C# 百分比计算突然变为负数

Question

我有一个程序可以下载一些讲座视频以供离线观看。我使用下面的代码来下载和计算百分比。大约 32-33% 的计算百分比总是负数，但以一种奇怪的方式 (33 -> -33 -> -32 -> -31 -> -30 等...)直到它再次达到 33。至此下载完成。我什至手动计算。调试输出也在代码下方。

这种情况一直在发生，所以我肯定做错了什么，但是在哪里呢？

private async Task CopyStream(
      Lecture lecture,
      Stream source,
      Stream destination,
      int sourceLength,
      CancellationToken token,
      int bufferSize = (16 * 1024))
    {
      var buffer = new byte[bufferSize];

      if (sourceLength <= 0) return;

      var totalBytesCopied = 0;
      var bytesRead = -1;

      while (bytesRead != 0)
      {
        bytesRead = await source.ReadAsync(buffer, 0, buffer.Length, token);

        if (bytesRead == 0) break;

        await destination.WriteAsync(buffer, 0, buffer.Length, token);

        totalBytesCopied += bytesRead;

        var progress = (int)Math.Round((double)(100 * totalBytesCopied) / sourceLength);

        Debug.WriteLine(
          $"Lecture={lecture.Title}, " +
          $"sourceLength={sourceLength}, " +
          $"totalBytesCopied={totalBytesCopied}, " +
          $"bytesRead={bytesRead}, progress={progress}");

        RaiseLectureDownloadProgressChangedEvent(null,
            new LectureDownloadProgressChangedEventArgs(lecture, progress));
      }
    }

Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21404088, bytesRead=16384, progress=33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21420472, bytesRead=16384, progress=33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21436856, bytesRead=16384, progress=33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21453240, bytesRead=16384, progress=33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21469624, bytesRead=16384, progress=33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21486008, bytesRead=16384, progress=-33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21502392, bytesRead=16384, progress=-33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21518776, bytesRead=16384, progress=-33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21535160, bytesRead=16384, progress=-33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21551544, bytesRead=16384, progress=-33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21567928, bytesRead=16384, progress=-33
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21584312, bytesRead=16384, progress=-32
Lecture=Lecture1, sourceLength=65777753, totalBytesCopied=21600696, bytesRead=16384, progress=-32

Answer 1

Lasse V. Karlsen 在评论中给出了正确的解释。

要修复它，请更改：

var progress = (int) Math.Round((double) (100 * totalBytesCopied) / sourceLength);

进入：

var progress = (int) Math.Round(((double)100) * totalBytesCopied / sourceLength);

或者简单地说：

var progress = (int) Math.Round(100.0 * totalBytesCopied / sourceLength);

正如他所说，这将迫使一切都以浮点形式完成。在这种情况下，整数将自动（隐式）转换为 double 。我删除了一个多余的括号（但你也可以保留它）。

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