Convert EntityCollection to a xml string in the format of fetchxml result - crm 2011 onpremises

Hi all, I have old  external webservice  which use fetchxml and get result in xml format.  (using very old way of connecting crm    "http://crmservername/mscrmservices/2007/crmservice.asmx" and organization name with token)

I need to upgrade this to use earlybound entity class and OrganizationServiceProxy  with crm 2011 organization service url . I have generate xrm.cs using CRMSvcUtil and other libraries. But there is no Fetch any more isn't it ?

so I have used RetrieveMultiple (new FetchExpression(fetchsrt)) 

now my question is how do I get this convert to string result in same format of fetchxml result.  So I don't need to change coding on applications that I am using this web service (as a gateway to handle crm data)

ideas pls