Object instantiation with new in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the new keyword do in C#?

The <code>new</code> keyword creates a new instance (object) of a class in memory, allowing you to use that object in your program.

beginner

How do you create a new object of a class named <code>Car</code>?

You write:
Car myCar = new Car();
This creates a new Car object and stores it in the variable myCar.

beginner

Why do you need to use new to create objects?

Using new tells the computer to allocate memory for the object and run its constructor to set it up properly.

intermediate

What happens if you declare a variable of a class type without using <code>new</code>?

The variable will be null (empty) and does not point to any object. Trying to use it will cause a NullReferenceException at runtime.

beginner

Can you instantiate an object without parentheses after the class name? For example: <code>new Car;</code>

No, in C# you must include parentheses even if the constructor has no parameters: new Car(); is correct.

What does new do in C#?

A Declares a variable

B Deletes an object

C Creates a new object instance

D Calls a method

new creates a new object instance in memory.

Which is the correct way to instantiate a class named Dog?

A Dog d = Dog();

B Dog d = new Dog();

C Dog d = new Dog;

D Dog d = new;

You must use new Dog(); with parentheses to create a new object.

What happens if you declare Car myCar; without new?

A myCar is null and unusable

B myCar is a new object

C myCar is a number

D myCar is a string

Without new, the variable is null and does not reference an object.

Why do you need parentheses after the class name when using new?

A Parentheses are optional

B To declare a variable

C To assign a value

D To call the constructor method

Parentheses call the constructor to initialize the new object.

Which statement is true about object instantiation?

A The <code>new</code> keyword allocates memory for the object

B You can create an object without <code>new</code>

C Objects are created automatically without code

D Variables hold objects directly without references

new allocates memory and creates the object instance.

Explain in your own words what happens when you use new to create an object in C#.

Think about what the computer does behind the scenes when you write <code>new ClassName()</code>.

Describe the difference between declaring a variable of a class type and instantiating an object with new.

Consider what the variable holds before and after using <code>new</code>.

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