ARTICLE AD BOX
I've implemented a custom XmlWriter that ensures that the namespaces in the root element are sorted before they're written out:
/// <summary> /// XmlWriter that ensures the namespace declarations in the root element are always sorted. /// </summary> class SortedNamespaceXmlWriter : XmlWriter { private readonly XmlWriter _baseWriter; private readonly List<(string prefix, string uri)> _namespaces = new List<(string prefix, string uri)>(); private int _elementIndex; private string _nsPrefix; private bool _inXmlNsAttribute; public SortedNamespaceXmlWriter(XmlWriter baseWriter) => _baseWriter = baseWriter; public override void WriteStartElement(string prefix, string localName, string ns) { _elementIndex++; FlushRootElementAttributesIfNeeded(); _baseWriter.WriteStartElement(prefix, localName, ns); } public override void WriteStartAttribute(string prefix, string localName, string ns) { if (prefix == "xmlns") { _inXmlNsAttribute = true; _nsPrefix = localName; } else { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteStartAttribute(prefix, localName, ns); } } public override void WriteEndAttribute() { if (_inXmlNsAttribute) _inXmlNsAttribute = false; else _baseWriter.WriteEndAttribute(); } public override void WriteString(string text) { if (_inXmlNsAttribute) _namespaces.Add((_nsPrefix, text)); else _baseWriter.WriteString(text); } private void FlushRootElementAttributesIfNeeded() { if (_elementIndex != 1 || _namespaces.Count == 0) return; _namespaces.Sort((a, b) => StringComparer.Ordinal.Compare(a.prefix, b.prefix)); foreach (var (prefix, uri) in _namespaces) { _baseWriter.WriteStartAttribute("xmlns", prefix, null); _baseWriter.WriteString(uri); _baseWriter.WriteEndAttribute(); } _namespaces.Clear(); } public override WriteState WriteState => _baseWriter.WriteState; public override void Flush() => _baseWriter.Flush(); public override string LookupPrefix(string ns) => _baseWriter.LookupPrefix(ns); public override void WriteBase64(byte[] buffer, int index, int count) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteBase64(buffer, index, count); } public override void WriteCData(string text) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteCData(text); } public override void WriteCharEntity(char ch) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteCharEntity(ch); } public override void WriteChars(char[] buffer, int index, int count) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteChars(buffer, index, count); } public override void WriteComment(string text) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteComment(text); } public override void WriteDocType(string name, string pubid, string sysid, string subset) => _baseWriter.WriteDocType(name, pubid, sysid, subset); public override void WriteEndDocument() => _baseWriter.WriteEndDocument(); public override void WriteEndElement() { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteEndElement(); } public override void WriteEntityRef(string name) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteEntityRef(name); } public override void WriteFullEndElement() { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteFullEndElement(); } public override void WriteProcessingInstruction(string name, string text) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteProcessingInstruction(name, text); } public override void WriteRaw(char[] buffer, int index, int count) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteRaw(buffer, index, count); } public override void WriteRaw(string data) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteRaw(data); } public override void WriteStartDocument() => _baseWriter.WriteStartDocument(); public override void WriteStartDocument(bool standalone) => _baseWriter.WriteStartDocument(); public override void WriteSurrogateCharEntity(char lowChar, char highChar) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteSurrogateCharEntity(lowChar, highChar); } public override void WriteWhitespace(string ws) { FlushRootElementAttributesIfNeeded(); _baseWriter.WriteWhitespace(ws); } }