XmlWriter forces stream to close

02 April 2019 - 4 mins read time
Tags: C# .NET XML Microsoft

Today a friend showed me a piece of code that was supposed to serialize some Model to XML string without any consideration for custom namespaces apart from defined in given Model. Ok, so we need an instance of XmlSerializer for Model’s type and we will use XmlWriter to write through some MemoryStream and read the contents at the end. Seems quite obvious therefore he wrote something like this:

private static string GetString(Model model)
{
  var serializer = new XmlSerializer(typeof(Model));

  var xmlWriterSettings = new XmlWriterSettings
  {
    CheckCharacters = true,
    CloseOutput = false
  };

  using (var stream = new MemoryStream())
  using (var writer = XmlWriter.Create(stream, xmlWriterSettings))
  using (var reader = new StreamReader(stream))
  {
    serializer.Serialize(writer, model);
    writer.Flush();

    stream.Position = 0;

    return reader.ReadToEnd();
  }
}

Then he run the code and he got an Exception:

System.ObjectDisposedException: Cannot access a closed Stream.
   at System.IO.MemoryStream.Write(Byte[] buffer, Int32 offset, Int32 count)
   at System.Xml.XmlUtf8RawTextWriter.FlushBuffer()
   at System.Xml.XmlUtf8RawTextWriter.Flush()
   at System.Xml.XmlWellFormedWriter.Close()
   at System.Xml.XmlWriter.Dispose(Boolean disposing)
   at System.Xml.XmlWriter.Dispose()

CQS with streams

At first it looks like there as a bug in .NET because the given solution seems so obvious but look again at above exception stack trace. It is not a problem with reader trying to read closed Stream. It is a problem with MemoryStream.Write that stumbles across closed stream. The reason for this problem seems to be oddly enough StreamReader itself. You want to write and read the same stream in one block and that is not what CQS is telling us to do. Let’s break those operations into separate blocks.

using (var stream = new MemoryStream())
{
  using (var writer = XmlWriter.Create(stream, xmlWriterSettings))
  {
    serializer.Serialize(writer, model);
  }

  stream.Position = 0;

  using (var reader = new StreamReader(stream))
  {
    return reader.ReadToEnd();
  }
}

The Dispose() method invokes Close(), which in turn wants to flush an already closed stream. This looks like a bug since there should be nothing left to flush.

The solution is to not close the MemoryStream before the XmlWriter disposes.

Forget the streams

But, you may say, I don’t like long code and I am not enforcing using stream to read data from writer in my solution. If so look at this example:

private static string GetString(Model model)
{
  var serializer = new XmlSerializer(typeof(Model));

  var xmlWriterSettings = new XmlWriterSettings
  {
    CheckCharacters = true
  };

  using(var sw = new StringWriter())
  using(XmlWriter writer = XmlWriter.Create(sw, xmlWriterSettings))
  {
    serializer.Serialize(writer, model);
    return sw.ToString();
  }
}

I’ve used StringWriter that internally writes to StringBuilder so it basically converts stream processing into safe string processing and from the very beginning clearly states intention => you want an XML string from Model.

Aftermath

At last I will leave you with just couple of thoughts:

  1. stream operations are not trivial therefore ask yourself if you really need them,
  2. main intent should be easily visible or code smells,
  3. let’s enjoy that Microsoft has opened .NET sources e.g. StringBuilder and we can do deep debugging without IL decompiler.

References: