Constructors and initialization in C Sharp (C#) - Time & Space Complexity

Time Complexity: Constructors and initialization

O(n)

Understanding Time Complexity

When we create objects using constructors, some steps run to set up the object.

We want to see how the time needed changes as we create more objects or initialize more data.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class Box
{
    public int[] Items;

    public Box(int size)
    {
        Items = new int[size];
        for (int i = 0; i < size; i++)
        {
            Items[i] = i * 2;
        }
    }
}

// Creating a Box object
Box myBox = new Box(1000);
    

This code creates a Box object and fills an array with values during construction.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop inside the constructor that fills the array.
• How many times: It runs once for each element in the array, so size times.

How Execution Grows With Input

As the size of the array grows, the number of steps to fill it grows too.

Input Size (n)	Approx. Operations
10	About 10 steps to fill the array
100	About 100 steps to fill the array
1000	About 1000 steps to fill the array

Pattern observation: The work grows directly with the size of the array; doubling the size doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to initialize grows in a straight line with the number of items we set up.

Common Mistake

[X] Wrong: "Constructors always run in constant time no matter what."

[OK] Correct: If a constructor does work like filling an array, the time depends on how much data it handles, so it grows with input size.

Interview Connect

Understanding how constructors scale helps you explain object setup costs clearly, a useful skill when discussing code efficiency.

Self-Check

"What if the constructor called another method that also loops over the array? How would the time complexity change?"

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