ফাইল ইনপুট/আউটপুট বা I/O (File I/O)

string path = "hello.txt";

// Write an entire file in one line
File.WriteAllText(path, "Hello from C#!\nThis is line 2.");
Console.WriteLine("File written!");

// Read the entire file in one line
string content = File.ReadAllText(path);
Console.WriteLine($"Content:\n{content}");

// Write lines (array of strings)
string[] shoppingList = { "Apples", "Bread", "Milk", "Eggs" };
File.WriteAllLines("shopping.txt", shoppingList);

// Read lines back as array
string[] lines = File.ReadAllLines("shopping.txt");
Console.WriteLine($"\nShopping list ({lines.Length} items):");
for (int i = 0; i < lines.Length; i++)
    Console.WriteLine($"  {i + 1}. {lines[i]}");

// Append to a file (adds to the end without erasing)
File.AppendAllText("shopping.txt", "\nCheese\nButter");
Console.WriteLine($"\nAfter appending:");
Console.WriteLine(File.ReadAllText("shopping.txt"));

// Check if file exists
Console.WriteLine($"\nhello.txt exists: {File.Exists(path)}");
Console.WriteLine($"ghost.txt exists: {File.Exists("ghost.txt")}");

File written!
Content:
Hello from C#!
This is line 2.

Shopping list (4 items):
  1. Apples
  2. Bread
  3. Milk
  4. Eggs

After appending:
Apples
Bread
Milk
Eggs
Cheese
Butter

hello.txt exists: True
ghost.txt exists: False

// Write with StreamWriter + using statement
using (var writer = new StreamWriter("journal.txt"))
{
    writer.WriteLine("Day 1: Started learning C#");
    writer.WriteLine("Day 2: Files are actually fun!");
    writer.WriteLine("Day 3: Built my first app");
}
// File is automatically closed here, even if an error occurred

// Read with StreamWriter — line by line
Console.WriteLine("Journal entries:");
using (var reader = new StreamReader("journal.txt"))
{
    string? line;
    int lineNum = 1;
    while ((line = reader.ReadLine()) != null)
    {
        Console.WriteLine($"  {lineNum}: {line}");
        lineNum++;
    }
}

// Modern C# shorthand: using declaration (no braces needed)
using var writer2 = new StreamWriter("notes.txt");
writer2.WriteLine("This using style disposes at end of scope");
writer2.WriteLine("Cleaner for simple cases!");
// writer2 is disposed when the enclosing scope ends

Console.WriteLine($"\nNotes: {File.ReadAllText("notes.txt")}");

Journal entries:
  1: Day 1: Started learning C#
  2: Day 2: Files are actually fun!
  3: Day 3: Built my first app

Notes: This using style disposes at end of scope
Cleaner for simple cases!

// Async file operations
await File.WriteAllTextAsync("async-test.txt", "Written asynchronously!");
string asyncContent = await File.ReadAllTextAsync("async-test.txt");
Console.WriteLine(asyncContent);

// Practical example: simple config reader
var config = new Dictionary<string, string>();
string configData = "theme=dark\nlanguage=en\nfont_size=14\nauto_save=true";
await File.WriteAllTextAsync("config.ini", configData);

string[] configLines = await File.ReadAllLinesAsync("config.ini");
foreach (string line in configLines)
{
    string[] parts = line.Split('=', 2);
    if (parts.Length == 2)
        config[parts[0].Trim()] = parts[1].Trim();
}

Console.WriteLine("\nConfig loaded:");
foreach (var pair in config)
    Console.WriteLine($"  {pair.Key} = {pair.Value}");

// Working with directories
string dir = "my_data";
if (!Directory.Exists(dir))
    Directory.CreateDirectory(dir);

await File.WriteAllTextAsync(Path.Combine(dir, "test.txt"), "Inside a folder!");

Console.WriteLine($"\nFiles in {dir}:");
foreach (string file in Directory.GetFiles(dir))
    Console.WriteLine($"  {Path.GetFileName(file)}");

Written asynchronously!

Config loaded:
  theme = dark
  language = en
  font_size = 14
  auto_save = true

Files in my_data:
  test.txt