c# using + XmlWriter.Create = "Cannot access a closed Stream."答案

【问题标题】：c# using + XmlWriter.Create = "Cannot access a closed Stream."c# using + XmlWriter.Create = "Cannot access a closed Stream."
【发布时间】：2011-03-30 01:06:17
【问题描述】：

为什么会这样：

using (var ms = new MemoryStream())
{
    using (var dummy = new StreamWriter(ms))
    {
        var sw = new StreamWriter(ms);
        sw.WriteLine("Hello World");
        sw.Flush();
        using (StreamReader rdr = new StreamReader(ms))
        {
            ms.Position = 0;
            textBoxExc.Text = rdr.ReadToEnd();
        }
    }
}

但这不起作用（“无法访问关闭的流。”）：唯一的区别是var dummy = XmlWriter.Create(ms) 而不是var dummy = new StreamWriter(ms)

using (var ms = new MemoryStream())
{
    using (var dummy = XmlWriter.Create(ms))
    {
        var sw = new StreamWriter(ms);
        sw.WriteLine("Hello World");
        sw.Flush();
        using (StreamReader rdr = new StreamReader(ms))
        {
            ms.Position = 0;
            textBoxExc.Text = rdr.ReadToEnd();
        }
    }
}

堆栈跟踪：

System.ObjectDisposedException was unhandled by user code
  Message=Cannot access a closed Stream.
  ObjectName=""
  StackTrace:
       w System.IO.__Error.StreamIsClosed()
       w System.IO.MemoryStream.Write(Byte[] buffer, Int32 offset, Int32 count)
       w System.Xml.XmlUtf8RawTextWriter.FlushBuffer()
       w System.Xml.XmlUtf8RawTextWriter.Flush()
       w System.Xml.XmlWellFormedWriter.Close()
       w System.Xml.XmlWriter.Dispose(Boolean disposing)
       w System.Xml.XmlWriter.Dispose()
       w SerializeTest.MainPage.buttonExc_Click(Object sender, RoutedEventArgs e)
       w System.Windows.Controls.Primitives.ButtonBase.OnClick()
       w System.Windows.Controls.Button.OnClick()
       w System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
       w System.Windows.Controls.Control.OnMouseLeftButtonUp(Control ctrl, EventArgs e)
       w MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, Int32 actualArgsTypeIndex, String eventName)
  InnerException:

这也不起作用（同样的错误）：

using (var ms = new MemoryStream())
{
     using (var writer = XmlWriter.Create(ms))
    {
        var serializer = new DataContractSerializer(typeof(T));
        serializer.WriteObject(writer, objectToSave);
        writer.Flush();
        ms.Position = 0;
        using (StreamReader rdr = new StreamReader(ms))
        {
            return rdr.ReadToEnd();
        }
    }
}

堆栈跟踪：

System.ObjectDisposedException was unhandled by user code
  Message=Cannot access a closed Stream.
  ObjectName=""
  StackTrace:
       w System.IO.__Error.StreamIsClosed()
       w System.IO.MemoryStream.Write(Byte[] buffer, Int32 offset, Int32 count)
       w System.Xml.XmlUtf8RawTextWriter.FlushBuffer()
       w System.Xml.XmlUtf8RawTextWriter.Flush()
       w System.Xml.XmlWellFormedWriter.Close()
       w System.Xml.XmlWriter.Dispose(Boolean disposing)
       w System.Xml.XmlWriter.Dispose()
       w SerializeTest.SerializeToStringTest[T](T objectToSave)
       w SerializeTest.MainPage.button2A_Click(Object sender, RoutedEventArgs e)
       w System.Windows.Controls.Primitives.ButtonBase.OnClick()
       w System.Windows.Controls.Button.OnClick()
       w System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
       w System.Windows.Controls.Control.OnMouseLeftButtonUp(Control ctrl, EventArgs e)
       w MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, Int32 actualArgsTypeIndex, String eventName)
  InnerException:

但这有效（唯一的区别是StreamReader w/o using）：

using (var ms = new MemoryStream())
{
     using (var writer = XmlWriter.Create(ms))
    {
        var serializer = new DataContractSerializer(typeof(T));
        serializer.WriteObject(writer, objectToSave);
        writer.Flush();
        ms.Position = 0;
        StreamReader rdr = new StreamReader(ms);
        return rdr.ReadToEnd();
    }
}

【问题讨论】：

Silverlight 4 上没有重现，我没想到会有。
您找到了升级的好理由。

标签： c# silverlight using xmlwriter

【解决方案1】：

只需将 MemoryStream 块的读取移动到一个级别并写入它。

using (var ms = new MemoryStream())
{
     using (var writer = XmlWriter.Create(ms))
    {
        var serializer = new DataContractSerializer(typeof(T));
        serializer.WriteObject(writer, objectToSave);
        writer.Flush();
        ms.Position = 0;        
    }
    using (StreamReader rdr = new StreamReader(ms))
    {
        return rdr.ReadToEnd();
    }
}

【讨论】：

完美，尝试将 XmlSchema 转换为字符串时遇到了同样的问题。一旦我按照此处所示移动了该块，一切都很好。
也为 .net core 工作过

【解决方案2】：

您能给我们提供异常的完整堆栈跟踪吗？我的第一个猜测是XmlWriter 仍然尝试在XmlWriter 的Dispose() 方法中访问流。

在您的第二个和第四个代码示例中，您将 StreamReader 放在 using 块中。这会导致在此块末尾调用StreamReader 的Dispose() 方法。此方法关闭阅读器和底层流。之后，XmlWriter 的Dispose() 方法就不能再访问流了。

更新： 根据stackstrace，看来我是对的。 Dispose() 方法调用Close()，后者又想要刷新已经关闭的流。这看起来像一个错误，因为应该没有任何东西可以刷新。

您已经给出了解决方案：在 XmlWriter 释放之前不要关闭内存流。

（我假设您知道 using-block 隐式处置使用的对象，处置 StreamReader 或 StreamWriter 隐式处置（并关闭）底层流。）

【讨论】：