🎯 C# Functions & Code Organization Project

Lesson Snapshot

Transform messy code into clean, reusable functions that work like building blocks! (Ages 8-18)

Folder & File Map

CSharp-Lesson-Four/
├── .git/
├── .gitignore
├── CSharp-Lesson-Four.csproj
└── Program.cs

How to use the code snippets

Each step below shows the complete contents of Program.cs after you finish that step. When a step says "replace your Program.cs with this", select all (Ctrl+A / Cmd+A) inside Program.cs, delete it, then paste the new code. That keeps you from ending up with two Main methods or leftover code from earlier steps.

Step-by-Step Build Guide

1. Fork and Clone the Repository ✅

• What to do: Go to https://github.com/PhillyCoderDojo/CSharp-Lesson-Four → Click "Fork" → Open GitHub Desktop → Clone your fork
• File(s) touched: Creating project folder
• Code snippet: None
• Why it matters: Get your own workspace for function practice
• Git command: git clone https://github.com/YOUR-USERNAME/CSharp-Lesson-Four.git

2. Open Project in Rider ✅

• What to do: Open Rider → File → Open → Select your cloned folder → Click OK
• File(s) touched: Loading project
• Code snippet: None
• Why it matters: Time to organize code like a pro!
• Git command: None

3. Create Your First Function 📦

• What to do: Replace your Program.cs with the code below. It adds a SayHello function and calls it three times from Main.
• File(s) touched: Program.cs
• Code snippet (full file):

namespace CSharp_Lesson_Four;

class Program
{
    static void SayHello()
    {
        Console.WriteLine("Hello from my function!");
    }

    static void Main(string[] args)
    {
        SayHello();
        SayHello();
        SayHello();
    }
}

• Why it matters: Functions let you reuse code instead of copying
• Git command: git add . && git commit -m "Create first function without parameters"

4. Add Parameters to Functions 🎛️

• What to do: Replace your Program.cs with the version below. The function now takes a name and an age.
• File(s) touched: Program.cs
• Code snippet (full file):

namespace CSharp_Lesson_Four;

class Program
{
    static void GreetPerson(string name, int age)
    {
        Console.WriteLine($"Hello {name}! You are {age} years old.");
    }

    static void Main(string[] args)
    {
        GreetPerson("Alex", 12);
        GreetPerson("Jordan", 15);
        GreetPerson("Casey", 10);
    }
}

• Why it matters: Parameters make functions flexible with different inputs
• Git command: git add . && git commit -m "Add parameters to function"

5. Create Functions That Return Values 🔄

• What to do: Replace your Program.cs with the version below. Now the functions calculate something and return the result.
• File(s) touched: Program.cs
• Code snippet (full file):

namespace CSharp_Lesson_Four;

class Program
{
    static int AddNumbers(int a, int b)
    {
        return a + b;
    }

    static double CalculateCircleArea(double radius)
    {
        return 3.14159 * radius * radius;
    }

    static void Main(string[] args)
    {
        int sum = AddNumbers(5, 3);
        Console.WriteLine($"5 + 3 = {sum}");

        double area = CalculateCircleArea(4.0);
        Console.WriteLine($"Circle area: {area}");
    }
}

• Why it matters: Return values let functions send results back
• Git command: git add . && git commit -m "Add functions with return values"

6. Build a Calculator with Functions 🧮

• What to do: Replace your Program.cs with the calculator version below. Each math operation is its own function.
• File(s) touched: Program.cs
• Code snippet (full file):

namespace CSharp_Lesson_Four;

class Program
{
    static double Add(double x, double y) => x + y;
    static double Subtract(double x, double y) => x - y;
    static double Multiply(double x, double y) => x * y;
    static double Divide(double x, double y) => x / y;

    static void DisplayMenu()
    {
        Console.WriteLine("=== Calculator ===");
        Console.WriteLine("1. Add  2. Subtract  3. Multiply  4. Divide");
    }

    static double GetNumber(string prompt)
    {
        Console.Write(prompt);
        return double.Parse(Console.ReadLine());
    }

    static void Main(string[] args)
    {
        DisplayMenu();
        Console.Write("Choose operation (1-4): ");
        int choice = int.Parse(Console.ReadLine());

        double num1 = GetNumber("First number: ");
        double num2 = GetNumber("Second number: ");

        double result = choice switch
        {
            1 => Add(num1, num2),
            2 => Subtract(num1, num2),
            3 => Multiply(num1, num2),
            4 => Divide(num1, num2),
            _ => 0
        };

