Output.xml

<?xml version="1.0" encoding="utf-8"?>
<ArrayOfItem xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Item Completed="true">
    <DoItem>fds</DoItem>
  </Item>
  <Item Completed="false">
    <DoItem>Find</DoItem>
  </Item>
</ArrayOfItem>

Source.cs

// Sample program:
using System;
using System.Collections.Generic;
using System.Xml;
using System.Xml.Serialization;

public class Item {
	public string DoItem;

	[XmlAttribute ()]
	public bool Completed;
}

class Demo {
	public static void Main ()
	{
		var list = new List<Item> {
			new Item {DoItem = "fds", Completed=true},
			new Item {DoItem = "Find", Completed=false},
		};

		Serialize (list);
	}

	static void Serialize(List<Item> list)
	{
		Type[] itemTypes = { typeof(Item) };
		XmlSerializer serializer = new XmlSerializer(typeof(List<Item>), itemTypes);
		// FileStream fs = new FileStream("ToDoList.xml", FileMode.Create);
		serializer.Serialize(Console.Out, list);
		// fs.Close();
	}
}