Methods that operate on state in C Sharp (C#) - Mini Project: Build & Apply

Methods that operate on state

📖 Scenario: You are creating a simple bank account program. The account has a balance that changes when you deposit or withdraw money.

🎯 Goal: Build a class with methods that change the account balance and show the current balance.

📋 What You'll Learn

Create a class called BankAccount with a private balance variable.

Add a method Deposit that adds money to balance.

Add a method Withdraw that subtracts money from balance.

Add a method GetBalance that returns the current balance.

Create an instance of BankAccount and use the methods to change and show the balance.

💡 Why This Matters

🌍 Real World

Bank accounts, game scores, or any object that keeps track of changing information use methods to update and read their state.

💼 Career

Understanding how to write methods that operate on an object's state is essential for software development, especially in object-oriented programming.

1

Create the BankAccount class with a balance variable

Create a class called BankAccount with a private integer variable balance set to 0.

C Sharp (C#)

class BankAccount
{
    // Your code here
}

Hint

Use class BankAccount and inside it declare private int balance = 0;.

2

Add Deposit and Withdraw methods

Add a public method Deposit that takes an integer amount and adds it to balance. Add a public method Withdraw that takes an integer amount and subtracts it from balance.

C Sharp (C#)

class BankAccount
{
    private int balance = 0;

    // Your code here
}

Hint

Write two methods: Deposit adds amount to balance, Withdraw subtracts amount from balance.

3

Add GetBalance method

Add a public method GetBalance that returns the current balance as an integer.

C Sharp (C#)

class BankAccount
{
    private int balance = 0;

    public void Deposit(int amount)
    {
        balance += amount;
    }

    public void Withdraw(int amount)
    {
        balance -= amount;
    }

    // Your code here
}

Hint

Write a method GetBalance that returns the value of balance.

4

Create an instance and show the balance

Create an instance of BankAccount called account. Call Deposit(100) and Withdraw(30) on account. Then print the result of account.GetBalance().

C Sharp (C#)

class BankAccount
{
    private int balance = 0;

    public void Deposit(int amount)
    {
        balance += amount;
    }

    public void Withdraw(int amount)
    {
        balance -= amount;
    }

    public int GetBalance()
    {
        return balance;
    }
}

// Your code here

Hint

Create account, call Deposit(100), Withdraw(30), then print account.GetBalance().

Practice

1. What is the main purpose of methods that operate on state in a C# class?

easy

A. To perform calculations without changing any data

B. To allow objects to keep and change their own data safely

C. To handle user input from the console

D. To create new classes from existing ones

Solution

1. Step 1: Understand what 'state' means in programming

   State refers to the data stored inside an object that can change over time.

2. Step 2: Identify the role of methods operating on state

   These methods allow the object to update or read its own data safely, keeping control inside the object.

3. Final Answer:

   To allow objects to keep and change their own data safely -> Option B

4. Quick Check:

   Methods on state = safe data change inside object [OK]

Hint: Think: methods change or read object's own data [OK]

Common Mistakes:

• Confusing methods on state with inheritance
• Thinking methods only perform calculations
• Believing methods handle external input only

2. Which of the following is the correct syntax for a method that changes an object's state in C#?

easy

A. public void UpdateName(string newName) { name = newName; }

B. void UpdateName(string newName) name = newName;

C. public UpdateName(string newName) { name = newName; }

D. public void UpdateName(string newName) => return name = newName;

Solution

1. Step 1: Check method declaration syntax

   In C#, methods must specify access modifier, return type, name, and parameters inside parentheses, with body in braces.

2. Step 2: Verify the method body updates the state correctly

   public void UpdateName(string newName) { name = newName; } correctly assigns newName to the field name inside braces.

3. Final Answer:

   public void UpdateName(string newName) { name = newName; } -> Option A

4. Quick Check:

   Correct method syntax = public void UpdateName(string newName) { name = newName; } [OK]

Hint: Remember method syntax: access + return type + name(params) { body } [OK]

Common Mistakes:

• Missing braces around method body
• Omitting return type
• Using return with void methods incorrectly

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. 3

B. 1

C. 0

D. 2

Solution

1. Step 1: Trace the Increment method calls

   Each call to Increment increases count by 1. Two calls increase count from 0 to 2.

2. Step 2: Check the GetCount method output

   GetCount returns the current count, which is 2 after two increments.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   2 increments = count 2 [OK]

Hint: Count increments twice, so output is 2 [OK]

Common Mistakes:

• Forgetting that count starts at 0
• Assuming Increment adds more than 1
• Confusing method names or outputs

4. Identify the error in this method that tries to update an object's state:

public void SetAge(int age) {
  int age = age;
}

medium

A. The method redeclares 'age' variable causing a conflict

B. The method is missing a return statement

C. The method should be static to update state

D. The method should not have parameters

Solution

1. Step 1: Analyze variable declarations inside the method

   The method declares a new local variable 'int age', which conflicts with the parameter 'age'.

2. Step 2: Understand how to update the object's field

   To update the object's state, assign the parameter to the field, e.g., this.age = age; without redeclaring.

3. Final Answer:

   The method redeclares 'age' variable causing a conflict -> Option A

4. Quick Check:

   Variable redeclaration error = The method redeclares 'age' variable causing a conflict [OK]

Hint: Don't redeclare parameter names inside method [OK]

Common Mistakes:

• Thinking missing return causes error in void method
• Assuming static needed to update instance state
• Believing parameters should be removed

5. You have a class BankAccount with a private field balance. You want to add a method Withdraw that subtracts an amount only if there is enough balance. Which method implementation correctly operates on the state safely?

hard

A. public decimal Withdraw(decimal amount) { return balance - amount; }

B. public void Withdraw(decimal amount) { balance -= amount; }

C. public void Withdraw(decimal amount) { if (amount <= balance) balance -= amount; else Console.WriteLine("Insufficient funds"); }

D. public void Withdraw(decimal amount) { if (amount < 0) balance += amount; }

Solution

1. Step 1: Check for safe state update conditions

   Method should only subtract amount if balance is enough to avoid negative balance.

2. Step 2: Verify method behavior on insufficient funds

   public void Withdraw(decimal amount) { if (amount <= balance) balance -= amount; else Console.WriteLine("Insufficient funds"); } checks amount <= balance and prints a message if not enough, preventing invalid state.

3. Final Answer:

   public void Withdraw(decimal amount) { if (amount <= balance) balance -= amount; else Console.WriteLine("Insufficient funds"); } -> Option C

4. Quick Check:

   Safe state update with condition = public void Withdraw(decimal amount) { if (amount <= balance) balance -= amount; else Console.WriteLine("Insufficient funds"); } [OK]

Hint: Check balance before subtracting to avoid negative state [OK]

Common Mistakes:

• Subtracting without checking balance
• Returning new value without updating state
• Adding amount when negative instead of subtracting