C# 非同步程式設計基礎（二·下）：BackgroundWorker 事件處理實作與檔案讀取進度回報

DoWork 事件：背景執行緒執行耗時操作

private void BackgroundWorker_DoWork(object sender, DoWorkEventArgs e) {
    long fileLength;
    using (var sr = new StreamReader(_filePath)) {
        fileLength = sr.ReadToEnd().Length; // 先讀完取得長度 (效能較差但準確)
    }
    long onePercent = fileLength / 100;
    long current = 0;
    using (var sr = new StreamReader(_filePath)) {
        while (!sr.EndOfStream) {
            sr.Read(); current++;
            while (current > onePercent * (i+1)) { // i 為已回報百分比
                ((BackgroundWorker)sender).ReportProgress(++i);
            }
        }
        e.Result = current; // 傳遞給 RunWorkerCompleted
    }
}

註：作者承認先讀完再處理效能不佳，因 StreamReader.BaseStream.Length 或 FileInfo.Length 導致許多檔案讀不到 100% 就結束，歡迎高手指教改進。

ProgressChanged 事件：UI 執行緒接收進度

private void BackgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e) {
    Console.WriteLine($"Reading: {e.ProgressPercentage}%");
}

RunWorkerCompleted 事件：完成/取消/錯誤統一處理

private void BackgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) {
    if (e.Cancelled) Console.WriteLine("已取消");
    else if (e.Error != null) Console.WriteLine($"錯誤: {e.Error.Message}");
    else Console.WriteLine($"完成，共讀取 {e.Result} 字元");
}