SerializationException:“类型 . . in assembly…未标记为serializa

在作为并行下载管理器的双工WCF服务中，我有Downloader类，它表示每个下载状态和一个包含Downloader实例的ObservableCollection。当我试图通过BinaryFormatter序列化ObservableCollection，然后我得到SerializationException消息:"类型'System.Net。ConnectStream' in assembly'System。版本4.0.0.0,Culture=neutral, PublicKey Token=b77a5c561934e089'未标记为可序列化"。下面是Downloader类的精简版本:

[Serializable()]
public class Downloader
{
     /// <summary>
     /// "Download status changed" event.
     /// </summary>
     [field: NonSerialized()]
     public event EventHandler<InServiceHandledDownloadStatusChangedEventArgs>    InServiceHandledDownloadStatusChanged;
     /// <summary>
     /// URL of downloaded resource.
     /// </summary>
     private String _targetUrl;  
     /// <summary>
     /// Path to save downloaded data on local drive.
     /// </summary>
     private String _pathToSave;
     /// <summary>
     /// The number of bytes downloaded from internet resource.
     /// </summary>
     private Int64 _downloadedBytesQuantity = 0;
     /// <summary>
     /// Current status of downloading ("Await", "Running", "Completed").
     /// </summary>
     private String _status = "Added";
     /// <summary>
     /// Task that performs download.
     /// </summary>
     [NonSerialized()]
     public Task TaskPerformingDownload;
     /// <summary>
     /// The source of cancellation token for cancelling of TaskPerformingDownload.
     /// </summary>
     [NonSerialized()]
     public CancellationTokenSource CancelDownloadTokenSource;
     /// <summary>
     ///  Gets or sets stream to read downloaded data from internet.
     /// </summary>
     public Stream ReadСontentStream { get; set; }
     /// <summary>
     /// Gets or sets stream to write downloaded data to local drive.
     /// </summary>
     public Stream SaveСontentStream { get; set; }
     /// <summary>
     /// This method executes in TaskPerformingDownload and performs downloading.
     /// of a resource from internet.
     /// </summary>
     /// <param name="p_CancellationToken">Cancellation Token</param>
     public void PerformDownload(Object p_CancellationToken)
     {
         try
         {
             // Get cancellation token.
             CancellationToken cancellationToken = (CancellationToken)p_CancellationToken;
             // Check was the task canceled?
             cancellationToken.ThrowIfCancellationRequested();
             . . . . . . . . 
             HttpWebRequest webResourceRequest = (HttpWebRequest)WebRequest.Create(TargetUrl);
             HttpWebResponse webResourceResponse = (HttpWebResponse)webResourceRequest.GetResponse();
             this.ReadСontentStream = webResourceResponse.GetResponseStream();
             this.SaveСontentStream = new FileStream(this.PathToSave, FileMode.Create, FileAccess.Write, FileShare.None);
             int bytesReceivedInChank = 0;
             byte[] downloadBuffer = new byte[2048];
             // The downloading loop.
             while ((bytesReceivedInChank = this.ReadСontentStream.Read(downloadBuffer, 0, downloadBuffer.Length)) > 0)
             {
                  if (cancellationToken.IsCancellationRequested)
                    cancellationToken.ThrowIfCancellationRequested();
                  this.SaveСontentStream.Write(downloadBuffer, 0, bytesReceivedInChank);
                  . . . . . . . .
             }
        }
        catch(Exception){. . . .}
        finally
        {
            if (this.ReadСontentStream != null)
            {
                this.ReadСontentStream.Close();
                this.ReadСontentStream.Dispose();
            }
            if (this.SaveСontentStream != null)
            {
                this.SaveСontentStream.Close();
                this.SaveСontentStream.Dispose();
            }
        }
     }
}

TaskPerformingDownload成员是执行一次下载的TPL任务。它是从StartDownload()合约方法启动的，WCF服务在客户端请求时调用该合约方法。本任务执行PerformDownload方法。WCF服务创建与必须执行的下载数量一样多的Downloader实例。每次下载一个Downloader实例。SerializationException异常带有消息:"Type'System.Net. net . "ConnectStream' in assembly'System。版本4.0.0.0,Culture=neutral, PublicKey Token=b77a5c561934e089'未标记为serializable"仅在我尝试序列化已完成下载的ObservableCollection时发生。当下载完成时，它的任务(TaskPerformingDownload成员)也完成了它的工作，不再执行。我尝试在完成的下载中处理任务，但它没有帮助，SerializationException异常仍然存在。但是如果ObservableCollection中只有新的下载，那么下载还没有运行(所以这个下载的TaskPerformingDownload成员还没有运行)，在这种情况下，ObservableCollection被序列化得很好，没有任何异常或错误。你能告诉我为什么会出现这种情况吗?这对我来说很重要。