<ItemsList xmlns:xsi=“www.w3.org/2001/XMLSchema-instance”>

<Item xsi:type="ItemWithNumber">
 <Name>Name1</Name>
 <Number>20</Number>
</Item>
<Item xsi:type="ItemWithNumber">
 <Name>Name1</Name>
 <Name>Name2</Name>
 <Number>20</Number>
</Item>

</ItemsList>