What's the most efficient way to locate and set element values in an XDocument?

Question

Given the following XML \'template\':

Answer 1

Does this do it for you? Good old Descendants property.

string xmlInput = ...;
XDocument myDoc = XDocument.Parse(xmlInput);
//
List<XElement> someElements = myDoc.Descendants("a").ToList();
someElements.ForEach(x => x.Value = "Foo");
//
Console.WriteLine(myDoc);

Hmm, I see you have an attribute in there. Can do that too:

string xmlInput = //...
XDocument myDoc = XDocument.Parse(xmlInput);
//
List<XText> someText =
  myDoc.Descendants()
  .Nodes()
  .OfType<XText>()
  .Where(x => x.Value.StartsWith("{") && x.Value.EndsWith("}"))
  .ToList();
//
List<XAttribute> someAttributes =
  myDoc.Descendants()
  .Attributes()
  .Where(x => x.Value.StartsWith("{") && x.Value.EndsWith("}"))
  .ToList();
//
someText.ForEach(x => x.Value = "Foo");
someAttributes.ForEach(x => x.Value = "Bar");
//
Console.WriteLine(myDoc);

Ah, now with what you're expecting, I will make it work:

List<XElement> e = myDoc.Descendants("a").ToList();
e.Where(x => x.Attribute("name").Value == "username").Single().Value = "abc";
e.Where(x => x.Attribute("name").Value == "password").Single().Value = "abc";