xml创建根节点时如何在根节点上加多个命名空间呢

<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encodi
ng/" >
</SOAP-ENV:Envelope> 

补充：.NET技术 ,  C#