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Linq to XML query works when returning single entity but needs to be of list<T>
<foo>
   <xxx>Some XXX</xxx>
   <bar Attr1="true">Some Bar</bar>
   <bar Attr2="true">Some Bar #2</bar>
   <bar>Some Bar #3</bar>
   <bar>Some Bar #4</bar>
</foo>

var foos = from query in xmlData.Descendants("foo")
       from bars in query.Elements("bar")
       select new FooClass
       {
    xxx = (string)query.Element("xxx"),
    barCollection = new BarClass
    {
        display = bars.Value,
        attr1Exists = (answers.Attribute("Attr1") != null ? true : false)
        attr2Exists = (answers.Attribute("Attr2") != null ? true : false)
        }
       };

var foos = from query in xmlData.Descendants("foo")
select new FooClass
{
    xxx = (string)query.Element("xxx"),
    bar = query.Elements("bar").Select(x => x.Value).ToList()
};

var foos = from query in xmlData.Descendants("foo")
select new FooClass
{
    xxx = (string)query.Element("xxx"),
    bar = query.Elements("bar").Select(x => new BarClass
    {
        display = x.Value,
        attr1Exists = x("Attr1") != null ? true : false),
        attr2Exists = x("Attr2") != null ? true : false)
        }

    ).ToList()
};

List<BarClass> mybars = Foo.Bars; // is this your aim?

var foos = from query in xmlData.Descendants("foo")
  select new FooClass
  {
       xxx = (string)query.Element("xxx"),
       Bars = query.Elements("bar").Select(x => new BarClass()
          {
             Attr1 = .. /// etc
             Attr2 = ....
          }).ToList();
  }