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C# - Append an XElement array to XElement

I have a c# application, where I'm doing a data compare of two xml files inside a method called RevisionTree. I return a list of elements(XElement) from this method. From the BuildXml method, call that method and save the list as tree. Next I create an xml root XElement. I then loop over each element from tree and add specified descendants (status, msg, date) to the root element, each one of these are XElement. So i should see an xml doument with root, then a list of repeating xml. However, when i try to save the to the writer i get the following error.
Error
Exception thrown: 'System.InvalidOperationException' in System.Private.Xml.dll
An unhandled exception of type 'System.InvalidOperationException' occurred in System.Private.Xml.dll
Token StartDocument in state Document would result in an invalid XML document.
Code
{
IEnumerable<XElement>
var tree = RevisionTree("C:\\Users\\Owner\\source\\repos\\SvnCore\\SvnCore\\old_logs.xml", "C:\\Users\\Owner\\source\\repos\\SvnCore\\SvnCore\\new_logs.xml");
using (XmlWriter writer = XmlWriter.Create("C:\\Users\\Owner\\source\\repos\\SvnCore\\SvnCore\\Temp.xml", xmlSettings))
{
writer.WriteStartDocument();
var root = new XElement("root");
foreach (var node in tree)
{
root.Add(new XElement("id", node.FirstAttribute));
root.Add(node.Descendants("status").FirstOrDefault());
root.Add(node.Descendants("msg").FirstOrDefault());
root.Add(node.Descendants("date").FirstOrDefault());
}
root.Save(writer);
writer.WriteEndElement();
writer.WriteEndDocument();
}
return true;
}

XElement.Save produces an entire document on its own -- you need XElement.WriteTo, which does not. So either (simplified):
var sb = new StringBuilder();
using (var sw = new StringWriter(sb))
using (XmlWriter writer = XmlWriter.Create(sw)) {
var root = new XElement("root");
root.Add(new XElement("id", "1"));
root.Save(writer); // no DocumentStart, no ElementStart
}
<?xml version="1.0" encoding="utf-16"?><root><id>1</id></root>
or (if you wanted to write multiple elements, or for some other reason want to control the document node yourself):
using (XmlWriter writer = XmlWriter.Create(sw)) {
writer.WriteStartDocument();
writer.WriteStartElement("root");
var notRoot = new XElement("notRoot");
notRoot.Add(new XElement("id", "1"));
notRoot.WriteTo(writer);
notRoot.WriteTo(writer);
}
<?xml version="1.0" encoding="utf-16"?><root><notRoot><id>1</id></notRoot><notRoot><id>1</id></notRoot></root>
Note that I'm omitting the End calls, since the XmlWriter will take care of that implicitly.
If you aren't doing anything interesting with the xmlSettings, the whole thing is even simpler since XElement.Save has an overload that accepts a file name directly, so you don't need an XmlWriter at all.

Correct way to add a namespace to an element using XmlDocument

I have to modify a incoming SOAP XML message to add a namespace to one of the elements so that the deserialisation will work. However when I add the xmlns attribute to the correct element, I get an error when I try to load the xml in to an XmlWriter via a stream (which I need to do to in my IClientMessageInspector implementation to replace the Message reply).
The prefix '' cannot be redefined from '' to 'http://www.example.com' within the same start element tag.
I have a work around, which is that after I've modifed the attribute, I reload the entire XML document from it's own OuterXML. This works for some reason, but makes me think there must be a 'correct' way to do this.
Here's a sample test that demonstrates the problem and my current solutions:
[Test]
public void XmlNamespaceTest()
{
var originalXmlString = "<?xml version=\"1.0\" encoding=\"utf-8\"?><TestElement><Child>thing</Child></TestElement>";
var desiredXmlString = "<?xml version=\"1.0\" encoding=\"utf-8\"?><TestElement xmlns=\"http://www.example.com\"><Child>thing</Child></TestElement>";
var doc = new XmlDocument();
doc.LoadXml(originalXmlString);
Assert.That(originalXmlString, Is.EqualTo(doc.OuterXml));
// Write this document via an XMLWriter
var ms = new MemoryStream();
var xmlWriter = XmlWriter.Create(ms);
doc.WriteTo(xmlWriter);
xmlWriter.Flush();
ms.Position = 0;
StreamReader sr = new StreamReader(ms);
var originalXmlViaXmlWriter = sr.ReadToEnd();
Assert.That(originalXmlString, Is.EqualTo(originalXmlViaXmlWriter));
// Add the namespace to the element
((XmlElement)doc.GetElementsByTagName("TestElement").Item(0))?.SetAttribute("xmlns", "http://www.example.com");
Assert.That(desiredXmlString, Is.EqualTo(doc.OuterXml));
// Now attempt to write this modified xml via an XMLWriter
ms.SetLength(0);
xmlWriter = XmlWriter.Create(ms);
//Uncomment line below to fix the error.
//doc.LoadXml(doc.OuterXml);
doc.WriteTo(xmlWriter);
xmlWriter.Flush();
ms.Position = 0;
sr = new StreamReader(ms);
var modifedXmlViaXmlWriter = sr.ReadToEnd();
Assert.That(desiredXmlString, Is.EqualTo(modifedXmlViaXmlWriter));
}

