dataset中有多个表，返回XML字符串设置主从关系

<?xml version="1.0" encoding="utf-8" ?>
<string xmlns="http://tempuri.org/">
<Getstore>
<Goods>
<product_no>FB22S58613</product_no>
<product_class>女鞋</product_class>
<SKU>
<product_no>FB22S58613</product_no>
<color>46</color>
<product_size>39</product_size>
<color1>46</color1>
<product_class>女鞋</product_class>
<number3>3</number3>
<number7>0</number7>
<number8>0</number8>
<number>50</number>
<number9>0</number9>
</SKU>
<SKU>
<product_no>FB22S58613</product_no>
<color>36</color>
<product_size>39</product_size>
<color1>36</color1>
<product_class>女鞋</product_class>
<number3>2</number3>
<number7>0</number7>
<number8>0</number8>
<number>50</number>
<number9>0</number9>
</SKU>
</Goods>
</Getstore>
</string>

DataSet ds = api.get_xmlstore(productno,color,productsize,users,user_key);
ds.Tables[0].TableName = "Goods";
ds.Tables[1].TableName = "SKU";
DataRelation relation = new DataRelation("SKU",
ds.Tables [0].Columns["product_no"],
ds.Tables[1].Columns["product_no"]);
relation.Nested = true;
ds.Relations.Add(relation);
ds.DataSetName = "Getstore";
XmlDataDocument xda = new XmlDataDocument(ds);