Introduction to C#: What It Is, Why It Exists, and Your First Program

// FILE: HowCSharpWorks.cs
// PURPOSE: Illustrate the journey from source code to running output.
// This is a minimal C# program — the absolute smallest valid program you can write.
// Every C# program MUST have an entry point: the Main method.
// Think of Main as the 'Start' button on your program.

using System; // Import the System namespace so we can use Console.WriteLine

class HowCSharpWorks
{
    // The Main method — the CLR looks for this exact signature to start your program
    static void Main(string[] args)
    {
        // Console.WriteLine prints a line of text to the terminal
        // and automatically moves to the next line afterwards
        Console.WriteLine("The compiler checked this code.");
        Console.WriteLine("The CLR is now running it on your specific machine.");
        Console.WriteLine("That two-step process is .NET in action.");
    }
}

// FILE: VariablesAndTypes.cs
// PURPOSE: Demonstrate declaring, assigning, and using the core C# data types.
// Each variable declaration follows the pattern: type variableName = value;

using System;

class VariablesAndTypes
{
    static void Main(string[] args)
    {
        // int stores whole numbers (no decimals)
        // Good for: age, count, score, quantity
        int playerAge = 27;

        // double stores numbers with decimal points (uses 64-bit precision)
        // Good for: prices, measurements, coordinates
        double productPrice = 19.99;

        // string stores text of any length — always wrapped in double quotes
        // Good for: names, messages, addresses, usernames
        string customerName = "Maria Santos";

        // bool stores ONLY true or false — nothing else
        // Good for: flags, switches, on/off states
        bool isAccountActive = true;

        // char stores a SINGLE character — always wrapped in single quotes
        // Good for: menu choices, grades, single symbols
        char gradeScore = 'A';

        // Printing variables: use + to join text and variables together
        // This is called string concatenation
        Console.WriteLine("Customer: " + customerName);
        Console.WriteLine("Age: " + playerAge);
        Console.WriteLine("Price: $" + productPrice);
        Console.WriteLine("Account active: " + isAccountActive);
        Console.WriteLine("Grade: " + gradeScore);

        // You can also use string interpolation — cleaner and easier to read
        // Prefix the string with $ and wrap variable names in { }
        Console.WriteLine($"\nSummary: {customerName} is {playerAge} years old.");
        Console.WriteLine($"Their account is active: {isAccountActive}");
    }
}

// FILE: TemperatureAdvisor.cs
// PURPOSE: A complete, real program that takes user input and gives advice.
// This combines: variables, data types, input, type conversion, and if/else.

using System;

class TemperatureAdvisor
{
    static void Main(string[] args)
    {
        Console.WriteLine("=== Daily Temperature Advisor ===");
        Console.WriteLine("Enter the current temperature in Celsius:");

        // Console.ReadLine() pauses the program and waits for user input
        // It ALWAYS returns a string — even if the user types a number
        string userInput = Console.ReadLine();

        // double.Parse() converts the string "23.5" into the actual number 23.5
        // Without this conversion, we couldn't do math or comparisons
        double currentTemperature = double.Parse(userInput);

        Console.WriteLine(); // Print a blank line for readability

        // if/else if/else — the program checks conditions top to bottom
        // The FIRST condition that is true gets executed; the rest are skipped
        if (currentTemperature >= 30.0)
        {
            // This block runs ONLY when temperature is 30 or higher
            Console.WriteLine("It's very hot outside!");
            Console.WriteLine("Tip: Stay hydrated and wear light clothing.");
        }
        else if (currentTemperature >= 20.0)
        {
            // This block runs when temp is between 20 and 29.9
            Console.WriteLine("The weather is warm and pleasant.");
            Console.WriteLine("Tip: A light jacket in the evening might help.");
        }
        else if (currentTemperature >= 10.0)
        {
            // This block runs when temp is between 10 and 19.9
            Console.WriteLine("It's a bit cool out there.");
            Console.WriteLine("Tip: Wear a jacket before heading out.");
        }
        else
        {
            // This block runs when NONE of the above conditions were true
            // i.e., temperature is below 10
            Console.WriteLine("It's cold! Bundle up.");
            Console.WriteLine("Tip: Heavy coat, scarf, and gloves recommended.");
        }

        // String interpolation neatly embeds the variable inside the message
        Console.WriteLine($"\nRecorded temperature: {currentTemperature}°C");
    }
}