        Console.WriteLine($"Result: {result}");
    }
}

• Why it matters: Each function has one clear job
• Git command: git add . && git commit -m "Create organized calculator with functions"

7. Add Input Validation Function 🛡️

• What to do: Replace your Program.cs with the version below. The new GetValidNumber function keeps asking until the user types a real number, and the calculator now uses it instead of GetNumber.
• File(s) touched: Program.cs
• Code snippet (full file):

namespace CSharp_Lesson_Four;

class Program
{
    static double Add(double x, double y) => x + y;
    static double Subtract(double x, double y) => x - y;
    static double Multiply(double x, double y) => x * y;
    static double Divide(double x, double y) => x / y;

    static void DisplayMenu()
    {
        Console.WriteLine("=== Calculator ===");
        Console.WriteLine("1. Add  2. Subtract  3. Multiply  4. Divide");
    }

    static double GetValidNumber(string prompt)
    {
        while (true)
        {
            Console.Write(prompt);
            if (double.TryParse(Console.ReadLine(), out double result))
            {
                return result;
            }
            Console.WriteLine("❌ Please enter a valid number!");
        }
    }

    static void Main(string[] args)
    {
        DisplayMenu();
        Console.Write("Choose operation (1-4): ");
        int choice = int.Parse(Console.ReadLine());

        double num1 = GetValidNumber("First number: ");
        double num2 = GetValidNumber("Second number: ");

        double result = choice switch
        {
            1 => Add(num1, num2),
            2 => Subtract(num1, num2),
            3 => Multiply(num1, num2),
            4 => Divide(num1, num2),
            _ => 0
        };

        Console.WriteLine($"Result: {result}");
    }
}

• Why it matters: Functions can handle errors gracefully
• Git command: git add . && git commit -m "Add input validation function"

8. Push Your Organized Code 🚀

• What to do: In GitHub Desktop → Commit changes → Push origin
• File(s) touched: Uploading to GitHub
• Code snippet: None
• Why it matters: Share your well-organized functions
• Git command: git push origin main

Run & Test ▶️

• Press Ctrl+F5 or click green ▶️ button in Rider
• Expected output: Calculator menu, prompts for numbers, shows calculation results

Common errors and fixes

• "Type 'Program' already defines a member called 'Main'" — You have two Main methods. Make sure you replaced the whole file at each step instead of pasting on top of the old code. There should be exactly one static void Main(string[] args) in Program.cs.
• "The entry point of the program is global code; ignoring 'Main(string[])' entry point" — Same root cause: there's a stray line of code outside any method (often a leftover Console.WriteLine("Hello, World!"); from the starter). Delete anything outside the class Program { ... } braces.
• "Method must have a return type" — A function is missing static or its return type. Make sure every function starts with static and either void, int, double, etc.

Bonus Challenge 🔥

Create a "Math Quiz Game" using functions! Add the helpers below to your Program class and call AskMathQuestion() from Main.

Hint:

static int GenerateRandomNumber(int min, int max)
{
    Random random = new Random();
    return random.Next(min, max + 1);
}

static bool AskMathQuestion()
{
    int a = GenerateRandomNumber(1, 10);
    int b = GenerateRandomNumber(1, 10);
    int correct = a + b;

    Console.Write($"What is {a} + {b}? ");
    int answer = int.Parse(Console.ReadLine());

    return answer == correct;
}