How can I deserialize an XML, when I only know the type of an ancestor class?

Question

Let's assume I want to deserialize this (I've removed the namespaces to make things simpler):

<TextField>
  <Caption>Location</Caption>
  <Name>Location</Name>
</TextField>

TextField inherits from FormField, so in my class definition of FormField looks something like this:

[KnownType(typeof(TextField))]
[DataContract(Name = "FormField"]
public abstract class FormField
{
    [DataMember]
    public string Name { get; set; }
}

TextField class looks like this:

[DataContract(Name = "TextField")]
public class TextField : FormField
{
    [DataMember]
    public string Caption { get; set; }
}

I tried deserializing using this:

internal static FormField Deserialize(string xml)
{
    var serializer = new DataContractSerializer(typeof(FormField)});
    using (var backing = new StringReader(xml))
    {
        using (var reader = new XmlTextReader(backing))
        {
            return serializer.ReadObject(reader) as FormField;
        }
    }
}

I get a SerializationException: "Expecting element 'FormField'..."

Answer 1

To solve my problem I added a container node to the XML, so that it looks like this:

<FormFieldContainer>
    <TextField>   
        <Caption>Location</Caption>
        <Name>Location</Name>
    </TextField>
</FormFieldContainer>

I created this class:

[DataContract(Name = "FormFieldContainer")]
internal class FormFieldContainer
{
    [DataMember]
    internal FormField FormField { get; set; }
}

And my deserialize method looks like this:

    internal static FormField Deserialize(string xml)
    {
        var serializer = new DataContractSerializer(typeof(FormFieldContainer));
        using (var backing = new StringReader(xml))
        {
            using (var reader = new XmlTextReader(backing))
            {
                return ((FormFieldContainer)serializer.ReadObject(reader)).FormField;
            }
        }
    }

If anyone has a better solution, please share it :)