corrected test example for issue 2710

gistfile1.txt

using System;
using System.Xml;
using System.Configuration;
using System.Xml.Serialization;
using System.Xml.Schema;
using System.IO;
using System.Globalization;

class Clobals
{
	public static void Main (string[] args)
	{
		Settings.Default.WindowPositions = new WindowPositionList ();
		//Settings.Default.Save ();
		Console.WriteLine(SerializedValue(Settings.Default.WindowPositions));
	}
	public static string SerializedValue(object propertyValue)
	{
		XmlSerializer serializer = new XmlSerializer (propertyValue.GetType ());
		using (StringWriter stream = new StringWriter(CultureInfo.InvariantCulture))
		{
			var settings = new XmlWriterSettings();
			settings.OmitXmlDeclaration = true;
			using (var writer = XmlWriter.Create (stream, settings))
			{
				var emptyNamepsaces = new XmlSerializerNamespaces(new[] { XmlQualifiedName.Empty });
				serializer.Serialize (writer, propertyValue, emptyNamepsaces);
			} // writer.Flush happens here
			var serializedValue = stream.ToString ();
			return serializedValue;
		}
	}
}

internal sealed partial class Settings : global::System.Configuration.ApplicationSettingsBase
{
	private static Settings defaultInstance = ((Settings)(global::System.Configuration.ApplicationSettingsBase.Synchronized (new Settings ())));

	public static Settings Default {
		get {
			return defaultInstance;
		}
	}

	[global::System.Configuration.UserScopedSettingAttribute ()]
	[global::System.Diagnostics.DebuggerNonUserCodeAttribute ()]
	public WindowPositionList WindowPositions {
		get {
			return ((WindowPositionList)(this ["WindowPositions"]));
		}
		set {
			this ["WindowPositions"] = value;
		}
	}

}

[Serializable]
public class WindowPositionList : IXmlSerializable
{
	public XmlSchema GetSchema ()
	{
		return null;
	}

	public void ReadXml (XmlReader reader)
	{
		reader.ReadStartElement ("WindowPositions");
		reader.ReadEndElement ();
	}

	public void WriteXml (XmlWriter writer)
	{
		writer.WriteStartElement ("WindowPositions");
		writer.WriteEndElement ();
	}
}

the output is

<WindowPositionList><WindowPositions /></WindowPositionList>

without XML declaration first line.