 10. So the output is 'x = 5, y = 10'.

❓ Predict Output

advanced

Output of static and instance constructors

What is the output when this C# program runs?

C Sharp (C#)

using System;
class Demo {
    static Demo() {
        Console.WriteLine("Static constructor called");
    }

    public Demo() {
        Console.WriteLine("Instance constructor called");
    }
}

class Program {
    static void Main() {
        Demo d1 = new Demo();
        Demo d2 = new Demo();
    }
}

A

Instance constructor called
Instance constructor called
Static constructor called

B

Instance constructor called
Static constructor called
Instance constructor called

C

Static constructor called
Instance constructor called

D

Static constructor called
Instance constructor called
Instance constructor called

💡 Hint

Static constructors run once before any instance is created or static member accessed.

The static constructor runs once before the first instance is created, printing 'Static constructor called'. Then each instance constructor runs printing 'Instance constructor called'. So the output is the static constructor message once, then two instance constructor messages.

❓ Predict Output

advanced

Output of readonly field initialization

What is the output of this C# program?

C Sharp (C#)

using System;
class Test {
    private readonly int number;

    public Test(int num) {
        number = num;
        Console.WriteLine($"Number is {number}");
    }

    public void ChangeNumber(int newNum) {
        // number = newNum; // Uncommenting this line causes error
    }
}

class Program {
    static void Main() {
        Test t = new Test(10);
        t.ChangeNumber(20);
        Console.WriteLine("Done");
    }
}

A

Number is 10
Done

B

Number is 20
Done

C Compilation error due to assignment in ChangeNumber method

D

Number is 0
Done

💡 Hint

Readonly fields can only be assigned in declaration or constructor.

The readonly field 'number' is assigned in the constructor to 10. The ChangeNumber method does not change it because the assignment is commented out. So the output is 'Number is 10' and then 'Done'.

❓ Predict Output

expert

Output of complex constructor and field initialization order

What is the output of this C# program?

C Sharp (C#)

using System;
class Base {
    public Base() {
        Console.WriteLine("Base constructor");
        Print();
    }
    public virtual void Print() {
        Console.WriteLine("Base Print");
    }
}

class Derived : Base {
    private int x = 5;
    public Derived() {
        Console.WriteLine("Derived constructor");
    }
    public override void Print() {
        Console.WriteLine($"Derived Print: x = {x}");
    }
}

class Program {
    static void Main() {
        Derived d = new Derived();
    }
}

A

Base constructor
Derived Print: x = 5
Derived constructor

B

Derived constructor
Base constructor
Derived Print: x = 0

C

Base constructor
Derived Print: x = 0
Derived constructor

D

Derived constructor
Base constructor
Derived Print: x = 5

💡 Hint

During base constructor, derived fields are not yet initialized.

When the base constructor calls the virtual method Print, the derived override runs but the derived field 'x' is still at default 0 because field initializers run after base constructor. Then the derived constructor runs printing 'Derived constructor'.