.NET: XDocument missing XDeclaration? [duplicate]

Question

Is there any way to have an XDocument print the xml version when using the ToString method? Have it output something like this:

<?xml version="1.0"?>
<!DOCTYPE ELMResponse [
]>
<Response>
<Error> ...

I have the following:

var xdoc = new XDocument(new XDocumentType("Response", null, null, "\n"), ...

which will print this which is fine, but it is missing the "<?xml version" as stated above.

<!DOCTYPE ELMResponse [
]>
<Response>
<Error> ...

I know that you can do this by outputting it manually my self. Just wanted to know if it was possible by using XDocument.

Answer 1

By using XDeclaration. This will add the declaration.

But with ToString() you will not get the desired output.

You need to use XDocument.Save() with one of his methods.

Full sample:

var doc = new XDocument(
        new XDeclaration("1.0", "utf-16", "yes"), 
        new XElement("blah", "blih"));

var wr = new StringWriter();
doc.Save(wr);
Console.Write(wr.ToString());

Answer 2

Late answer to an old question, but I shall try to provide more details than the other answers.

The thing you ask about, is called an XML declaration.

First of all, the XDocument has a property Declaration of type XDeclaration for this. You can either user another overload of the XDocument constructor:

var xdoc = new XDocument(
  new XDeclaration("1.0", null, null), // <--- here
  new XDocumentType("Response", null, null, "\n"), ... 
  );

or set the property later:

xdoc.Declaration = new XDeclaration("1.0", null, null);

But depending on how you save or write your XDocument later, the declaration (or parts of it) may be ignored. More on that later.

The XML declaration can have a number of appearances. Here are some valid examples:

<?xml version="1.0"?>                                        new XDeclaration("1.0", null, null)
<?xml version="1.1"?>                                        new XDeclaration("1.1", null, null)
<?xml version="1.0" encoding="us-ascii"?>                    new XDeclaration("1.0", "us-ascii", null)
<?xml version="1.0" encoding="utf-8"?>                       new XDeclaration("1.0", "utf-8", null)
<?xml version="1.0" encoding="utf-16"?>                      new XDeclaration("1.0", "utf-16", null)
<?xml version="1.0" encoding="utf-8" standalone="no"?>       new XDeclaration("1.0", "utf-8", "no")
<?xml version="1.0" encoding="utf-8" standalone="yes"?>      new XDeclaration("1.0", "utf-8", "yes")
<?xml version="1.0" standalone="yes"?>                       new XDeclaration("1.0", null, "yes")

Note that XDeclaration will happily accept invalid arguments, so it is up to you to get it right.

In many cases the first one, <?xml version="1.0"?>, the form you ask for, is perfect (it is not needed to give encoding if it is just UTF-8 (including ASCII), and it is not needed to specify standalone if its intended value is "no" or if there are no DTDs).

Note that xdoc.ToString() goes do the override from the XNode base class (in my version of .NET) and does not include the XML declaration. You can easily enough create a method to deal with that, like this:

public static string ToStringWithDecl(this XDocument d)
  => $"{d.Declaration}{Environment.NewLine}{d}";

Some of the other answers indicate that the XDeclaration will be respected if you use xdoc.Save or xdoc.WriteTo methods, but that is not quite true:

• They might include an XML declaration even if you have none in your XDocument
• They might specify the encoding used by the target file, stream, writer, string builder etc. instead of the encoding you gave, or instead of omitting the encoding if you did that in your XDeclaration
• They might change your version from e.g. 1.1 into 1.0

Of course, when you save/write to a file, it is a good thing that the declaration matches the true encoding of that file!

But sometimes when you write to a string in mememory, you do not want the utf-16 (even if you realize that .NET strings are in UTF-16 internally). You can use the extension method above instead. Or you can use the following hacked version of the method from EricSch's answer:

  string xdocString;
  using (var hackedWriter = new SuppressEncodingStringWriter())
  {
    xdoc.Save(hackedWriter);
    xdocString = hackedWriter.ToString();
  }

where you have:

// a string writer which claims its encoding is null in order to omit encoding in XML declarations
class SuppressEncodingStringWriter : StringWriter
{
  public sealed override Encoding Encoding => null;
}

---

Addition. Whoever updated the type StringWriter (and its base type TextWriter) to use nuallable reference types, decided that properties such as Encoding and FormatProvider should not be nullable. Maybe that was unfortunate? Anyway, if you have nullable reference types turned on, you may need to write null! instead of just null.