I have a collection in MongoDB which I'm trying to "FindAndModify" using C# driver. This collection holds types of a base class and its derived classed, as follows:

[BsonDiscriminator(RootClass = true)]
public class Father
{
public Guid Id

{ get; private set; }
}

public class Son : Father
{
public string SomeProperty { get; private set; }

}

When I'm trying to cast the BsonDocument to my base class, after the FindAndModify, where the result is "Son":

Father modifiedDocument = result.GetModifiedDocumentAs<Father>();

I get the following exception:

System.IO.FileFormatException: Element 'SomeProperty' does not match any field or property of class Father.

Any idea why? -Can't I perform a down cast here?

Thanks,
Nir.