Imports System
Imports System.IO
Imports System.Collections
Imports System.Collections.Generic
Imports System.Runtime.Serialization
Imports ServiceStack
Imports ServiceStack.DataAnnotations
Imports Test.ServiceModel
Namespace Global
    Namespace Test.ServiceModel
        Public Partial Class HelloMap
            Public Overridable Property Names As List(Of String) = New List(Of String)
        End Class
    End Namespace
End Namespace
To override the Content-type in your clients, use the HTTP Accept Header, append the .xml suffix or ?format=xml
The following are sample HTTP requests and responses. The placeholders shown need to be replaced with actual values.
POST /xml/reply/HelloMap HTTP/1.1 
Host: test.servicestack.net 
Accept: application/xml
Content-Type: application/xml
Content-Length: length
<HelloMap xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/Test.ServiceModel">
  <Names xmlns:d2p1="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
    <d2p1:string>String</d2p1:string>
  </Names>
</HelloMap>
HTTP/1.1 200 OK
Content-Type: application/xml
Content-Length: length
<ArrayOfKeyValueOfstringArrayResulttM8isCsu xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
  <KeyValueOfstringArrayResulttM8isCsu>
    <Key>String</Key>
    <Value xmlns:d3p1="http://schemas.datacontract.org/2004/07/Test.ServiceModel">
      <d3p1:Result>String</d3p1:Result>
    </Value>
  </KeyValueOfstringArrayResulttM8isCsu>
</ArrayOfKeyValueOfstringArrayResulttM8isCsu>