อ่านและเขียนไฟล์ Text ใน C# - StreamReader และ StreamWriter

StreamReader และ StreamWriter เป็น Class หลักสำหรับอ่าน/เขียน Text File แบบ Streaming (Memory Efficient)

อ่านไฟล์ Text (StreamReader)

using System;
using System.IO;

public class Program
{
    public static void Main()
    {
        string path = @"D:\new.txt";

        try
        {
            // using = Auto Close/Dispose
            using (StreamReader sr = new StreamReader(path, Encoding.UTF8))
            {
                string line;
                while ((line = sr.ReadLine()) != null)
                {
                    Console.WriteLine(line);
                }
            }
        }
        catch (FileNotFoundException)
        {
            Console.WriteLine("File not found");
        }
        catch (IOException ex)
        {
            Console.WriteLine($"IO Error: {ex.Message}");
        }
    }
}

เขียนไฟล์ Text (StreamWriter)

using System;
using System.IO;

public class Program
{
    public static void Main()
    {
        string[] names = { "Jack", "Tom", "Alice" };
        string path = "students.txt";

        // เขียนไฟล์ (Overwrite)
        using (StreamWriter sw = new StreamWriter(path, false, Encoding.UTF8))
        {
            foreach (string name in names)
            {
                sw.WriteLine(name);
            }
        }

        // อ่านกลับมาแสดง
        Console.WriteLine("File Contents:");
        using (StreamReader sr = new StreamReader(path, Encoding.UTF8))
        {
            string line;
            while ((line = sr.ReadLine()) != null)
            {
                Console.WriteLine(line);
            }
        }
    }
}

ผลลัพธ์

File Contents:
Jack
Tom
Alice

StreamWriter Options

// Append Mode (true = Append, false = Overwrite)
using (var sw = new StreamWriter(path, append: true, Encoding.UTF8))
{
    sw.WriteLine("New Line Appended");
}

// AutoFlush - Flush ทุก Write (ช้ากว่า แต่ปลอดภัย)
using (var sw = new StreamWriter(path) { AutoFlush = true })
{
    sw.WriteLine("Immediate");
}

// NewLine Customization
using (var sw = new StreamWriter(path))
{
    sw.NewLine = "\n"; // Unix Line Ending
    sw.WriteLine("Line 1");
    sw.WriteLine("Line 2");
}

Modern APIs (.NET Core / .NET 5+)

// Async
await using (var sw = new StreamWriter(path))
{
    await sw.WriteLineAsync("Async Write");
}

await using (var sr = new StreamReader(path))
{
    while (await sr.ReadLineAsync() is { } line)
    {
        Console.WriteLine(line);
    }
}

// One-liners (Small Files)
await File.WriteAllTextAsync(path, content);
await File.WriteAllLinesAsync(path, lines);
string content = await File.ReadAllTextAsync(path);
string[] lines = await File.ReadAllLinesAsync(path);

// Streaming Lines (Lazy)
foreach (string line in File.ReadLines(path)) { } // IEnumerable
await foreach (string line in File.ReadLinesAsync(path)) { } // IAsyncEnumerable

Encoding Handling

// UTF-8 (Default, No BOM)
new StreamReader(path, new UTF8Encoding(false))

// UTF-8 with BOM
new StreamReader(path, Encoding.UTF8)

// Detect Encoding from BOM
new StreamReader(path, Encoding.UTF8, detectEncodingFromByteOrderMarks: true)

// Legacy Thai (TIS-620 / Windows-874)
new StreamReader(path, Encoding.GetEncoding("TIS-620"))

Error Handling Best Practices

public static async Task SafeReadAllLinesAsync(string path)
{
    try
    {
        return await File.ReadAllLinesAsync(path);
    }
    catch (FileNotFoundException)
    {
        throw new FileNotFoundException($"File not found: {path}");
    }
    catch (UnauthorizedAccessException)
    {
        throw new UnauthorizedAccessException($"Access denied: {path}");
    }
    catch (IOException ex) when (ex.HResult == -2147024864) // ERROR_SHARING_VIOLATION
    {
        throw new IOException($"File is locked by another process: {path}", ex);
    }
}

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