Where is my XDeclaration?

问题:

For some reason the following code produces XML that does not contain a declaration:

        XDocument xDocument = new XDocument(new XDeclaration("1.0", "utf-8", "yes"),
            new XElement("project",
                new XAttribute("number", project.ProjectNumber),
                new XElement("store",
                    new XAttribute("number", project.StoreNumber)
                ),

                // User Element
                new XElement("user", 
                    new XAttribute("owner-id", project.OwnerID ?? 0),
                    new XElement("email", new XCData(project.OwnerEmail ?? "")),
                    new XElement("project-name", new XCData(project.ProjectName ?? ""))
                ),

                // Nested Project Element
                new XElement("nested-project", 
                    new XAttribute("version", new Version(1, 0)),
                    new XElement("project", 
                        new XAttribute("version", new Version(1, 0)),
                        xProjectItems = new XElement("project-items")
                    ),
                    new XElement("price-per-part", project.PricePerPart),
                    new XElement("sheet-quantity", project.SheetQuantity),
                    new XElement("edge-length", project.EdgeLength),
                    new XElement("price", project.Price),
                    new XElement("status", project.Status),
                    xMaterialItems = new XElement("alternative-material-items"),
                    xProductItems = new XElement("project-product-items")
                )
            )
        );

        String strXML = xDocument.ToString();

It has produced a declaration before. Am I missing something obvious?

Thanks.

回答1:

The XDeclaration will be available when you use one of the XDocument.Save methods. For example:

var doc = new XDocument (
            new XDeclaration ("1.0", "utf-8", "yes"),
            new XElement ("test", "data")
        );

string path = Path.Combine(Path.GetTempPath(), "temp.xml");
doc.Save(path);
Console.WriteLine(File.ReadAllText(path));

Alternately you could use this approach:

var sw = new StringWriter();
doc.Save(sw);
string result = sw.GetStringBuilder().ToString();
Console.WriteLine(result);

EDIT: note that some approaches will convert the utf-8 designation to utf-16. If you want to force it to be utf-8 you would have to use this different approach:

using (var mem = new MemoryStream())  
using (var writer = new XmlTextWriter(mem, System.Text.Encoding.UTF8))
{
    writer.Formatting = Formatting.Indented;
    doc.WriteTo(writer);
    writer.Flush();
    mem.Flush();
    mem.Seek(0, SeekOrigin.Begin);
    using (var reader = new StreamReader(mem))
    {
        var xml = reader.ReadToEnd();
        Console.WriteLine(xml);
    }
}

回答2:

The documentation does not state explicitly that xDocument.ToString() will output the XML declaration, it only says: "Returns the indented XML for this node.".

In my testing, I found that the following methods will output the declaration:

Use ToString on the Declaration property:

string strXML = string.Concat(xDocument.Declaration.ToString(), "\r\n",
                              xDocument.ToString());

or use the Save method:

string strXml;
using(var ms = new MemoryStream())
{
    xDocument.Save(ms);
    ms.Position = 0;
    using(var sr = new StreamReader(ms))
    {
        strXml = sr.ReadToEnd();
    }
}

回答3:

My answer (inspired by the answer by @rsbarro):

string xml = xDocument.Declaration.ToString() +
    xDocument.ToString();

- or -

string xml = xDocument.Declaration.ToString() +
    xDocument.ToString(SaveOptions.DisableFormatting);

As to why I believe use of the + operator is appropriate in this situation, see this answer to the question What's the best string concatenation method using C#?.