在XML序列化时去除默认命名空间xmlns:xsd和xmlns:xsi

//Create our own namespaces for the output
XmlSerializerNamespaces ns = new XmlSerializerNamespaces ();
//Add an empty namespace and empty value
ns.Add ("", "");
//Create the serializer
XmlSerializer slz = new XmlSerializer (someType);
//Serialize the object with our own namespaces (notice the overload)
slz.Serialize (myXmlTextWriter, someObject, ns);

此外，在评论中还提到了去除开头的<?xml version="1.0" encoding="utf-8"?>的方法：
XmlWriterSettings settings = new XmlWriterSettings ();
// Remove the <?xml version="1.0" encoding="utf-8"?>
settings.OmitXmlDeclaration = true;
XmlWriter writer = XmlWriter.Create ("output_file_name.xml", settings);

另外，如果出现开头没有encoding="utf-8"时，应该使用：
XmlWriterSettings settings = new XmlWriterSettings ();
settings.Encoding = Encoding.UTF8;
XmlWriter writer = XmlWriter.Create ("output_file_name.xml", settings);