Reading and Writing Files

A file is a named block of data that stays on disk after your program ends. The types that read and write files live in the System.IO namespace. A namespace is a group of related types; writing using System.IO; at the top of a file lets you name those types without the namespace in front.

Whole-file calls

File.ReadAllText(path) opens the file, reads every character into one string, and closes the file. File.WriteAllText(path, text) creates the file, writes the string, and closes it. If the file already exists it is truncated, so the old contents are gone. File.AppendAllText(path, text) adds to the end instead. File.ReadAllLines(path) returns an array of strings, one per line.

These calls are shorthand. Each one opens a stream, does the work and closes it for you; the next lesson shows that longer form. Use the shorthand whenever the whole file fits comfortably in memory.

Building paths

Never glue path pieces together with +. Windows separates folders with a backslash, Linux and macOS with a forward slash. Path.Combine("data", "notes.txt") inserts the right one for the machine running the code. Path.GetFileName, Path.GetExtension and Path.GetFullPath take a path apart again.

Folders

Directory.CreateDirectory(path) creates every missing folder in the path, and does nothing if they all exist. Directory.EnumerateFiles(path, "*.txt") lists matching files, and the order it returns them in is not guaranteed, so sort if the order matters.

Any of these can throw: FileNotFoundException, DirectoryNotFoundException or UnauthorizedAccessException. Every reading and writing method on File also has an Async twin, such as File.ReadAllTextAsync, which becomes useful once Stage 4 introduces await. The Path and Directory members shown here have no async versions.

using System;
using System.IO;

// Path.Combine picks the right separator for this operating system.
string folder = Path.Combine("data", "notes");
Directory.CreateDirectory(folder);   // no error if it already exists

string file = Path.Combine(folder, "today.txt");

File.WriteAllText(file, "milk\neggs\n");   // replaces the file if it exists
File.AppendAllText(file, "bread\n");       // adds to the end

string text = File.ReadAllText(file);
Console.Write(text);

string[] lines = File.ReadAllLines(file);
Console.WriteLine($"lines: {lines.Length}");
Console.WriteLine($"name: {Path.GetFileName(file)}");
Console.WriteLine($"exists: {File.Exists(file)}");

// Output:
// milk
// eggs
// bread
// lines: 3
// name: today.txt
// exists: True

using System;
using System.IO;

string path = Path.Combine("data", "ids.txt");
Directory.CreateDirectory("data");
File.WriteAllLines(path, new[] { "17", "4", "23" });

// ReadAllLines builds an array, so every line sits in memory at once.
string[] all = File.ReadAllLines(path);
Console.WriteLine($"count: {all.Length}");

// ReadLines hands back one line at a time, so a huge file still works.
int total = 0;
foreach (string line in File.ReadLines(path))
{
    total += int.Parse(line);
}

Console.WriteLine($"total: {total}");

// Output:
// count: 3
// total: 44

using System;
using System.IO;
using System.Linq;

string folder = Path.Combine("data", "reports");
Directory.CreateDirectory(folder);
File.WriteAllText(Path.Combine(folder, "june.csv"), "x");
File.WriteAllText(Path.Combine(folder, "may.csv"), "x");
File.WriteAllText(Path.Combine(folder, "notes.txt"), "x");

// EnumerateFiles returns full paths, in no guaranteed order, so sort them.
foreach (string path in Directory.EnumerateFiles(folder, "*.csv").Order())
{
    Console.WriteLine(Path.GetFileName(path));
}

// Output:
// june.csv
// may.csv

using System;
using System.IO;

string path = Path.Combine("data", "settings.json");
Directory.CreateDirectory("data");

// A raw string literal (C# 11) holds quotes without backslashes.
File.WriteAllText(path, """{"theme":"dark"}""");

// Write the new content beside the real file, then swap it in.
string temp = path + ".tmp";
File.WriteAllText(temp, """{"theme":"light"}""");
File.Move(temp, path, overwrite: true);

Console.WriteLine(File.ReadAllText(path));
Console.WriteLine(File.Exists(temp));

// Output:
// {"theme":"light"}
// False