C# 文件讀寫實戰：文本模式、二進位模式與通訊錄完整案例

.NET 提供豐富的讀寫 API，針對不同資料性質選擇正確模式，事半功倍。

文本模式：StreamReader / StreamWriter

• 適用：日誌、設定檔、CSV、JSON、XML、原始碼等人類可讀資料
• 自動處理編碼（預設 UTF-8，支援 BOM 偵測）、換行符正規化
• 靜態工廠：File.OpenText(path)、File.CreateText(path)

讀取範例

using var sr = File.OpenText(@"C:\data\log.txt");
while (sr.Peek() >= 0) {
    string line = sr.ReadLine();
    Console.WriteLine(line);
}

寫入範例

using var sw = File.CreateText(@"C:\data\output.txt");
sw.WriteLine("Web Address:");
sw.WriteLine("https://example.com");
sw.WriteLine("https://github.com");

二進位模式：BinaryReader / BinaryWriter

• 適用：高效序列化、自訂檔案格式、網路封包、影像/音訊資料
• 強型別方法對稱：ReadInt32/Write、ReadDouble/Write、ReadString/Write、ReadBytes/Write
• 嚴格依賴讀寫順序與型別一致，不自處理編碼

完整案例：通訊錄管理系統

struct Record {
    public string Name;
    public ushort Age;
    public string Phone;
    public string Address;
}

class PhoneBook {
    private readonly string _path;
    public PhoneBook(string path) => _path = path;

    public void Add() {
        using var fs = new FileStream(_path, FileMode.OpenOrCreate, FileAccess.Write);
        using var bw = new BinaryWriter(fs);
        bw.BaseStream.Seek(0, SeekOrigin.End);

        Console.Write("姓名："); var name = Console.ReadLine();
        Console.Write("年齡："); ushort age = ushort.Parse(Console.ReadLine());
        Console.Write("電話："); var phone = Console.ReadLine();
        Console.Write("地址："); var addr = Console.ReadLine();

        bw.Write(name);
        bw.Write(age);
        bw.Write(phone);
        bw.Write(addr);
    }

    public void List() {
        using var fs = new FileStream(_path, FileMode.Open, FileAccess.Read);
        using var br = new BinaryReader(fs);
        Console.WriteLine($"{"姓名",-10}{"年齡",-6}{"電話",-15}{"地址"}");
        while (fs.Position < fs.Length) {
            try {
                var r = new Record {
                    Name = br.ReadString(),
                    Age = br.ReadUInt16(),
                    Phone = br.ReadString(),
                    Address = br.ReadString()
                };
                Console.WriteLine($"{r.Name,-10}{r.Age,-6}{r.Phone,-15}{r.Address}");
            } catch (EndOfStreamException) { break; }
        }
    }

    public void Erase() {
        File.WriteAllBytes(_path, Array.Empty());
        Console.WriteLine("通訊錄已清空");
    }
}

// 選單驅動
class Program {
    static void Main() {
        var pb = new PhoneBook(@"C:\data\phonebook.dat");
        while (true) {
            Console.WriteLine("1:列表 2:新增 3:清空 0:退出");
            switch (Console.ReadLine()) {
                case "1": pb.List(); break;
                case "2": pb.Add(); break;
                case "3": pb.Erase(); break;
                case "0": return;
            }
        }
    }
}

格式化輸出技巧

// 複合格式：{索引,對齊寬度:格式} 負寬度=靠左
Console.WriteLine($"{r.Name,-10}{r.Age,-6}{r.Phone,-15}{r.Address}");
// 等價於
Console.WriteLine("{0,-10}{1,-6}{2,-15}{3}", r.Name, r.Age, r.Phone, r.Address);

選型決策表

掌握模式選擇與 API 組合，是高效處理檔案資料的關鍵。