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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.

Inserting new data into xml from datatable

I have an xml file about 900MB and i want to insert data from datatable to that existing xml.
There is a way to do this by using load and save like below;
XDocument xdoc = XDocument.Load(FilePath);
var root = xdoc.Descendants("DocumentElement").FirstOrDefault()
if (root != null){
root.Add(new XElement("tag", "value"));
xdoc.Save(FilePath);
}
However, its cost too much. Loading 900MB xml into datatable can cause memory exceptions and poor performance for couple of new rows.
How can i add new rows to big existing xml from datatable?

XDocument (and XmlDocument as well) loads whole file into memory. You should use XmlWriter with combination of XNode.WriteTo method
Example from XNode.WriteTo documentation:
StringBuilder sb = new StringBuilder();
XmlWriterSettings xws = new XmlWriterSettings();
xws.OmitXmlDeclaration = true;
xws.Indent = true;
using (XmlWriter xw = XmlWriter.Create(sb, xws))
{
XElement child2 = new XElement("AnotherChild",
new XElement("GrandChild", "different content"));
child2.WriteTo(xw);
xw.WriteEndElement();
}
Console.WriteLine(sb.ToString());
You can read big xml documents in the same way, check example in XNode.ReadFrom documentation

How to include Declaration with XElement.ToString()

I am trying to write an XML response for my web service however I can't figure out how to make the declaration appear in the response.
My code is like so :
StringBuilder sBuilder = new StringBuilder();
using (XmlWriter writer = XmlWriter.Create(sBuilder))
{
writer.WriteStartDocument();
writer.WriteStartElement("ReportResponse");
Response.WriteXml(writer);
writer.WriteEndElement();
writer.WriteEndDocument();
}
var response = XElement.Parse(sBuilder.ToString());
return response;
Response is just a POCO for storing response data.
I am aware that the Save method includes the declaration and the ToString() method does not. I need to write my declaration back with ToString().
I really just want to return custom XML from my REST Service without casting my string 100 times to return valid XML. Is this even possible or am just spinning my wheels ?

If you want to include xml declaration, you can do it this way:
XDocument xdoc = XDocument.Parse(xmlString);
StringBuilder builder = new StringBuilder();
using (TextWriter writer = new StringWriter(builder))
{
xdoc.Save(writer);
}
Console.WriteLine(builder);
Update:
I've noticed, that StringWriter spoils encoding. So one more option is to do so:
string docWithDeclaration = xdoc.Declaration + xdoc.ToString();

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!

XDocument.Save() without header

I am editing csproj files with Linq-to-XML and need to save the XML without the <?XML?> header.
As XDocument.Save() is missing the necessary option, what's the best way to do this?

You can do this with XmlWriterSettings, and saving the document to an XmlWriter:
XDocument doc = new XDocument(new XElement("foo",
new XAttribute("hello","world")));
XmlWriterSettings settings = new XmlWriterSettings();
settings.OmitXmlDeclaration = true;
StringWriter sw = new StringWriter();
using (XmlWriter xw = XmlWriter.Create(sw, settings))
// or to write to a file...
//using (XmlWriter xw = XmlWriter.Create(filePath, settings))
{
doc.Save(xw);
}
string s = sw.ToString();

A simpler solution than the accepted answer is to use XDocument.ToString() to get the XML text without the header.
Example:
// Load the file
XDocument xDocument = XDocument.Load(fileName);
// Edit the XML...
// Save the edited XML text to file
File.WriteAllText(fileName, xDocument.ToString());