The basic rules for data ordering include:

• If a data contract type is a part of an inheritance hierarchy, data members of its base types are always first in the order.
  
  
• Next in order are the current type’s data members that do not have the Order property of the DataMemberAttribute attribute set, in alphabetical order.
  
  
• Next are any data members that have the Order property of the DataMemberAttribute attribute set. These are ordered by the value of the Order property first and then alphabetically if there is more than one member of a certain Order value. Order values may be skipped.
  
  

Alphabetical order is established by calling the CompareOrdinal method.

Consider the following code.

<DataContract()> _
Public Class BaseType
    <DataMember()> Public zebra As String
End Class

<DataContract()> _
Public Class DerivedType
    Inherits BaseType
    <DataMember(Order:=0)> Public bird As String
    <DataMember(Order:=1)> Public parrot As String
    <DataMember()> Public dog As String
    <DataMember(Order:=3)> Public antelope As String
    <DataMember()> Public cat As String
    <DataMember(Order:=1)> Public albatross As String
End Class

The XML produced is similar to the following.

<DerivedType>
    <!-- Zebra is a base data member, and appears first. -->
    <zebra/> 

    <!-- Cat has no Order, appears alphabetically first. -->
    <cat/>
    
   <!-- Dog has no Order, appears alphabetically last. -->
    <dog/> 

    <!-- Bird is the member with the smallest Order value -->
    <bird/>

    <!-- Albatross has the next Order value, alphabetically first. -->
    <albatross/>

    <!-- Parrot, with the next Order value, alphabetically last. -->
     <parrot/>

    <!-- Antelope is the member with the highest Order value. Note that 
    Order=2 is skipped -->
     <antelope/> 
</DerivedType>

Reference

Concepts