Question

I am trying to create a class that derives from ObservableCollection and restricts itself to only being used with a particular base class (BaseMetadata). It also needs to implement the IXmlSerializable interface as I am adding a persistence capability to the collection.

Here is the class definition of the collection…

public class CollectionMetadata<T> : ObservableCollection<T> where T : BaseMetadata,
                                     IXmlSerializable
{
    XmlSchema IXmlSerializable.GetSchema()
    {
        return null;
    }

    public void ReadXml(XmlReader reader)
    {
    }

    public void WriteXml(XmlWriter writer)
    {
    }
}

…the BaseMetadata can be simplied to an empty class and still produce the error…

public class BaseMetadata
{
}

…I get the following error….

CollectionMetadata<T>.IXmlSerializable.GetSchema()': 
    containing type does not implementinterface 
    'System.Xml.Serialization.IXmlSerializable'

…on the following line from the above code…

XmlSchema IXmlSerializable.GetSchema()

I must be missing something really obvious?

Answer 1

In it’s current form, you’re applying the IXmlSerializable as a constraint on the T; not declaring it as an interface of CollectionMetadata<T>. So your explicit interface implementation is moaning, because it can’t find the interface.

You just need to move the IXmlSerializable interface to before the constraint:

public class CollectionMetadata<T> : ObservableCollection<T>, IXmlSerializable
                                     where T : BaseMetadata

As a side note – it’s potentially a really good thing that you used an explicit interface implementation for this, otherwise this could have been a real head-scratcher later on. Because as a public method, the compiler wouldn’t moan at this point, but would have complained if you’d tried to pass the collection as IXmlSerializable.