How to serialize an object array in C#?

Question

I generated ac# class using xsd.exe with this xml file:

<?xml version="1.0" encoding="UTF-8"?>
<Mary>
    <Frank>
        <Joe>
            <Susan>
                <Stuff>data</Stuff>
            </Susan>
            <Susan>
                <Stuff>data</Stuff>
            </Susan>
        </Joe>
        <Joe>
            <Susan>
                <Stuff>data</Stuff>
            </Susan>
            <Susan>
                <Stuff>data</Stuff>
            </Susan>
        </Joe>
    </Frank>
</Mary>

The C# class that was generated can be viewed here.

I can initialize the object with data:

var susan = new MaryFrankJoeSusan(){Stuff = "my data"};
var frank = new MaryFrank(){Joe = new MaryFrankJoeSusan[1][]};
frank.Joe[0] = new MaryFrankJoeSusan[1]{susan};
var mary = new Mary { Items = new MaryFrank[1] { frank } };

I'm using the following to serialize it to disk:

var serializer = new XmlSerializer(typeof(Mary));

using (Stream stream = new FileStream(@"C:\out.xml", FileMode.Create))
{
    var settings = new XmlWriterSettings { Indent = true, NewLineOnAttributes = true, OmitXmlDeclaration = true};
    using (XmlWriter writer = new XmlTextWriter(stream, Encoding.Unicode))
    {
        serializer.Serialize(writer, mary);
        writer.Close();
    }
}

However I am get the following error when the serializer is initialized:

error CS0030: Cannot convert type 'MaryFrankJoeSusan[]' to 'MaryFrankJoeSusan'

How do I serialize the entire Mary object to disk?

Answer 1

Something is off with those generated classes.

The problem is happening because MaryFrank.Joe is declared as a two-dimensional array of MaryFrankJoeSusan objects, but it is decorated with a XmlArrayItemAttribute which is telling the serializer that each item of that 2D array are of type MaryFrankJoeSusan when they are of course MaryFrankJoeSusan[] .

If you change this line in the generated classes:

[System.Xml.Serialization.XmlArrayItemAttribute("Susan", typeof(MaryFrankJoeSusan),
 Form=System.Xml.Schema.XmlSchemaForm.Unqualified, IsNullable=false)]

to this:

[System.Xml.Serialization.XmlArrayItemAttribute("Susan", typeof(MaryFrankJoeSusan[]),
 Form=System.Xml.Schema.XmlSchemaForm.Unqualified, IsNullable=false)]

then it will serialize without error. However, you won't get the results you are looking for. Instead of this:

<Mary>
    <Frank>
        <Joe>
            <Susan>
                <Stuff>my data</Stuff>
            </Susan>
        </Joe>
    </Frank>
</Mary>

you will get this (note the extra MaryFrankJoeSusan tag):

<Mary>
    <Frank>
        <Joe>
            <Susan>
                <MaryFrankJoeSusan>
                    <Stuff>my data</Stuff>
                </MaryFrankJoeSusan>
            </Susan>
        </Joe>
    </Frank>
</Mary>

The real problem seems to be that the xsd.exe tool has generated the class structure incorrectly to begin with. It is not creating a class in the heirarchy to represent Joe, but is instead trying to combine Joe and Susan together, which doesn't really work here.

I ran your original XML from the question through the tool to generate an XSD schema, and I got this:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="Mary" xmlns="" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
  <xs:element name="Mary" msdata:IsDataSet="true" msdata:UseCurrentLocale="true">
    <xs:complexType>
      <xs:choice minOccurs="0" maxOccurs="unbounded">
        <xs:element name="Frank">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Joe" minOccurs="0" maxOccurs="unbounded">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element name="Susan" minOccurs="0" maxOccurs="unbounded">
                      <xs:complexType>
                        <xs:sequence>
                          <xs:element name="Stuff" type="xs:string" minOccurs="0" />
                        </xs:sequence>
                      </xs:complexType>
                    </xs:element>
                  </xs:sequence>
                </xs:complexType>
              </xs:element>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:choice>
    </xs:complexType>
  </xs:element>
</xs:schema>

which looks OK to me. Then I took that same schema and ran it through the tool again to generate C# classes. I would have expected to get something similar to this:

