How do you force explicit tag closing with Linq XML?

Explicitly setting the XElement value to an empty string should work. LINQ-to-XML already treats nodes without content (like new XElement("foo")) differently from nodes with content of length zero (like new XElement("foo", string.Empty)), as you can see from the documentation on XElement.IsEmpty.

But in case that doesn't work, or in case you need to fine tune some other aspect of the XML output, you can derive a custom XmlWriter:

public class MyWriter : XmlWriter{    private readonly XmlWriter inner;    public MyWriter(XmlWriter inner)    {        this.inner = inner;    }    public void Dispose()    {        ((IDisposable) inner).Dispose();    }    public override void WriteStartDocument()    {        inner.WriteStartDocument();    }    public override void WriteStartDocument(bool standalone)    {        inner.WriteStartDocument(standalone);    }    public override void WriteEndDocument()    {        inner.WriteEndDocument();    }    public override void WriteDocType(string name, string pubid, string sysid, string subset)    {        inner.WriteDocType(name, pubid, sysid, subset);    }    public override void WriteStartElement(string prefix, string localName, string ns)    {        inner.WriteStartElement(prefix, localName, ns);    }    public override void WriteEndElement()    {        inner.WriteFullEndElement();    }    public override void WriteFullEndElement()    {        inner.WriteFullEndElement();    }    public override void WriteStartAttribute(string prefix, string localName, string ns)    {        inner.WriteStartAttribute(prefix, localName, ns);    }    public override void WriteEndAttribute()    {        inner.WriteEndAttribute();    }    public override void WriteCData(string text)    {        inner.WriteCData(text);    }    public override void WriteComment(string text)    {        inner.WriteComment(text);    }    public override void WriteProcessingInstruction(string name, string text)    {        inner.WriteProcessingInstruction(name, text);    }    public override void WriteEntityRef(string name)    {        inner.WriteEntityRef(name);    }    public override void WriteCharEntity(char ch)    {        inner.WriteCharEntity(ch);    }    public override void WriteWhitespace(string ws)    {        inner.WriteWhitespace(ws);    }    public override void WriteString(string text)    {        inner.WriteString(text);    }    public override void WriteSurrogateCharEntity(char lowChar, char highChar)    {        inner.WriteSurrogateCharEntity(lowChar, highChar);    }    public override void WriteChars(char[] buffer, int index, int count)    {        inner.WriteChars(buffer, index, count);    }    public override void WriteRaw(char[] buffer, int index, int count)    {        inner.WriteRaw(buffer, index, count);    }    public override void WriteRaw(string data)    {        inner.WriteRaw(data);    }    public override void WriteBase64(byte[] buffer, int index, int count)    {        inner.WriteBase64(buffer, index, count);    }    public override void Close()    {        inner.Close();    }    public override void Flush()    {        inner.Flush();    }    public override string LookupPrefix(string ns)    {        return inner.LookupPrefix(ns);    }    public override WriteState WriteState    {        get { return inner.WriteState; }    }}

The relevant method is this one:

public override void WriteEndElement(){    inner.WriteFullEndElement(); // always write both start and close tags}

I can't reproduce your error. This works as expected in both 4.0 and 3.5 netFX:

namespace ExplicitXmlClosingTags{    using System.Xml;    using System.Xml.Linq;    class Program    {        static void Main(string[] args)        {            const string ElementRoot = "RootElement";            const string ElementChild = "ChildElement";            const string AttributeChild = "ChildAttribute";            XDocument xDoc = new XDocument();            XElement root = new XElement(ElementRoot);            XElement child = new XElement(ElementChild, string.Empty);            root.Add(child);            child.SetAttributeValue(AttributeChild, "AttrValue");            xDoc.Add(root);            XmlWriterSettings xws = new XmlWriterSettings();            xws.Indent = true;            using (XmlWriter xw = XmlWriter.Create("out.xml", xws))            {                xDoc.Save(xw);                }        }    }}

producing following content:

<?xml version="1.0" encoding="utf-8"?><RootElement>  <ChildElement ChildAttribute="AttrValue"></ChildElement></RootElement>

set the value of the XElement to String.Empty

OR

setting the IsEmpty property to false for all elements not having any child nodes

    foreach (XElement childElement in        from x in document.DescendantNodes().OfType<XElement>()        where x.IsEmpty        select x)    {        childElement.IsEmpty = false;    }