Question

What I want
retrieve task whch “due” element is overdue.

What the XML is

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<tasks>
  <task>    
    <title>11111</title>
    <due>2012/06/18</due>
  </task>
  <task>
    <title>2121211212</title>    
    <due></due>
  </task>
</tasks>

What I code

        var res = from q in xml.Root.Descendants("task")
                  where q.Element("due").IsEmpty == false & (Convert.ToDateTime(q.Element("due").Value)).Date < DateTime.Now.Date
                  select q

What the error is

An exception of type ‘System.FormatException’ occurred in mscorlib.dll but was not handled in user code

Additional information: String was not recognized as a valid DateTime.
If there is a handler for this exception, the program may be safely continued.

-_-
If I delete the task which element”due” is empty, the error went away.
but Don’t I just filter the empty element with the code below?!

q.Element("due").IsEmpty == false

Why and how to solve it?

Answer 1

Per XElement.IsEmpty

Note that an element that contains a start and end tag with no content between the tags is not considered to be an empty element. It has content with no length. Only an element that contains only a start tag, and is expressed as a terminated empty element, is considered to be empty.

You could use string.IsNullOrWhiteSpace to check whether the element contains text characters:

var res = from q in xml.Root.Descendants("task")
          where q.Element("due").IsEmpty == false &&
                string.IsNullOrWhiteSpace(q.Element("due").Value) == false &&
                Convert.ToDateTime(q.Element("due").Value).Date < DateTime.Now.Date
          select q