使用C#上传CSV至FTP服务器时遭遇“无法访问已关闭流”错误

问题场景 #

开发CSV文件上传至FTP服务器的应用时，遇到错误：

can not access the closed stream

相关代码示例：

public Stream WriteCSV<T>(List<T> content) where T : class
{
    var stream = new MemoryStream();
    using (StreamWriter writer = new StreamWriter(stream))
    {
        using (CsvWriter csv = new CsvWriter(writer, CultureInfo.InvariantCulture))
        {
            csv.WriteRecords(content);
            writer.Flush();
        }
    }
    return stream;
}

// calling method
var _writeService = new WriteService();
using var stream = _writerService.WriteCSV(partLocatorFiltered.ToList());
var fileUploadPath = $"/input/partslocator042/KE5206205_TEST_CSV.csv";
await _iFtpClientService.SendFileAsync(_options?.FTP, stream, fileUploadPath);

错误原因 #

StreamWriter的默认行为是：当它被using块释放时，会自动关闭它所包裹的底层MemoryStream。你的代码中，using (StreamWriter writer = new StreamWriter(stream))块结束后，stream已经被关闭，后续调用SendFileAsync时尝试访问已关闭的流，就会触发该错误。

解决方法 #

1. 让StreamWriter释放时不关闭底层流 #

创建StreamWriter时，指定leaveOpen: true参数，这样using块结束后只会释放StreamWriter本身，不会关闭底层的MemoryStream：

public Stream WriteCSV<T>(List<T> content) where T : class
{
    var stream = new MemoryStream();
    // 关键：设置leaveOpen: true，保留底层流不关闭
    using (StreamWriter writer = new StreamWriter(stream, leaveOpen: true))
    {
        using (CsvWriter csv = new CsvWriter(writer, CultureInfo.InvariantCulture))
        {
            csv.WriteRecords(content);
            writer.Flush();
        }
    }
    // 重置流的位置到起始处，确保FTP能读取到完整内容
    stream.Position = 0;
    return stream;
}

2. 额外注意点 #

写入完成后必须将stream.Position设置为0，因为写入操作会把流的指针移到末尾，如果不重置，FTP上传时会从流的末尾开始读取，最终上传空白文件。

内容的提问来源于stack exchange，提问作者Kamran Khan