Question

I’m getting an exception

The type KML.Placemark was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically.

When I try to serialize my objects. I know of two different solutions to this exception, but neither is working in this case.

Some background:

I have a Class-structure that closely follows the GoogleEarth/OpenGIS KML format (for drawing on top of GoogleEarth).

My root type is KMLDocument which contains a set of KMLObjects:

public class KMLDocument
{
    public KMLObject[] members;
}

And KMLObject is the base-type for Feature, which is the base-type for Placemark

---

The Problem:

When I construct the serializer for KMLDocument, it won’t directly know about derived types like Placemark unless I explicitly tell it. So I do it this way:

XmlSerializer serializer = new XmlSerializer(typeof(KMLDocument),
    new Type[] { typeof(KMLObject),
                 typeof(Feature),
                 typeof(Placemark) } );

I also attach Attributes to the KMLDocument class to make sure it knows about all important types:

[XmlRootAttribute("kml", Namespace="http://www.opengis.net/kml/2.2")]
[XmlInclude(typeof(KMLObject))]
[XmlInclude(typeof(Feature))]
[XmlInclude(typeof(Placemark))]
public class KMLDocument
{  ....   }

But, despite telling the serializer about Placemark two different ways, when I call
serialize, I get the exception:

static void Main(string[] args)
{
    KMLDocument kml = new KMLDocument();
    kml.AddPlacemark("MyPlacemark", "MyTest");

    XmlSerializer serializer = new XmlSerializer(typeof(KMLDocument),
        new Type[] { typeof(KMLObject),
                     typeof(Feature), 
                     typeof(Placemark) } );
     serializer.Serialize(new StreamWriter("MyKML.kml"), kml);  // Exception on this line!
}

If I add a dummy-variable of type Placemark, suddenly the serializer can find the type, and it works right:

public class KMLDocument
{
    public KMLObject[] members;
    public Placemark dummy_var;  // Should NOT be needed!
}

What am I missing? Why are both my XmlSerializer-Constructor and my Attributes failing to provide the important information?

Answer 1

Placemark and Feature are subclasses of KMLObject. The members field holds a mixed array of Placemarks and Features.

The members field has to be tagged with an XmlArrayItemAttribute in order to specify that the elements it contains are polymorphic.

http://msdn.microsoft.com/en-us/library/system.xml.serialization.xmlarrayitemattribute.aspx