[Serializable]
[XmlRoot(Namespace = "", ElementName = "Mary")]
public class Mary
{
    [XmlElement("Frank")]
    public Frank[] Frank { get; set; }
}
[Serializable]
public class Frank
{
    [XmlElement("Joe")]
    public Joe[] Joe { get; set; }
}
[Serializable]
public class Joe
{
    [XmlElement("Susan")]
    public Susan[] Susan { get; set; }
}
[Serializable]
public class Susan
{
    [XmlElement("Stuff")]
    public string Stuff { get; set; }
}

but instead I got the same broken classes that you linked in the question. So it looks like a bug in the xsd tool to me.

To get it to work, you can either use the hand-coded classes I made above, changing your initialization code to this:

var susan = new Susan { Stuff = "my data" };
var joe = new Joe { Susan = new Susan[] { susan } };
var frank = new Frank { Joe = new Joe[] { joe } };
var mary = new Mary { Frank = new Frank[] { frank } };

--OR--

another alternative is to alter the xsd. Replace the xs:sequence indicators for Frank and Joe elements with xs:choice instead, like this:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="Mary" xmlns="" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
  <xs:element name="Mary" msdata:IsDataSet="true" msdata:UseCurrentLocale="true">
    <xs:complexType>
      <xs:choice minOccurs="0" maxOccurs="unbounded">
        <xs:element name="Frank">
          <xs:complexType>
            <xs:choice> <!-- was xs:sequence -->
              <xs:element name="Joe" minOccurs="0" maxOccurs="unbounded">
                <xs:complexType>
                  <xs:choice> <!-- was xs:sequence -->
                    <xs:element name="Susan" minOccurs="0" maxOccurs="unbounded">
                      <xs:complexType>
                        <xs:sequence>
                          <xs:element name="Stuff" type="xs:string" minOccurs="0" />
                        </xs:sequence>
                      </xs:complexType>
                    </xs:element>
                  </xs:choice> <!-- was /xs:sequence -->
                </xs:complexType>
              </xs:element>
            </xs:choice> <!-- was /xs:sequence -->
          </xs:complexType>
        </xs:element>
      </xs:choice>
    </xs:complexType>
  </xs:element>
</xs:schema>

With this schema, the generated classes come out much better: there is a class to represent Joe now. (I've simplified the generated code here and removed some of the attributes for brevity):

[System.SerializableAttribute()]
[System.Xml.Serialization.XmlRootAttribute(Namespace="", IsNullable=false)]
public partial class Mary {
    [System.Xml.Serialization.XmlElementAttribute("Frank", Form=System.Xml.Schema.XmlSchemaForm.Unqualified)]
    public MaryFrank[] Items { get; set; }
}

[System.SerializableAttribute()]
public partial class MaryFrank {
    [System.Xml.Serialization.XmlElementAttribute("Joe", Form=System.Xml.Schema.XmlSchemaForm.Unqualified)]
    public MaryFrankJoe[] Items { get; set; }
}

[System.SerializableAttribute()]
public partial class MaryFrankJoe {
    [System.Xml.Serialization.XmlElementAttribute("Susan", Form=System.Xml.Schema.XmlSchemaForm.Unqualified)]
    public MaryFrankJoeSusan[] Items { get; set; }
}

[System.SerializableAttribute()]
public partial class MaryFrankJoeSusan {
    [System.Xml.Serialization.XmlElementAttribute(Form=System.Xml.Schema.XmlSchemaForm.Unqualified)]
    public string Stuff { get; set; }
}

The setup code then becomes:

var susan = new MaryFrankJoeSusan() { Stuff = "my data" };
var joe = new MaryFrankJoe() { Items = new MaryFrankJoeSusan[] { susan } };
var frank = new MaryFrank() { Items = new MaryFrankJoe[] { joe } };
var mary = new Mary { Items = new MaryFrank[] { frank } };

And we get the expected output:

<?xml version="1.0" encoding="utf-16"?>
<Mary xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <Frank>
        <Joe>
            <Susan>
                <Stuff>my data</Stuff>
            </Susan>
        </Joe>
    </Frank>
</Mary>