Object instantiation with new in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Object instantiation with new

Class Definition

↓

Call new Keyword

↓

Allocate Memory for Object

↓

Call Constructor

↓

Return Object Reference

↓

Assign to Variable

↓

Use Object

This flow shows how using 'new' creates a new object: memory is allocated, constructor runs, and a reference is returned and stored.

Execution Sample

C Sharp (C#)

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

Person p = new Person("Alice");

This code creates a new Person object with the name "Alice" and stores it in variable p.

Execution Table

Step	Action	Evaluation	Result
1	Call new Person("Alice")	Allocates memory for Person	Memory allocated
2	Call Person constructor with name="Alice"	Sets Name field	Name = "Alice"
3	Return reference to new Person object	Reference created	Reference to Person object
4	Assign reference to variable p	p now points to new object	p = reference to Person("Alice")

💡 Object created and reference assigned to variable p

Variable Tracker

Variable	Start	After Step 4
p	null	Reference to Person object with Name="Alice"

Key Moments - 2 Insights

Why do we use the 'new' keyword to create an object?

The 'new' keyword tells the program to allocate memory and call the constructor to create a new object, as shown in steps 1 and 2 of the execution table.

What does the variable 'p' hold after instantiation?

Variable 'p' holds a reference (or address) to the new Person object, not the object itself, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 2?

A The constructor sets the Name field

B Memory is allocated for the object

C The reference is assigned to variable p

D The object is destroyed

💡 Hint

Check the 'Action' and 'Result' columns at step 2 in the execution table.

At which step does the variable 'p' get its value?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look for the step where the reference is assigned to 'p' in the execution table.

If we omit 'new' and write 'Person p = Person("Alice");', what would happen?

A The object is created normally

B Compilation error because 'new' is missing

C The variable 'p' holds null

D The constructor is called twice

💡 Hint

Remember that 'new' is required to create objects in C#, as shown in the concept flow.

Concept Snapshot

Object instantiation with new in C#:
- Use 'new ClassName(args)' to create an object.
- 'new' allocates memory and calls the constructor.
- The constructor initializes the object.
- The variable holds a reference to the object.
- Without 'new', object creation fails.

Full Transcript

This lesson shows how to create a new object in C# using the 'new' keyword. First, the program allocates memory for the object. Then, it calls the constructor to set up the object's data. After that, it returns a reference to the new object. Finally, this reference is stored in a variable. The variable does not hold the object itself but a pointer to it. Using 'new' is necessary to create objects; without it, the code will not compile.

Practice

1. What does the new keyword do in C# when used like new MyClass()?

easy

A. It calls a static method of MyClass.

B. It deletes an existing object of MyClass.

C. It converts MyClass to a string.

D. It creates a new object instance of the class MyClass.

Solution

1. Step 1: Understand the role of new

   The new keyword in C# is used to create a fresh object from a class blueprint.

2. Step 2: Apply to the example new MyClass()

   This expression creates a new instance of the class MyClass by calling its constructor.

3. Final Answer:

   It creates a new object instance of the class MyClass. -> Option D

4. Quick Check:

   new creates object = C [OK]

Hint: Remember: new means create a fresh object [OK]

Common Mistakes:

• Thinking new deletes or modifies existing objects
• Confusing new with method calls
• Forgetting parentheses after class name

2. Which of the following is the correct syntax to create a new object of class Person?

easy

A. Person p = new Person;

B. Person p = Person.new();

C. Person p = new Person();

D. Person p = Person();

Solution

1. Step 1: Check correct use of new keyword and parentheses

   In C#, to create a new object, you must use new ClassName() with parentheses.

2. Step 2: Analyze each option

   Person p = new Person(); uses new Person(); correctly. Person p = Person.new(); uses wrong syntax with dot notation. Person p = new Person; misses parentheses. Person p = Person(); misses new.

3. Final Answer:

   Person p = new Person(); -> Option C

4. Quick Check:

   Correct syntax = A [OK]

Hint: Always use new ClassName() with parentheses [OK]

Common Mistakes:

• Omitting parentheses after class name
• Using dot notation with new
• Forgetting the new keyword

3. What will be the output of this code?

class Box {
  public int size;
  public Box(int s) { size = s; }
}

var b = new Box(5);
Console.WriteLine(b.size);

medium

A. 5

B. 0

C. null

D. Compilation error

Solution

1. Step 1: Understand the constructor call

   The constructor Box(int s) sets the field size to the passed value s. Here, new Box(5) sets size = 5.

2. Step 2: Check the output of Console.WriteLine(b.size)

   This prints the value of b.size, which was set to 5 by the constructor.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Constructor sets size = 5 [OK]

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

Common Mistakes:

• Assuming default 0 instead of constructor value
• Confusing null with int fields
• Thinking code won't compile

4. Identify the error in this code snippet:

class Car {
  public string model;
  public Car(string m) { model = m; }
}

Car c = new Car;

medium

A. Class Car has no constructor defined.

B. Missing parentheses after Car in object creation.

C. model field is not initialized.

D. Cannot assign new Car to variable c.

Solution

1. Step 1: Check object instantiation syntax

   In C#, when creating a new object, parentheses must follow the class name even if no arguments are passed.

2. Step 2: Analyze the code snippet

   The code uses new Car; without parentheses, which causes a syntax error.

3. Final Answer:

   Missing parentheses after Car in object creation. -> Option B

4. Quick Check:

   new requires parentheses () [OK]

Hint: Always add () after new ClassName [OK]

Common Mistakes:

• Omitting parentheses after new keyword
• Assuming default constructor exists without parentheses
• Ignoring compiler error messages

5. You want to create two independent objects of class Student with different names. Which code correctly does this?

hard

A. Student s1 = new Student("Alice"); Student s2 = new Student("Bob");

B. Student s1 = Student("Alice"); Student s2 = Student("Bob");

C. Student s1, s2 = new Student("Alice"), new Student("Bob");

D. Student s1 = new Student; Student s2 = new Student;

Solution

1. Step 1: Understand object creation with parameters

   To create objects with different names, call the constructor with the name string for each object separately using new Student(name).

2. Step 2: Analyze each option

   Student s1 = new Student("Alice"); Student s2 = new Student("Bob"); correctly creates two objects with different names. Student s1 = Student("Alice"); Student s2 = Student("Bob"); misses new. Student s1, s2 = new Student("Alice"), new Student("Bob"); has invalid syntax for multiple declarations. Student s1 = new Student; Student s2 = new Student; misses parentheses and parameters.

3. Final Answer:

   Student s1 = new Student("Alice"); Student s2 = new Student("Bob"); -> Option A

4. Quick Check:

   Use new with constructor for each object [OK]

Hint: Create each object with new and constructor call [OK]

Common Mistakes:

• Forgetting new keyword
• Trying to create multiple objects in one line incorrectly
• Omitting constructor parameters