C# Program

using System;  // Importing the System namespace

namespace HelloWorldApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello, World!");
        }
    }
}

🔁 Quick Recap

• C# is used widely in back-end and app development

• The Main() method is where your program starts

• All code is written inside classes and namespaces

🧠 Lesson 1 Quiz & Coding Challenge

🔸 Quiz:

1. What is the purpose of Main() in a C# program?

2. What does using System; allow you to do?

3. True or False: C# requires all code to be inside a class.

🔸 Coding Challenge:

Write a C# program that prints:

Welcome to C#!
Let's start coding.

🧩 What is a Variable?

A variable stores data in memory so you can use or manipulate it later.

Think of it like a labeled box where you store a value.

🧱 Declaring a Variable

int age = 20; // age is the variable
string name = "Renz"; // name is the variable

🔁 Reassigning a Variable

int age = 20;
age = 25; // updates the value of age

🧪 Common Data Types in C#

🧠 Lesson 2 Quiz & Coding Challenge

🔸 Quiz:

1. What’s the difference between char and string?

2. Can you change the type of a variable after declaring it? Why or why not?

3. What value type would you use to store:

   • A user’s age

   • A login status (logged in or not)

   • A student's name

🔸 Coding Challenge:

Create a program that:

• Declares these variables:

  • A string fullName

  • An int for age

  • A bool for isStudent

• Assigns values to them

• Prints them to the console like this:

Name: John Doe
Age: 21
Student: True

📘 Lesson 3: Control Flow (if, else, switch)

🔍Why It Matters

• Control flow lets your program "think"—it can make decisions, follow different paths, and act based on input or conditions. Essential for back-end logic and user input handling.

🧱 The if-else Statement

int score = 85;

if (score >= 90)
{
    Console.WriteLine("Grade: A");
}
else if (score >= 80)
{
    Console.WriteLine("Grade: B");
}
else
{
    Console.WriteLine("Grade: C or below");
}

✅ How it works:

• Conditions are checked top-down

• The first true condition gets executed

• If none match, else runs as the fallback

🎛 The switch Statement

string day = "Monday";

switch (day)
{
    case "Monday":
        Console.WriteLine("Start of the week");
        break;
    case "Friday":
        Console.WriteLine("Almost weekend!");
        break;
    default:
        Console.WriteLine("Regular day");
        break;
}

✅ How it works:

• Great for checking one variable against multiple values

• Use break to prevent fall-through

• default is optional but recommended

💡 Pro Tips

• Use if-else for ranges (like scores, ages)

• Use switch for fixed values (days, options, commands)

• Don’t forget {} around if blocks—even for one line. It avoids bugs later.

🔁 Quick Recap

• Use if-else for branching based on logic

• Use switch for comparing one variable to many values

• Be mindful of brackets and break statements

🔸 Coding Challenge:

Write a program that:

• Declares an int temperature

• Uses if-else to print:

  • "It's hot!" if temperature >= 30

  • "It's warm." if temperature >= 20

  • "It's cold." otherwise

• Then, use a switch to check the value of a string variable weatherCondition

  • "sunny" → Print "Wear sunglasses"

  • "rainy" → Print "Bring an umbrella"

  • Default → Print "Check the forecast"

📘 Lesson 4: Methods & Reusability

What is a Method?

• A method is a reusable block of code that performs a specific task.

• It helps keep your code organized, modular, and DRY (Don't Repeat Yourself).

🧱 Basic Method Structure

returnType MethodName(parameterList)
{
    // code block
    return value;
}

🔹 Example: Method without return

void GreetUser()
{
    Console.WriteLine("Hello, Renz!");
}

🔹 Method with Parameter and Return

int Add(int a, int b)
{
    return a + b;
}

🧩 Parameters vs Arguments

Example:

🧪 Return Types vs void

Example:

You just call it like:

Console.WriteLine(Greet());

🧠 How to Call a Method

🧠 When to Use static and private Methods in C#

🔹 static Method

You use static when:

• The method does not rely on instance variables or objects.

• You call the method directly using the class, like in Main().

• Example use: Utility/helper functions (e.g., formatting, math, conversions).

🧠 Think of static like a standalone tool — it doesn’t care who’s using it, it just works.

🔹 private Method

You use private when:

• You want to hide the method from other classes.

• It's only used inside the current class.

• It helps keep your code organized and secure by limiting access.

🧠 Think of private like a secret helper — only its friends inside the same class can ask for help.