File I/O

## Learning Objectives - Read and write text files - Use StreamReader and StreamWriter - Work with file paths - Handle file operations safely ## File Class (Static Methods) ### Simple Operations ```csharp using System.IO; // Read all text string content = File.ReadAllText("file.txt"); // Write all text File.WriteAllText("output.txt", "Hello, World!"); // Read all lines string[] lines = File.ReadAllLines("file.txt"); // Write all lines File.WriteAllLines("output.txt", new[] { "Line 1", "Line 2" }); ``` ### File Methods ```csharp File.Exists("file.txt"); // Check if exists File.Copy("source.txt", "dest.txt"); // Copy File.Copy("source.txt", "dest.txt", true); // Overwrite File.Move("source.txt", "dest.txt"); // Move/Rename File.Delete("file.txt"); // Delete File.GetCreationTime("file.txt"); // Date created File.GetLastWriteTime("file.txt"); // Date modified File.GetLastAccessTime("file.txt"); // Date accessed ``` ## StreamReader ### Reading Text ```csharp using System.IO; // Read entire file using (StreamReader reader = new StreamReader("file.txt")) { string content = reader.ReadToEnd(); Console.WriteLine(content); } // Read line by line using (StreamReader reader = new StreamReader("file.txt")) { string? line; while ((line = reader.ReadLine()) != null) { Console.WriteLine(line); } } // Read single character using (StreamReader reader = new StreamReader("file.txt")) { int c = reader.Read(); // Returns int, -1 at EOF } ``` ### StreamReader Methods ```csharp reader.Peek(); // Look at next char without consuming reader.Read(); // Read single char reader.ReadLine(); // Read until newline reader.ReadToEnd(); // Read rest of file reader.EndOfStream; // true if at end ``` ## StreamWriter ### Writing Text ```csharp using System.IO; // Create/overwrite file using (StreamWriter writer = new StreamWriter("output.txt")) { writer.Write("Hello"); writer.WriteLine(" World"); writer.WriteLine("Line 3"); } // Append to file using (StreamWriter writer = new StreamWriter("output.txt", append: true)) { writer.WriteLine("Appended line"); } ``` ### StreamWriter Methods ```csharp writer.Write("text"); // Write without newline writer.WriteLine("text"); // Write with newline writer.Flush(); // Force write to disk writer.AutoFlush = true; // Auto flush after each write ``` ## Using Statement ### Automatic Disposal ```csharp // Correct - using ensures disposal using (StreamReader reader = new StreamReader("file.txt")) { string content = reader.ReadToEnd(); } // reader is disposed here // Wrong - file may stay locked StreamReader reader2 = new StreamReader("file.txt"); string content = reader2.ReadToEnd(); reader2.Dispose(); // Must manually dispose ``` ### Nested Using ```csharp using (StreamReader reader = new StreamReader("input.txt")) using (StreamWriter writer = new StreamWriter("output.txt")) { string line; while ((line = reader.ReadLine()) != null) { writer.WriteLine(line.ToUpper()); } } ``` ## File Paths ### Path Class ```csharp using System.IO; string path = @"C:\Users\John\Documents\file.txt"; Path.GetFileName(path); // "file.txt" Path.GetFileNameWithoutExtension(path); // "file" Path.GetExtension(path); // ".txt" Path.GetDirectoryName(path); // "C:\Users\John\Documents" Path.GetFullPath(path); // Full path Path.Combine("folder", "file.txt"); // "folder\file.txt" ``` ### Path Methods ```csharp Path.ChangeExtension(path, ".md"); // Change extension Path.HasExtension(path); // true Path.IsPathRooted(path); // true if absolute Path.TrimEndingExtension(path); // Without extension ``` ## Directory Operations ### Directory Class ```csharp using System.IO; // Create directory Directory.CreateDirectory("newfolder"); // Check exists Directory.Exists("folder"); // true/false // Get files string[] files = Directory.GetFiles("folder"); string[] txtFiles = Directory.GetFiles("folder", "*.txt"); // Get directories string[] dirs = Directory.GetDirectories("folder"); // Delete (must be empty) Directory.Delete("folder"); Directory.Delete("folder", recursive: true); // With contents ``` ### DirectoryInfo ```csharp DirectoryInfo dir = new DirectoryInfo("."); Console.WriteLine(dir.Name); // Current folder name Console.WriteLine(dir.FullName); // Full path Console.WriteLine(dir.Parent); // Parent directory Console.WriteLine(dir.Root); // Root directory foreach (FileInfo file in dir.GetFiles()) { Console.WriteLine($"{file.Name} ({file.Length} bytes)"); } ``` ## FileInfo ### Detailed File Information ```csharp using System.IO; FileInfo file = new FileInfo("file.txt"); Console.WriteLine(file.Exists); // true/false Console.WriteLine(file.Name); // "file.txt" Console.WriteLine(file.Extension); // ".txt" Console.WriteLine(file.Length); // Size in bytes Console.WriteLine(file.CreationTime); // When created Console.WriteLine(file.LastWriteTime); // When modified Console.WriteLine(file.LastAccessTime); // When accessed Console.WriteLine(file.DirectoryName); // Containing folder ``` ## Working with JSON ### System.Text.Json (C# 8+) ```csharp using System.Text.Json; // Serialize class Person { public string Name { get; set; } public int Age { get; set; } } Person person = new Person { Name = "Alice", Age = 30 }; string json = JsonSerializer.Serialize(person); // Deserialize Person? p = JsonSerializer.Deserialize(json); // With options var options = new JsonSerializerOptions { WriteIndented = true, PropertyNameCaseInsensitive = true }; string prettyJson = JsonSerializer.Serialize(person, options); ``` ## Working with CSV ### Manual Parsing ```csharp string[] lines = File.ReadAllLines("data.csv"); foreach (string line in lines) { string[] fields = line.Split(','); Console.WriteLine($"Name: {fields[0]}, Age: {fields[1]}"); } ``` ### Writing CSV ```csharp using System.Text; var sb = new StringBuilder(); sb.AppendLine("Name,Age,City"); foreach (Person p in people) { sb.AppendLine($"{p.Name},{p.Age},{p.City}"); } File.WriteAllText("output.csv", sb.ToString()); ``` ## Exception Handling with Files ### Safe File Operations ```csharp try { string content = File.ReadAllText("file.txt"); } catch (FileNotFoundException) { Console.WriteLine("File not found"); } catch (IOException ex) { Console.WriteLine($"IO Error: {ex.Message}"); } // Safer approach if (File.Exists("file.txt")) { string content = File.ReadAllText("file.txt"); } ``` ## Summary - File class: static methods for simple operations - StreamReader/StreamWriter: detailed control - Always use `using` statement for proper disposal - Path class: manipulate file paths safely - Directory class: folder operations - FileInfo/DirectoryInfo: object-oriented approach - System.Text.Json for JSON serialization