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'.

Practice

1. What is the main purpose of a constructor in a C# class?

easy

A. To define methods that return values

B. To declare variables inside a class

C. To initialize new objects with starting values

D. To inherit properties from another class

Solution

1. Step 1: Understand what constructors do

   Constructors are special methods that run when an object is created to set initial values.

2. Step 2: Compare options with constructor purpose

   Only To initialize new objects with starting values describes initializing new objects, which matches the constructor's role.

3. Final Answer:

   To initialize new objects with starting values -> Option C

4. Quick Check:

   Constructor purpose = initialize objects [OK]

Hint: Constructors set initial values when creating objects [OK]

Common Mistakes:

• Confusing constructors with regular methods
• Thinking constructors return values
• Mixing constructors with inheritance

2. Which of the following is the correct syntax for a constructor in C# for a class named Car?

easy

A. public Car() { }

B. void Car() { }

C. public void Car() { }

D. public Car(void) { }

Solution

1. Step 1: Recall constructor syntax rules

   Constructors have the same name as the class and no return type, but must have an access modifier like public.

2. Step 2: Check each option

   public Car() { } matches: public + class name + parentheses + no return type. Others have void return or wrong syntax.

3. Final Answer:

   public Car() { } -> Option A

4. Quick Check:

   Constructor syntax = public ClassName() [OK]

Hint: Constructor name = class name, no return type [OK]

Common Mistakes:

• Adding a return type like void
• Using incorrect parameter syntax
• Omitting access modifier

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

class Person {
  public string Name;
  public Person(string name) {
    Name = name;
  }
}

var p = new Person("Anna");
Console.WriteLine(p.Name);

medium

A. Compilation error

B. Anna

C. null

D. Name

Solution

1. Step 1: Understand constructor usage

   The constructor sets the Name field to the passed string "Anna" when creating the Person object.

2. Step 2: Check output of Console.WriteLine

   Since p.Name was set to "Anna", printing p.Name outputs "Anna".

3. Final Answer:

   Anna -> Option B

4. Quick Check:

   Constructor sets Name = "Anna" so output = Anna [OK]

Hint: Constructor sets fields; output shows assigned value [OK]

Common Mistakes:

• Assuming default null value instead of assigned
• Confusing field name with value
• Expecting compilation error due to constructor

4. Identify the error in this C# class constructor and how to fix it:

class Book {
  public string Title;
  public Book(string title) {
    title = Title;
  }
}

medium

A. Title should be private, not public

B. Constructor name should be lowercase book

C. Missing return type void in constructor

D. The assignment is reversed; should be Title = title;

Solution

1. Step 1: Analyze the assignment inside constructor

   The code assigns title = Title, which sets the parameter to the field's value, not the other way around.

2. Step 2: Correct the assignment direction

   It should assign the field Title to the parameter value: Title = title; to initialize properly.

3. Final Answer:

   The assignment is reversed; should be Title = title; -> Option D

4. Quick Check:

   Field = parameter to initialize correctly [OK]

Hint: Assign field = parameter inside constructor [OK]

Common Mistakes:

• Reversing assignment direction
• Changing constructor name incorrectly
• Adding return type to constructor

5. Given this class with two constructors:

class Rectangle {
  public int Width, Height;
  public Rectangle() {
    Width = 1;
    Height = 1;
  }
  public Rectangle(int size) {
    Width = size;
    Height = size;
  }
}

var r1 = new Rectangle();
var r2 = new Rectangle(5);
Console.WriteLine(r1.Width + "," + r1.Height);
Console.WriteLine(r2.Width + "," + r2.Height);

What is the output?

hard

A. 1,1
5,5

B. 0,0
5,5

C. 1,1
1,1

D. Compilation error due to constructor overload

Solution

1. Step 1: Understand constructor overloading

   The class has two constructors: one with no parameters sets Width and Height to 1; the other sets both to the given size.

2. Step 2: Trace object creation and output

   r1 uses the no-parameter constructor, so Width=1, Height=1. r2 uses the int parameter constructor with 5, so Width=5, Height=5.

3. Final Answer:

   1,1 5,5 -> Option A

4. Quick Check:

   Overloaded constructors set different sizes correctly [OK]

Hint: Overloaded constructors run based on arguments [OK]

Common Mistakes:

• Assuming default values are zero
• Thinking constructor overload causes error
• Mixing up which constructor runs