Instance fields and state in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an instance field named 'count' of type int.

C Sharp (C#)

public class Counter {
    private [1] count;
}

Drag options to blanks, or click blank then click option'

Aint

Bstatic

Cstring

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' as a type for a field.

Using 'static' keyword instead of a type.

Using 'string' when the field should hold numbers.

The instance field 'count' should be declared with the type int to store integer values.

2fill in blank

medium

Complete the code to initialize the instance field 'count' to zero in the constructor.

C Sharp (C#)

public class Counter {
    private int count;
    public Counter() {
        [1] = 0;
    }
}

Drag options to blanks, or click blank then click option'

Athis

BCounter

Cvoid

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name 'Counter' instead of the field name.

Using 'this' alone without the field name.

Using 'void' which is not a variable.

The instance field 'count' is assigned the value 0 inside the constructor by using its name directly.

3fill in blank

hard

Fix the error in the method to correctly increment the instance field 'count'.

C Sharp (C#)

public class Counter {
    private int count;
    public void Increment() {
        [1]++;
    }
}

Drag options to blanks, or click blank then click option'

AIncrement

BCounter

Ccount

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to increment the class name 'Counter'.

Using the method name 'Increment' as a variable.

Using 'this' alone without the field name.

The method should increment the instance field 'count' by using its name directly.

4fill in blank

hard

Fill both blanks to create a method that returns the current value of the instance field 'count'.

C Sharp (C#)

public class Counter {
    private int count;
    public [1] GetCount() {
        return [2];
    }
}

Drag options to blanks, or click blank then click option'

Aint

Bvoid

Ccount

DCounter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' as return type when the method returns a value.

Returning the class name instead of the field.

Returning nothing or wrong variable.

The method should return an integer, so its return type is int, and it returns the instance field count.

5fill in blank

hard

Fill all three blanks to define a class with an instance field, a constructor that sets it, and a method that returns it.

C Sharp (C#)

public class Person {
    private string [1];
    public Person(string [2]) {
        this.name = [2];
    }
    public string GetName() {
        return [3];
    }
}

Drag options to blanks, or click blank then click option'

Aname

Dperson

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the field and parameter causing confusion.

Not using 'this' to distinguish the field from the parameter.

Returning a variable that does not exist.

The instance field is named 'name'. The constructor parameter is also 'name'. The method returns the field 'name'.

Practice

1. What is the main purpose of an instance field in a C# class?

easy

A. To store data unique to each object created from the class

B. To define a method that all objects share

C. To create a temporary variable inside a method

D. To hold data shared by all objects of the class

Solution

1. Step 1: Understand what instance fields represent

   Instance fields hold data that belongs to each individual object, not shared across all objects.

2. Step 2: Differentiate from static fields and methods

   Static fields hold shared data, methods define behavior, and local variables are temporary inside methods.

3. Final Answer:

   To store data unique to each object created from the class -> Option A

4. Quick Check:

   Instance field = unique object data [OK]

Hint: Instance fields hold unique data per object, not shared [OK]

Common Mistakes:

• Confusing instance fields with static fields
• Thinking methods are instance fields
• Mixing local variables with instance fields

2. Which of the following is the correct way to declare an instance field named count of type int inside a C# class?

easy

A. static int count;

B. int count() {}

C. void count;

D. int count;

Solution

1. Step 1: Identify correct syntax for instance field declaration

   Instance fields are declared with a type and name, without static keyword or parentheses.

2. Step 2: Check each option

   static int count; is static, not instance. void count; uses void which is invalid for fields. int count() {} looks like a method, not a field.

3. Final Answer:

   int count; -> Option D

4. Quick Check:

   Instance field syntax = type + name [OK]

Hint: Instance fields: type and name, no parentheses or static [OK]

Common Mistakes:

• Using static keyword for instance fields
• Adding parentheses like a method
• Using void as a type for fields

3. What will be the output of this C# code?

class Counter {
  private int count = 0;
  public void Increment() {
    count++;
  }
  public int GetCount() {
    return count;
  }
}

var c = new Counter();
c.Increment();
c.Increment();
Console.WriteLine(c.GetCount());

medium

A. 0

B. 2

C. 1

D. Compilation error

Solution

1. Step 1: Trace the method calls on the object

   The object c calls Increment() twice, each increasing count by 1.

2. Step 2: Check the value returned by GetCount()

   After two increments, count is 2, so GetCount() returns 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   2 increments = count 2 [OK]

Hint: Each Increment adds 1; two calls mean count is 2 [OK]

Common Mistakes:

• Forgetting that count starts at 0
• Assuming Increment does not change count
• Confusing method return types

4. Identify the error in this C# class that tries to track a score:

class Game {
  int score;
  public void AddPoints(int points) {
    score = score + points;
  }
  public int GetScore() {
    return score;
  }
}

var g = new Game();
g.AddPoints(5);
Console.WriteLine(g.GetScore());

medium

A. No error; code runs and prints 5

B. score should be declared static

C. AddPoints method should return int

D. score is not initialized and may have a default value

Solution

1. Step 1: Check field initialization rules in C#

   Instance fields like score default to 0 if not explicitly initialized.

2. Step 2: Verify method behavior and output

   AddPoints adds points correctly, and GetScore returns the updated score. The code prints 5 as expected.

3. Final Answer:

   No error; code runs and prints 5 -> Option A

4. Quick Check:

   Uninitialized int defaults to 0 in C# [OK]

Hint: Instance int fields default to 0 if not set [OK]

Common Mistakes:

• Thinking uninitialized int fields cause errors
• Believing AddPoints must return a value
• Confusing static and instance fields

5. You want to create a class BankAccount that remembers the balance for each account. Which design correctly uses instance fields to track the balance and safely update it?

class BankAccount {
  private decimal balance;

  public BankAccount(decimal initial) {
    balance = initial;
  }

  public void Deposit(decimal amount) {
    if (amount > 0) {
      balance += amount;
    }
  }

  public bool Withdraw(decimal amount) {
    if (amount > 0 && amount <= balance) {
      balance -= amount;
      return true;
    }
    return false;
  }

  public decimal GetBalance() {
    return balance;
  }
}

hard

A. Incorrect: Deposit and Withdraw should be static methods

B. Incorrect: balance should be static to share across accounts

C. Correct design: instance field stores balance, methods update and read it safely

D. Incorrect: balance should be public to allow direct access

Solution

1. Step 1: Check if balance is instance field and encapsulated

   Balance is private instance field, unique per object, which is correct for tracking each account.

2. Step 2: Verify methods safely update and provide access

   Deposit and Withdraw check amounts before changing balance, and GetBalance returns current balance safely.

3. Final Answer:

   Correct design: instance field stores balance, methods update and read it safely -> Option C

4. Quick Check:

   Instance field + safe methods = correct state management [OK]

Hint: Use private instance fields with methods to control access [OK]

Common Mistakes:

• Making balance static, sharing state wrongly
• Using static methods that can't access instance fields
• Making balance public, breaking encapsulation