Class declaration syntax in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple class instance property

What is the output of this C# code?

C Sharp (C#)

class Person {
    public string Name = "Alice";
}

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

A null

B Name

C Alice

D Person

💡 Hint

Look at the value assigned to the Name field inside the Person class.

The class Person has a public field Name initialized to "Alice". Creating an instance and printing p.Name outputs "Alice".

📝 Syntax

intermediate

Identify the syntax error in class declaration

Which option contains a syntax error in the class declaration?

A class Plane { public void Fly() { Console.WriteLine("Flying"); } }

B class Bike { private string model; }

C public class Car { public int Speed; }

D public sealed class Boat { }

💡 Hint

Check if all braces are properly closed.

Option A is missing the closing brace for the class Plane, causing a syntax error.

🔧 Debug

advanced

Why does this class instantiation cause an error?

What error will this code produce and why?

C Sharp (C#)

class Animal {
    private Animal() { }
}

var a = new Animal();

A Compile-time error: Missing semicolon

B Compile-time error: 'Animal.Animal()' is inaccessible due to its protection level

C No error, outputs 'Animal created'

D Runtime error: NullReferenceException

💡 Hint

Check the accessibility of the constructor.

The constructor is private, so it cannot be called outside the class. Trying to create an instance causes a compile-time accessibility error.

🧠 Conceptual

advanced

Effect of 'sealed' keyword on class inheritance

What happens if you try to inherit from a sealed class in C#?

C Sharp (C#)

sealed class Vehicle { }
class Car : Vehicle { }

A Car inherits Vehicle successfully

B Runtime error: InvalidCastException

C Warning but compiles and runs

D Compile-time error: cannot derive from sealed type 'Vehicle'

💡 Hint

The sealed keyword prevents inheritance.

A sealed class cannot be used as a base class. Attempting to inherit from it causes a compile-time error.

🚀 Application

expert

Number of members in a class with properties and methods

How many members does this class have?

C Sharp (C#)

public class Calculator {
    public int Add(int a, int b) => a + b;
    public int Subtract(int a, int b) => a - b;
    public int Result { get; private set; }
    private void Reset() { Result = 0; }
}

A 4

B 3

C 5

D 6

💡 Hint

Count all methods and properties declared inside the class.

The class has 2 public methods (Add, Subtract), 1 property (Result), and 1 private method (Reset), totaling 4 members.

Practice

1. What is the correct way to declare a class named Car in C#?

easy

A. Car class { }

B. class Car { }

C. class = Car { }

D. class: Car { }

Solution

1. Step 1: Understand class declaration syntax

   In C#, a class is declared using the keyword class followed by the class name and curly braces.

2. Step 2: Match the correct syntax

   class Car { } uses class Car { }, which is the correct syntax for declaring a class named Car.

3. Final Answer:

   class Car { } -> Option B

4. Quick Check:

   Class declaration = class ClassName { } [OK]

Hint: Remember: class keyword + name + braces [OK]

Common Mistakes:

• Putting class name before keyword
• Using = or : instead of space
• Missing curly braces

2. Which of the following is a syntax error when declaring a class in C#?

easy

A. public class Person { }

B. class Animal { }

C. class 123Car { }

D. internal class House { }

Solution

1. Step 1: Check class name rules

   Class names must start with a letter or underscore, not a number.

2. Step 2: Identify invalid class name

   class 123Car { } uses class 123Car { }, which starts with digits, causing a syntax error.

3. Final Answer:

   class 123Car { } -> Option C

4. Quick Check:

   Class names cannot start with numbers [OK]

Hint: Class names must start with letter or underscore [OK]

Common Mistakes:

• Starting class name with a digit
• Using spaces in class name
• Using reserved keywords as class names

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

class Dog {
  public string Name = "Buddy";
}

class Program {
  static void Main() {
    Dog d = new Dog();
    System.Console.WriteLine(d.Name);
  }
}

medium

A. Buddy

B. Name

C. Dog

D. Compilation error

Solution

1. Step 1: Understand class field initialization

   The class Dog has a public field Name initialized to "Buddy".

2. Step 2: Trace the program output

   In Main, a Dog object is created and d.Name is printed, so output is "Buddy".

3. Final Answer:

   Buddy -> Option A

4. Quick Check:

   Field value printed = Buddy [OK]

Hint: Prints field value assigned in class [OK]

Common Mistakes:

• Confusing class name with field value
• Expecting method output instead of field
• Assuming compilation error without reason

4. Identify the error in this class declaration:

class Book
{
  string title;
  void SetTitle(string t)
  {
    title = t;
  }
}

medium

A. Missing access modifiers for field and method

B. Class name should be lowercase

C. Method SetTitle must return a value

D. Field title must be static

Solution

1. Step 1: Check access modifiers in class members

   By default, class members are private, but it's good practice to specify access modifiers explicitly.

2. Step 2: Identify missing access modifiers

   Field title and method SetTitle lack access modifiers like private or public, which can cause confusion or errors in some contexts.

3. Final Answer:

   Missing access modifiers for field and method -> Option A

4. Quick Check:

   Always specify access modifiers [OK]

Hint: Always add public/private to fields and methods [OK]

Common Mistakes:

• Assuming lowercase class names are required
• Thinking void methods must return a value
• Believing fields must be static

5. You want to create a class Student with a field name and a method GetName that returns the student's name. Which is the correct complete class declaration?

hard

A. class Student { public string name; public void GetName() { return name; } }

B. class Student { string name; string GetName() { name; } }

C. class Student { public string name; string GetName() { return name; } }

D. class Student { public string name; public string GetName() { return name; } }

Solution

1. Step 1: Check field and method access modifiers

   The field name and method GetName should be public to be accessible outside the class.

2. Step 2: Verify method return type and body

   GetName returns a string, so its return type must be string and it must return name.

3. Step 3: Identify correct option

   class Student { public string name; public string GetName() { return name; } } correctly declares name as public string and GetName as public string method returning name.

4. Final Answer:

   class Student { public string name; public string GetName() { return name; } } -> Option D

5. Quick Check:

   Public field + public string method returning field [OK]

Hint: Method return type must match returned value type [OK]

Common Mistakes:

• Missing return statement in method
• Wrong method return type (void instead of string)
• Missing public keyword for accessibility