c# - Consecutive XML serialization causes - Token StartElement in state EndRootElement would result in an invalid XML document

Question

I am writing to the file XML serialization of the object, generated by validator.MatchPossiblyValid(string input)method. First call, serializes and write to the file. However, the second call fails with an exception: System.InvalidOperationException: 'Token StartElement in state EndRootElement would result in an invalid XML document. Make sure that the ConformanceLevel setting is set to ConformanceLevel.Fragment or ConformanceLevel.Auto if you want to write an XML fragment. '

XmlSerializerNamespaces emptyNS = new XmlSerializerNamespaces(new[] { XmlQualifiedName.Empty });
var serializer = new XmlSerializer(typeof(PDPCustomerInfoInvalid));
var settings = new XmlWriterSettings();
settings.OmitXmlDeclaration = true;
//settings.Indent = true;
using (var stream = new System.IO.StreamWriter(args[1], true))
{
    using (var writer = XmlWriter.Create(stream, settings))
        {

                serializer.Serialize(writer, validator.MatchPossiblyValid("STRING FOR PARSING"), emptyNS);
                stream.Write(Environment.NewLine);
                stream.Flush();
                //Line below throws the exception
                serializer.Serialize(writer, validator.MatchPossiblyValid("STRING FOR PARSING"), emptyNS);
                stream.Write(Environment.NewLine);
                stream.Flush();

        }
}

Answer 1

You are trying to use a single XmlWriter to create an XML file with multiple root elements. However, the XML standard requires exactly one root element per XML document. Your XmlWriter is throwing the exception to indicate that the XML being created is invalid. (MCVE here.)

If you really need to concatenate two XML documents into a single file, you could use separate XmlWriters created with XmlWriterSettings.CloseOutput set to false:

using (var stream = new System.IO.StreamWriter(args[1], true))
{
    var settings = new XmlWriterSettings();
    settings.OmitXmlDeclaration = true;
    //settings.Indent = true;
    settings.CloseOutput = false;

    using (var writer = XmlWriter.Create(stream, settings))
    {
        serializer.Serialize(writer, validator.MatchPossiblyValid("STRING FOR PARSING"), emptyNS);
    }

    stream.Write(Environment.NewLine);
    stream.Flush();

    using (var writer = XmlWriter.Create(stream, settings))
    {
        serializer.Serialize(writer, validator.MatchPossiblyValid("STRING FOR PARSING"), emptyNS);
    }
    //Line below throws the exception
    stream.Write(Environment.NewLine);
    stream.Flush();             
}

Sample fiddle.

Or, better yet, don't do this at all, since an "XML Document" with multiple roots is, as stated above, not valid. Instead, serialize both objects inside some container element.