XmlText attribute in base class breakes serialization

Question

I have a base class with a property called Name, which has an XmlText attribute.

When an inherited class is serialized I get an exception saying:

There was an error reflecting type '[type name]'. ---> System.InvalidOperationException: Cannot serialize object of type '[type name]'. Base type '[base type name]' has simpleContent and can only be extended by adding XmlAttribute elements. Please consider changing XmlText member of the base class to string array.

Here are my class definitions, the error occures when serializing to xml.

[Serializable]
public class LookupItem
{
    [System.Xml.Serialization.XmlAttribute()]
    public int ID { get; set; }

    [System.Xml.Serialization.XmlText()]
    public string Name { get; set; }
}

[Serializable]
public class Vendor : LookupItem
{
    public string ContactNumber { get; set; }
}

[Serializable]
public class Vendors : List<Vendor>
{

}

Answer 1

It seems to happen because the LookupItem class has no child element in its XML representation. Because of that, XmlSerializer considers it to be an simpleContent XML element. If you add a new property to LookupItem, which is to be serialized as an XML element, it works fine.

I just spent a few minutes looking at the available XML serialization attributes, but I couldn't find any which would allow to specify that the type is NOT a simpleContent element...

So I think you could make it work by adding a dummy property or field that you never use in the LookupItem class. If you're never assign a value to it, it will remain null and will not be serialized, but it will prevent your class from being treated as simpleContent. I know it's a dirty workaround, but I see no other easy way...

public class LookupItem
{
    [System.Xml.Serialization.XmlAttribute()]
    public int ID { get; set; }

    [System.Xml.Serialization.XmlText()]
    public string Name { get; set; }

    public object _dummy;
}

(BTW, note that the Serializable attribute is not necessary for XML serialization.)

Anyway, do you really need to serialize Name as XML text ? That's quite uncommon, usually XML elements have either simple content (text), or child elements... A mix of both makes the XML harder to read, so unless you are compelled to do that, I suggest you serialize Name as an attribute or child element.