File I/O in C#

1. Quick & Easy: File Class

Small files? One-liners. Kunal reads today's menu.

string text = File.ReadAllText("menu.txt"); // Whole file
string[] lines = File.ReadAllLines("menu.txt"); // Array of lines
File.WriteAllText("receipt.txt", "Order #123 paid"); // Overwrite

2. Big Files: Stream It

5GB log file? Don't load all to RAM. Read line by line. Prerna processes customer data without crashing her laptop.

using var reader = new StreamReader("biglog.txt");
string line;
while ((line = reader.ReadLine())!= null) {
    Process(line); // One line at a time
} // using auto-closes file

3. Paths: Use Path.Combine

Never hardcode \ or /. Different OS, different slash.

string path = Path.Combine("orders", "2025", "jan.txt");
// Windows: orders\2025\jan.txt
// Linux: orders/2025/jan.txt

1. Why File I/O Matters

Apps read configs, write logs, save user data, process uploads. Bad file handling = locked files, memory spikes, data corruption. Good handling = fast, safe, scalable.

2. Synchronous vs Asynchronous I/O

// Sync: blocks thread during disk wait
string data = File.ReadAllText("data.txt");

// Async: releases thread, scales better
string data = await File.ReadAllTextAsync("data.txt");

Web server handling 1000 requests: sync ties up 1000 threads waiting on disk. Async uses maybe 10 threads. Huge difference.

3. StreamReader/StreamWriter: Control + Performance

Streams give you buffering, encoding, and line-by-line control. Default buffer = 4KB.

using var writer = new StreamWriter("output.txt", append: true);
await writer.WriteLineAsync("New order");
await writer.FlushAsync(); // Force write to disk

4. FileInfo & DirectoryInfo: Metadata + Operations

var file = new FileInfo("report.pdf");
if (file.Exists && file.Length > 1_000_000) {
    file.Delete(); // Over 1MB, delete it
}

Directory.CreateDirectory("backups"); // Safe even if exists

5. Encoding & Gotchas

Default is UTF-8 without BOM. Reading ANSI files? Specify encoding or you get weird chars.

var text = File.ReadAllText("legacy.txt", Encoding.GetEncoding(1252));

6. Memory: Buffers & Disposal

File.ReadAllText loads entire file into a string on heap. 1GB file = 2GB string due to UTF-16. Crash. StreamReader reads 4KB chunks into buffer, reuses it. Memory = O(1). Sanju forgot using. File handle stays open until GC. Next process can't access it. using calls Dispose which calls Close which releases OS handle immediately.