How do I access to single property of an object of multiple XmlElementAttribute?

Question

public class A {
    private object itemField;

    [System.Xml.Serialization.XmlElementAttribute("type", typeof(TypeItemClass))]
    [System.Xml.Serialization.XmlElementAttribute("B", typeof(ClassB))]
    [System.Xml.Serialization.XmlElementAttribute("C", typeof(Class C)]
    public object Item {
        get {
            return this.itemField;
        }
        set {
            this.itemField = value;
        }
    }

Now i want to access a field in ClassB. How can i do that?

var ff = outerclass.Item...(then what should i do here to access ClassB)

I tried different solutions, but not able to find right one