Got it working after discussing with my senior.  (thank god he call from vacation to check on me).

He explain that the results of the query are not an XML document...

Still can't get this to work.

The XML is

<Table><Row><Column>-10000.00</Column></Row><Row><Column>3000.00</Column></Row><Row><Column>4200.00</Column></Row><Row><Column>6800.00</Column></Row></Table>

I modified my C# to:

using System;
using System.Collections;
using System.Data;
using System.Data.SqlTypes;
using System.IO;
using System.Xml;
using Microsoft.SqlServer.Server;
 
public partial class SQLFunctions
{

 ...