According to this you can't change an elements namespace in an XmlDocument. This is also what #LocEngineer found in his comment. The referenced article mentions that the only way to do this is to save and reload the XmlDocument, which is exactly what you are doing.
If you are in a position to use XDoxument instead, it is possible. See this answer for a solution.

How to check if XMLReader has valid XML without reading so as to write complete using XMLWriter?

I'm trying to write a string(which is nothing but XMLNodes) into a new XML File using XMLWriter. Few of the strings are valid XML content while few of the string aren't.
String Input:
1.
<Test>
<A a="Hello"></A>
<B b="Hello"></B>
</Test>
Hello
This is Sample String but not XML
Code :
using (XmlWriter writer = XmlWriter.Create(#"C:\\Test.XML"))
{
writer.WriteStartDocument();
string scontent2 = "Hello This is Sample String but not XML";
XmlReaderSettings settings = new XmlReaderSettings();
settings.ConformanceLevel = ConformanceLevel.Fragment;
try{
using (StringReader stringReader = new StringReader(scontent))
using (XmlReader xmlReader = XmlReader.Create(stringReader, settings))
{
writer.WriteStartElement("Test");
writer.WriteNode(xmlReader, true);
writer.WriteEndElement();
}catch(XMLException exception){}
}
Expected Output:
The Test element must also not be created if the Exception occurs. If I use, scontent.Read() or any such, the problem is since the pointer moves to a node, the writer.WriteNode(scontent,true) wont write entire nodes(if there are more than two nodes) For ex. <A a="Hello"></A><B b="Hello"></B>. In this case, I've write all nodes using WriteNode for which XMLReader must be in Initial State(XmlReader.State).

Creating an XML Element object from an XML Writer in C#

I'm writing a Windows service in C#. I've got an XmlWriter which is contains the output of an XSLT transformation. I need to get the XML into an XMLElement object to pass to a web service.
What is the best way to do this?

You do not need an intermediate string, you can create an XmlWriter that writes directly into an XmlNode:
XmlDocument doc = new XmlDocument();
using (XmlWriter xw = doc.CreateNavigator().AppendChild()) {
// Write to `xw` here.
// Nodes written to `xw` will not appear in the document
// until `xw` is closed/disposed.
}
and pass xw as the output of the transform.
NB. Some parts of the xsl:output will be ignored (e.g. encoding) because the XmlDocument will use its own settings.

Well, an XmlWriter doesn't contain the output; typically, you have a backing object (maybe a StringBuilder or MemoryStream) that is the dumping place. In this case, StringBuilder is probably the most efficient... perhaps something like:
StringBuilder sb = new StringBuilder();
using (XmlWriter writer = XmlWriter.Create(sb))
{
// TODO write to writer via xslt
}
string xml = sb.ToString();
XmlDocument doc = new XmlDocument();
doc.LoadXml(xml);
XmlElement el = doc.DocumentElement;

If you provide a writer, you provide a repository where an output generator is transferring data, thus the replay of Richard is good, you don't really need a string builder to send data from a reader to an XmlDocument!

How do I serialize an object into an XDocument?

I have a class that is marked with DataContract attributes and I would like to create an XDocument from objects of that class. Whats the best way of doing this?
I can do it by going via an XmlDocument but this seems like an unnecessary step.

You can create an XmlWriter directly into the XDocument:
XDocument doc = new XDocument();
using (var writer = doc.CreateWriter())
{
// write xml into the writer
var serializer = new DataContractSerializer(objectToSerialize.GetType());
serializer.WriteObject(writer, objectToSerialize);
}
Console.WriteLine(doc.ToString());

this is how i do it, which gives clean xml without all the namespace stuff in it,
XDocument xdoc = new XDocument(new XDeclaration("1.0", "utf-8", "yes"));
using (var writer = xdoc.CreateWriter())
{
System.Xml.Serialization.XmlSerializer x =
new System.Xml.Serialization.XmlSerializer(objecttoserialize.GetType());
x.Serialize(writer, objecttoserialize);
}
Debug.WriteLine(xdoc.ToString());