为何StringBuilder的Replace方法无法替换旧字符串为新字符串？

你的StringBuilder.Replace失效的核心原因是匹配字符串不包含换行符：

• 在LogStartDownload中，你通过logBuilder.AppendLine($"[Status] Downloading in progress")添加状态行，AppendLine会自动在字符串末尾追加Environment.NewLine（Windows下为\r\n），因此logBuilder中实际存储的是"[Status] Downloading in progress\r\n"。
• 但你在LogDownloadCompleted中调用Replace时，搜索的是不带换行的"[Status] Downloading in progress"，自然找不到匹配项，替换操作也就不会生效。

另外，调用逻辑中存在重复调用LogDownloadCompleted的情况，且GetLog会清空logBuilder，但这不是本次替换失效的主因。

方案1：修正匹配字符串（最简单直接） #

把Replace的搜索字符串改为包含换行符的版本，确保和AppendLine添加的内容完全一致：

public void LogDownloadCompleted(bool isSuccess, TimeSpan elapsedTime, bool isCancelled)
{
    this.elapsedTime = elapsedTime;
    string searchStr = $"[Status] Downloading in progress{Environment.NewLine}";
    if (!isCancelled)
    {
        logBuilder.Replace(searchStr, $"[Status]: Download {(isSuccess ? "Completed Successfully" : "Failed")}{Environment.NewLine}");
    }
    else
    {
        logBuilder.Replace(searchStr, $"[Status]: Download Cancelled{Environment.NewLine}");
    }
    // 后续代码不变
    logBuilder.AppendLine($"[Elapsed Time]: {FormatTimeSpan(elapsedTime)}");
    logBuilder.AppendLine($"[End Time]: {DateTime.Now}");
    logBuilder.AppendLine();
}

方案2：避免使用Replace，改用状态追踪（更健壮） #

如果后续日志结构可能调整，建议单独维护下载状态，最后统一生成日志，避免依赖字符串替换：

public class Logger
{
    private StringBuilder logBuilder = new StringBuilder();
    private TimeSpan elapsedTime;
    // 新增状态变量追踪当前下载状态
    private string currentStatus = string.Empty;

    public void LogStartDownload(string fileName, string fileUrl)
    {
        logBuilder.AppendLine($"[Start Time]: {DateTime.Now}");
        logBuilder.AppendLine($"[File Name]: {fileName}");
        logBuilder.AppendLine($"[File URL]: {fileUrl}");
        // 仅记录状态，暂不写入logBuilder
        currentStatus = "[Status] Downloading in progress";
    }

    public void LogDownloadProgress(double percentage, double speed, long totalBytes, long contentLength, TimeSpan elapsedTime)
    {
        this.elapsedTime = elapsedTime;
        logBuilder.AppendLine($"[Progress]: {percentage}%");
        logBuilder.AppendLine($"[Downloaded]: {FormatBytes(totalBytes)} / {FormatBytes(contentLength)}");
        logBuilder.AppendLine($"[Download Speed]: {FormatBytes(speed)}/s");
        logBuilder.AppendLine($"[Time Elapsed]: {FormatTimeSpan(elapsedTime)}");
    }

    public void LogDownloadCompleted(bool isSuccess, TimeSpan elapsedTime, bool isCancelled)
    {
        this.elapsedTime = elapsedTime;
        // 更新最终状态
        if (!isCancelled)
        {
            currentStatus = $"[Status]: Download {(isSuccess ? "Completed Successfully" : "Failed")}";
        }
        else
        {
            currentStatus = "[Status]: Download Cancelled";
        }
        // 将状态插入到日志开头（可根据需求调整位置）
        logBuilder.Insert(0, currentStatus + Environment.NewLine);
        
        logBuilder.AppendLine($"[Elapsed Time]: {FormatTimeSpan(elapsedTime)}");
        logBuilder.AppendLine($"[End Time]: {DateTime.Now}");
        logBuilder.AppendLine();
    }

    // 其余代码不变
}

额外优化：修正调用逻辑中的重复调用 #

在DownloadManager_DownloadCompleted中，存在两次调用LogDownloadCompleted的情况，会导致日志重复，建议调整为只调用一次：

private void DownloadManager_DownloadCompleted(object? sender, DownloadingCore.DownloadeCompletedEventArgs e)
{
    progressBar1.Value = 0;
    _stopwatch.Stop();

    if (e.IsSuccess && downloadsCounter > 0)
    {
        downloadsCounter--;
    }

    bool needLogCompleted = true;
    if(isCancelled && fileSavePath != null)
    {
        if (File.Exists(fileSavePath))
        {
            File.Delete(fileSavePath);
        }
        logger.LogDownloadCancelled();
        needLogCompleted = false;
    }

    if (needLogCompleted)
    {
        logger.LogDownloadCompleted(e.IsSuccess, _elapsedTime, isCancelled);
    }

    if (downloadsCounter == 0)
    {
        logger.LogDownloadCompleted();
        Download.Enabled = true;
    }

    logger.GetLog(richTextBoxLogger);
}

内容的提问来源于stack exchange，提问作者Shien