Constructors and initialization in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constructor in C#?

A constructor is a special method in a class that runs automatically when an object is created. It sets up the object with initial values.

beginner

How do you define a constructor in a C# class?

You define a constructor by creating a method with the same name as the class and no return type. For example: <pre>public class Car { public Car() { } }</pre>

beginner

What is the purpose of constructor parameters?

Constructor parameters let you pass values when creating an object, so you can set different initial states for each object.

intermediate

What happens if you don't define a constructor in a C# class?

C# provides a default parameterless constructor automatically, which initializes fields to default values like 0 or null.

intermediate

Explain constructor chaining in C#.

Constructor chaining means calling one constructor from another in the same class using the <code>: this(...)</code> syntax to reuse initialization code.

What is the name of a constructor in C#?

A Init

B Same as the class name

C Main

D Constructor

In C#, the constructor must have the same name as the class.

What does a parameterless constructor do?

A Requires parameters to create an object

B Cannot be used in C#

C Initializes object with default values

D Deletes the object

A parameterless constructor sets fields to default values like 0 or null.

How do you call one constructor from another in the same class?

A Using new keyword

B Using base keyword

C Using super()

D Using : this(...) syntax

Constructor chaining uses : this(...) to call another constructor in the same class.

What happens if you define a constructor with parameters but no parameterless constructor?

A No default constructor is created automatically

B Parameterless constructor is still created

C Class cannot be instantiated

D Compiler error always occurs

If you define any constructor, C# does not create a default parameterless constructor automatically.

Which of these is NOT true about constructors?

A Constructors can be called like normal methods

B Constructors can be overloaded

C Constructors have no return type

D Constructors initialize new objects

Constructors cannot be called like normal methods; they run automatically when creating an object.

Describe what a constructor is and why it is useful in C#.

Think about how you prepare a new object when you make it.

Explain constructor chaining and give an example of when you might use it.

Imagine you want to avoid repeating code in multiple constructors.

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