How can I force the use of an xsi:type attribute?

Question

How can I force .NET\'s XmlSerializer to add an xsi:type=\"FooClass\" to a member/node of type FooClass?

The scenario

Answer 1

I had no trouble producing the following:

    
        Hello
        5
    

from

[XmlInclude(typeof(FooClass))]
//[XmlType(TypeName = "FooBase", Namespace = "urn:namespace", IncludeInSchema = true)]
public class FooBaseClass
{
    public string FooPropertyA { get; set; }
}

//[XmlType(TypeName = "Foo", Namespace = "urn:namespace", IncludeInSchema = true)]
public class FooClass : FooBaseClass
{
    public int FooPropertyB { get; set; } 
}

public class BazClass
{
    public FooBaseClass Foo { get; set; }
}

(note the XmlType attributes are commented out. I wanted to see what happened if a namespace was specified)

Please show the code you used, and the XML it produced.