xml and condition by dateTime

XML

<CalendarFairs> 
    <CalendarFair>
        <DateStart>2011-04-05T00:00:00</DateStart>
       <DateEnd>2011-04-09T00:00:00</DateEnd>
        <Title>aaaa</Title>   
       <IdExecutive>1</IdExecutive>
   </CalendarFair>
  <CalendarFair>
      <DateStart>2011-04-16T00:00:00</DateStart>
      <DateEnd>2011-04-19T00:00:00</DateEnd>
      <Title>bbb</Title>
      <IdExecutive>2</IdExecutive>
  </CalendarFair>
<CalendarFairs>

Code

  var elements = from element in doc.Descendants("CalendarFair")
                           where DateTime.Parse (element.Elements ("DateStart").ToString())==DateTime.Now 
                           select new
                           {
                               dateStart = element.Element("DateStart").Value,
                               dateEnd=element.Element("DateEnd").Value,
                               title=element.Element("Title").Value,
                               idExcutive = element.Element("IdExecutive").Value ,
          开发者_如何学Go                 };

foreach (var item in elements)//send this error

            {}
System.FormatException: The string was not recognized as a valid DateTime. There is a 
unknown word starting at index 0.

why error?

Try to change it as follows:

var elements = from element in doc.Descendants("CalendarFair")
               let start = element.Element("DateStart").Value
               where DateTime.Parse (start)==DateTime.Now.Date
               select new
               {
                   dateStart = start,
                   dateEnd=element.Element("DateEnd").Value,
                   title=element.Element("Title").Value,
                   idExcutive = element.Element("IdExecutive").Value ,
               };

EDIT: based on the XML you have posted the query above works pretty well. Try to test it with this input:

<CalendarFairs> 
    <CalendarFair>
        <DateStart>2011-04-05T00:00:00</DateStart>
       <DateEnd>2011-04-09T00:00:00</DateEnd>
        <Title>aaaa</Title>   
       <IdExecutive>1</IdExecutive>
   </CalendarFair>
  <CalendarFair>
      <DateStart>2011-03-20T00:00:00</DateStart>
      <DateEnd>2011-04-19T00:00:00</DateEnd>
      <Title>bbb</Title>
      <IdExecutive>2</IdExecutive>
  </CalendarFair>
</CalendarFairs>

Note that I have inserted today's start date. Actually I think the result was empty just because there weren't entries with actual date.

It sounds like one or more of your input <DateStart> strings is not in a valid DateTime format.

Can you post some sample input XML?

It may be that you need to provide the date format using ParseExact - http://msdn.microsoft.com/en-us/library/w2sa9yss.aspx