.net XmlSerialize throws "WriteStartDocument cannot be called on writers created with ConformanceLevel.Fragment"

Question

Am trying to serialize a class, writing to an XML file as multiple fragments, i.e, write each object of the class as an individual fragment, without the XML header/root. Below is a sample code:

[Serializable]
public class Test
{
    public int X { get; set; }
    public String Y { get; set; }
    public String[] Z { get; set; }

    public Test()
    {
    }

    public Test(int x, String y, String[] z)
    {
        X = x;
        Y = y;
        Z = z;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Test t1 = new Test(1, "t1", new[] { "a", "b" });
        Test t2 = new Test(2, "t2", new[] { "c", "d", "e" });

        XmlSerializer serializer = new XmlSerializer(typeof(Test));
        //using (StreamWriter writer = new StreamWriter(@"f:\test\test.xml"))
        {
            XmlWriter xmlWriter = XmlWriter.Create(@"f:\test\test.xml",
                                                   new XmlWriterSettings()
                                                       {ConformanceLevel = ConformanceLevel.Fragment,
                                                        OmitXmlDeclaration = true,
                                                        Indent = true});
            serializer.Serialize(xmlWriter, t1);
            serializer.Serialize(xmlWriter, t2);
            xmlWriter.Close();
        }
    }
}

In the first call to serialize, I get the exception:

WriteStartDocument cannot be called on writers created with ConformanceLevel.Fragment

What am I missing here?

Answer 1

there is a workaround to this problem. When the xml writer has been used before you use the serializer, then the header won't be written. The following does work, but will add an empty comment tag on the first line of the xml file

improved code as suggested by oleksa

static void Main(string[] args)
    {
        Test t1 = new Test(1, "t1", new[] { "a", "b" });
        Test t2 = new Test(2, "t2", new[] { "c", "d", "e" });

        XmlSerializer serializer = new XmlSerializer(typeof(Test));
        //using (StreamWriter writer = new StreamWriter(@"f:\test\test.xml"))
        {
            XmlWriter xmlWriter = XmlWriter.Create(@"test.xml",
                                                   new XmlWriterSettings()
                                                   {
                                                       ConformanceLevel = ConformanceLevel.Fragment,
                                                       OmitXmlDeclaration = false,
                                                       Indent = true,
                                                       NamespaceHandling = NamespaceHandling.OmitDuplicates
                                                   });
            xmlWriter.WriteWhitespace("");
            serializer.Serialize(xmlWriter, t1);
            serializer.Serialize(xmlWriter, t2);
            xmlWriter.Close();
        }
    }