File I/O — C# System.IO

S5.0 What This Teaches

• Reading and writing text files with File convenience methods
• Streaming reads and writes with StreamReader / StreamWriter
• Working with paths using Path and DirectoryInfo
• Directory enumeration
• Reading and writing binary data

S5.1 File Convenience Methods

using System.IO;

// Write all text at once (creates or overwrites)
File.WriteAllText("notes.txt", "Hello, file!");

// Append text
File.AppendAllText("notes.txt", "\nSecond line.");

// Read all text
string content = File.ReadAllText("notes.txt");
Console.WriteLine(content);

// Read all lines into an array
string[] lines = File.ReadAllLines("notes.txt");
foreach (string line in lines)
    Console.WriteLine(line);

// Write multiple lines
File.WriteAllLines("list.txt", new[] { "alpha", "beta", "gamma" });

S5.2 StreamReader and StreamWriter

// Write line by line
using var writer = new StreamWriter("output.txt");
for (int i = 1; i <= 5; i++)
    writer.WriteLine($"Line {i}");
// writer.Dispose() called automatically at end of scope

// Read line by line - efficient for large files
using var reader = new StreamReader("output.txt");
string? line;
while ((line = reader.ReadLine()) != null)
    Console.WriteLine(line);

// Append to existing file
using var appender = new StreamWriter("output.txt", append: true);
appender.WriteLine("Appended line");

S5.3 Path and Directory Operations

string full = Path.GetFullPath("notes.txt");
string dir  = Path.GetDirectoryName(full)!;
string name = Path.GetFileName(full);        // "notes.txt"
string stem = Path.GetFileNameWithoutExtension(full);  // "notes"
string ext  = Path.GetExtension(full);       // ".txt"

// Build paths in a platform-safe way
string path = Path.Combine(dir, "subdir", "file.txt");

// Test existence
bool fileExists = File.Exists("notes.txt");
bool dirExists  = Directory.Exists("mydir");

// Create and delete
Directory.CreateDirectory("newdir");
Directory.Delete("newdir", recursive: false);
File.Delete("notes.txt");

// Enumerate files
foreach (string f in Directory.EnumerateFiles(".", "*.txt"))
    Console.WriteLine(Path.GetFileName(f));

S5.4 Binary Files

// Write bytes
byte[] data = { 1, 2, 3, 4, 5 };
File.WriteAllBytes("data.bin", data);

// Read bytes
byte[] read = File.ReadAllBytes("data.bin");
Console.WriteLine(read.Length);   // 5

// BinaryWriter / BinaryReader for typed data
using var bw = new BinaryWriter(File.OpenWrite("record.bin"));
bw.Write(42);        // int (4 bytes)
bw.Write(3.14);      // double (8 bytes)
bw.Write("hello");   // length-prefixed string

using var br = new BinaryReader(File.OpenRead("record.bin"));
Console.WriteLine(br.ReadInt32());   // 42
Console.WriteLine(br.ReadDouble());  // 3.14
Console.WriteLine(br.ReadString());  // hello

S5.5 Example - All Together

// File I/O - write CSV and read it back, counting lines.

using System.IO;

string csvPath = "scores.csv";

// Write CSV
var records = new[] { ("Alice", 92), ("Bob", 78), ("Carol", 95) };
using (var w = new StreamWriter(csvPath))
{
    w.WriteLine("Name,Score");
    foreach (var (name, score) in records)
        w.WriteLine($"{name},{score}");
}

// Read and process
int count = 0;
int total = 0;
using (var r = new StreamReader(csvPath))
{
    r.ReadLine();   // skip header
    string? line;
    while ((line = r.ReadLine()) != null)
    {
        var parts = line.Split(',');
        total += int.Parse(parts[1]);
        count++;
    }
}
Console.WriteLine($"Average score: {total / (double)count:F1}");

Average score: 88.3

S5.6 Exercise

S5.7 Common Mistakes

string path = dir + "\\" + name;   // breaks on Linux (different separator)
string path = Path.Combine(dir, name);  // correct - portable

S5.8 Key Terms