📝 Lesson 4.1: Working with Files and Streams

Why Files?

Data stored in variables lives only in memory — when the program ends, it's gone. To persist data between runs, you write it somewhere durable. The simplest option is a file on disk.

.NET gives you two levels of tools, and you'll use both:

• The File class — simple one-call methods for reading/writing an entire file. Perfect for small files.
• Streams (StreamReader/StreamWriter) — read or write a little at a time, ideal for large files.

The File Class

The File class has convenient static methods that handle the whole read or write in one call:

using System.IO;   // (modern templates often include this implicitly)

// Write a whole string to a file (creates or overwrites)
File.WriteAllText("greeting.txt", "Hello, file!");

// Read it all back
string content = File.ReadAllText("greeting.txt");
Console.WriteLine(content);            // Hello, file!

// Append without overwriting
File.AppendAllText("greeting.txt", "\nA second line.");

For line-based data, use the array-oriented versions — they pair naturally with collections and LINQ:

string[] lines = { "Ada", "Grace", "Alan" };
File.WriteAllLines("names.txt", lines);

string[] readBack = File.ReadAllLines("names.txt");
Console.WriteLine($"{readBack.Length} names");   // 3 names
foreach (string name in readBack)
{
    Console.WriteLine(name);
}

Building Paths Safely

File paths differ across operating systems (Windows uses \, macOS/Linux use /). Never hand-build paths with string concatenation — use the Path class, which does it correctly everywhere:

// ❌ Fragile — wrong separator on some systems
string bad = "data" + "\\" + "notes.txt";

// ✅ Correct on every OS
string good = Path.Combine("data", "notes.txt");

Console.WriteLine(Path.GetFileName(good));        // notes.txt
Console.WriteLine(Path.GetExtension(good));       // .txt
Console.WriteLine(Path.GetFileNameWithoutExtension(good));   // notes

string docs = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
string filePath = Path.Combine(docs, "notes.txt");

Existence & Exceptions

Reading a file that doesn't exist throws a FileNotFoundException (from Lesson 1.1). Guard against it — either check first, or wrap the read in try/catch:

string path = "notes.txt";

// Option 1: check before reading
if (File.Exists(path))
{
    string text = File.ReadAllText(path);
    Console.WriteLine(text);
}
else
{
    Console.WriteLine("No notes file yet.");
}

// Option 2: handle the exception
try
{
    string text = File.ReadAllText(path);
    Console.WriteLine(text);
}
catch (FileNotFoundException)
{
    Console.WriteLine("File not found.");
}
catch (IOException ex)
{
    Console.WriteLine($"A file error occurred: {ex.Message}");
}

Streams and using

The File.ReadAllText helpers load the entire file into memory at once — fine for small files, wasteful (or impossible) for huge ones. A stream reads or writes a bit at a time, so memory use stays low regardless of file size.

// Read a large file line by line with a StreamReader
using (StreamReader reader = new StreamReader("big-log.txt"))
{
    string? line;
    while ((line = reader.ReadLine()) != null)   // null means end of file
    {
        Console.WriteLine(line);
    }
}   // reader is automatically closed and disposed here

Notice the using statement. A StreamReader holds an operating-system file handle that must be released. using guarantees that happens — even if an exception is thrown inside the block:

Modern C# also offers a tidier using declaration — no braces; the resource is disposed at the end of the enclosing scope:

void WriteReport()
{
    using StreamWriter writer = new StreamWriter("report.txt");
    writer.WriteLine("Line 1");
    writer.WriteLine("Line 2");
}   // writer disposed here, at the end of the method

Exercise & Quiz

string path = Path.Combine(".", "notes.txt");

// TODO: if the file exists, read and print existing notes

Console.Write("Enter a new note: ");
string note = Console.ReadLine() ?? "";

// TODO: append the note (+ newline) to the file, inside try/catch

string path = Path.Combine(".", "notes.txt");

try
{
    if (File.Exists(path))
    {
        Console.WriteLine("Your notes so far:");
        Console.WriteLine(File.ReadAllText(path));
    }
    else
    {
        Console.WriteLine("No notes yet.");
    }

    Console.Write("Enter a new note: ");
    string note = Console.ReadLine() ?? "";

    File.AppendAllText(path, note + Environment.NewLine);
    Console.WriteLine("Note saved!");
}
catch (IOException ex)
{
    Console.WriteLine($"Could not access the notes file: {ex.Message}");
}

Your notes so far:
Buy milk
Walk dog

Enter a new note: _

Summary

📚 Additional Resources

• File class — API reference
• File and stream I/O — .NET
• The using statement — reference

🚀 What's Next?

You can save and load text — but real apps store structured data. In Lesson 4.2: JSON Serialization, you'll convert your objects (records and classes) to and from JSON, so you can save and reload them whole.