xml.Linq：“名称不能以 '?' 开头字符，十六进制值 0x3F"

Question

I try to create the below line above all of the files by using System.Xml.Linq

<?xml version="1.0" encoding="utf-8"?>

and here is the code

 var firstLine = new XElement(
            "?xml", 
            new XAttribute("version", "1.0"), 
            new XAttribute("encoding", "UTF-8"),
            "?");

but after the run, I get the below error

result Message: System.Xml.XmlException: Name cannot begin with the '?' character, hexadecimal value 0x3F.

I wonder if anyone knows how I can solve this?

Answer 1

That's an XML Declaration, represented by the XDeclaration type :

An example, from this doc :

XDocument doc = new XDocument(  
    new XDeclaration("1.0", "utf-8", "yes"),  
    new XElement("Root", "content")  
);  

Note that XDocument.ToString() will omit the declaration. Use XDocument.Save , eg:

using (var writer = new StringWriter())
{
    doc.Save(writer);
    Console.WriteLine(writer.ToString());
}

Note however that you'll get encoding="utf-16" in this case, because strings in .NET are UTF-16. If you want to serialize the XDocument to a UTF-8 byte array, then eg:

using (var stream = new MemoryStream())
{
    using (var writer = new StreamWriter(stream, Encoding.UTF8))
    {
        doc.Save(writer);
    }
    var utf8ByteArray = stream.ToArray();
    Console.WriteLine(Encoding.UTF8.GetString(utf8ByteArray));
}

Answer 2

<?xml version="1.0" encoding="utf-8"?>. 

to do so you need an XDeclaration with XDocument

XDocument doc = new XDocument(  
    new XDeclaration("1.0", "utf-8", "yes")
);