File Handling

Learn how to read from and write to files using the System.IO namespace in C#.

The System.IO Namespace

C#'s `System.IO` namespace provides everything needed to work with files and directories — from simple one-line helpers to fine-grained streaming APIs.

Writing to a File

using System.IO;

string path = "notes.txt";
File.WriteAllText(path, "Hello, file system!");

// Append more content
File.AppendAllText(path, "\nSecond line.");

Reading from a File

string content = File.ReadAllText(path);
Console.WriteLine(content);

// Or read line by line
foreach (string line in File.ReadAllLines(path))
{
    Console.WriteLine($"Line: {line}");
}

Output

Click Run to see what this code prints.

Handling File Errors

File operations can fail for many reasons — a missing file, no permissions, or another process holding it open — so always wrap them in exception handling.

try
{
    string content = File.ReadAllText("missing.txt");
}
catch (FileNotFoundException)
{
    Console.WriteLine("That file doesn't exist.");
}

FAQs

What's the difference between File.ReadAllText and StreamReader?

File.ReadAllText loads the entire file into memory at once — simple, fine for small files. StreamReader reads incrementally, better for very large files.

Summary

System.IO provides File and Directory helpers for common tasks.
File.WriteAllText/ReadAllText handle simple whole-file operations.
Always handle exceptions around file I/O — the file system is inherently unreliable.

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