Variables and Data Types

// Syntax: dataType variableName = value;
int age = 25;
string name = "John Doe";
double price = 99.99;
bool isActive = true;

// Valid variable names
int studentAge;
string firstName;
double _privateValue;
bool isReady2Go;

// Invalid variable names (will cause compilation errors)
// int 2students;     // Cannot start with a number
// string first-name; // Cannot contain hyphens
// bool class;        // Cannot use reserved keywords

// Integer types
int wholeNumber = 42;           // 32-bit signed integer
long bigNumber = 1234567890L;   // 64-bit signed integer
short smallNumber = 100;        // 16-bit signed integer
byte tinyNumber = 255;          // 8-bit unsigned integer

// Floating-point types
float singlePrecision = 3.14f;      // 32-bit floating point
double doublePrecision = 3.14159;   // 64-bit floating point (default)
decimal highPrecision = 99.99m;     // 128-bit decimal (for financial calculations)

// Display the values
Console.WriteLine($"Integer: {wholeNumber}");
Console.WriteLine($"Double: {doublePrecision}");
Console.WriteLine($"Decimal: {highPrecision}");

// Character type (single character)
char grade = 'A';
char symbol = '#';

// String type (sequence of characters)
string message = "Hello, C#!";
string multiLine = @"This is a
multi-line
string using verbatim syntax";

// String interpolation
string firstName = "Alice";
int age = 30;
string greeting = $"Hello, my name is {firstName} and I'm {age} years old.";

Console.WriteLine(greeting);

// Boolean type (true or false)
bool isLoggedIn = true;
bool hasPermission = false;

// Boolean expressions
bool isAdult = age >= 18;
bool canVote = isAdult && isLoggedIn;

Console.WriteLine($"Can vote: {canVote}");

// The compiler automatically determines the type
var number = 42;        // Inferred as int
var text = "Hello";     // Inferred as string
var price = 19.99;      // Inferred as double
var isReady = true;     // Inferred as bool

// You can still see the inferred type in your IDE
Console.WriteLine(number.GetType());    // System.Int32
Console.WriteLine(text.GetType());      // System.String

// Using the const keyword
const double PI = 3.14159;
const string COMPANY_NAME = "TechCorp";
const int MAX_USERS = 1000;

// Using readonly (can be assigned in constructor)
readonly DateTime creationDate = DateTime.Now;

Console.WriteLine($"Pi value: {PI}");
Console.WriteLine($"Company: {COMPANY_NAME}");

// Automatic conversion (no data loss)
int smallNumber = 100;
long bigNumber = smallNumber;    // int to long (safe)
double decimalNumber = smallNumber;  // int to double (safe)

Console.WriteLine($"Long: {bigNumber}, Double: {decimalNumber}");

// Manual conversion (potential data loss)
double pi = 3.14159;
int wholePi = (int)pi;          // Truncates decimal part
Console.WriteLine($"Original: {pi}, Truncated: {wholePi}");

// Using Convert class
string numberText = "123";
int convertedNumber = Convert.ToInt32(numberText);
bool convertedBool = Convert.ToBoolean("true");

Console.WriteLine($"Converted number: {convertedNumber}");
Console.WriteLine($"Converted boolean: {convertedBool}");

// Safe conversion that doesn't throw exceptions
string userInput = "not a number";

if (int.TryParse(userInput, out int result))
{
    Console.WriteLine($"Successfully converted: {result}");
}
else
{
    Console.WriteLine("Conversion failed. Invalid input.");
}

// TryParse with different types
string boolText = "true";
if (bool.TryParse(boolText, out bool boolResult))
{
    Console.WriteLine($"Boolean value: {boolResult}");
}

// Nullable value types
int? nullableInt = null;
bool? nullableBool = null;
DateTime? nullableDate = null;

// Checking for null values
if (nullableInt.HasValue)
{
    Console.WriteLine($"Value: {nullableInt.Value}");
}
else
{
    Console.WriteLine("Value is null");
}

// Null coalescing operator
int actualValue = nullableInt ?? 0;  // Use 0 if null
Console.WriteLine($"Actual value: {actualValue}");

string text = "  Hello, World!  ";

// String properties and methods
Console.WriteLine($"Length: {text.Length}");
Console.WriteLine($"Trimmed: '{text.Trim()}'");
Console.WriteLine($"Upper case: {text.ToUpper()}");
Console.WriteLine($"Lower case: {text.ToLower()}");
Console.WriteLine($"Contains 'World': {text.Contains("World")}");

// String manipulation
string replaced = text.Replace("World", "C#");
string[] words = text.Trim().Split(' ');

Console.WriteLine($"Replaced: {replaced}");
Console.WriteLine($"First word: {words[0]}");

// Array of integers
int[] numbers = { 1, 2, 3, 4, 5 };
Console.WriteLine($"First number: {numbers[0]}");
Console.WriteLine($"Array length: {numbers.Length}");

// Array of strings
string[] names = new string[3];
names[0] = "Alice";
names[1] = "Bob";
names[2] = "Charlie";

Console.WriteLine($"Second name: {names[1]}");

using System;

// Create variables of different types
// Convert between types
// Display the results with proper formatting
// Handle user input safely

Console.Write("Enter your age: ");
string ageInput = Console.ReadLine();

if (int.TryParse(ageInput, out int age))
{
    // Calculate birth year
    int currentYear = DateTime.Now.Year;
    int birthYear = currentYear - age;
    
    Console.WriteLine($"You were born in approximately {birthYear}");
    Console.WriteLine($"In 10 years, you'll be {age + 10} years old");
}
else
{
    Console.WriteLine("Please enter a valid number for your age.");
}