LINQ to XML - why do I get a NullReferenceException while executing my query?

Question

My code:

string WHXMLHistory = "<?xml version=\"1.0\"?>\r\n<Workflows xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">\r\n  <Items>\r\n    <WorkflowsWorkflow>\r\n      <WorklfowID>230c30f4-f69f-48a0-8d77-39896cb88ae4</WorklfowID>\r\n      <Step>\r\n        <WorkflowsWorkflowStep>\r\n          <StartStep>2012-04-02T17:51:34.272725+04:00</StartStep>\r\n          <Curcle>1</Curcle>\r\n          <Task>\r\n            <WorkflowsWorkflowStepTask>\r\n              <ID>90</ID>\r\n              <To>NIDAN\\ostepanova;NIDAN\\kvinogradov;NIDAN\\skechutkina</To>\r\n              <RealTo>NIDAN\\ostepanova;NIDAN\\kvinogradov;NIDAN\\skechutkina</RealTo>\r\n              <Title>Департамент качества</Title>\r\n              <Description>Согласование документа Департамент качества</Description>\r\n              <StartDate>2012-04-02T17:51:34.272725+04:00</StartDate>\r\n              <Duration>24</Duration>\r\n            </WorkflowsWorkflowStepTask>\r\n          </Task>\r\n          <ID>90</ID>\r\n       
 </WorkflowsWorkflowStep>\r\n      </Step>\r\n      <Name>Согласование</Name>\r\n    </WorkflowsWorkflow>\r\n    <WorkflowsWorkflow>\r\n      <WorklfowID>d66ac93d-3df2-42f1-869c-848ec86f8c12</WorklfowID>\r\n      <Step>\r\n        <WorkflowsWorkflowStep>\r\n          <StartStep>2012-04-03T15:27:36.2514978+04:00</StartStep>\r\n          <Curcle>1</Curcle>\r\n          <Task>\r\n            <WorkflowsWorkflowStepTask>\r\n              <ID>90</ID>\r\n              <To>NIDAN\\ostepanova;NIDAN\\kvinogradov;NIDAN\\skechutkina</To>\r\n              <RealTo>NIDAN\\ostepanova;NIDAN\\kvinogradov;NIDAN\\skechutkina</RealTo>\r\n              <Title>Департамент качества</Title>\r\n              <Description>Согласование документа Департамент качества</Description>\r\n              <StartDate>2012-04-03T15:27:36.2514978+04:00</StartDate>\r\n              <Duration>24</Duration>\r\n            </WorkflowsWorkflowStepTask>\r\n          </Task>\r\n          <ID>90</ID>\r\n        </WorkflowsWorkflowStep>\r\n      </Step>\r\n  
    <Name>Согласование</Name>\r\n    </WorkflowsWorkflow>\r\n    <WorkflowsWorkflow>\r\n      <WorklfowID>5c070d9a-afef-46bf-bab7-c27032eded24</WorklfowID>\r\n      <Step>\r\n        <WorkflowsWorkflowStep>\r\n          <StartStep>2012-04-03T15:30:08.5907302+04:00</StartStep>\r\n          <Curcle>1</Curcle>\r\n          <Task>\r\n            <WorkflowsWorkflowStepTask>\r\n              <ID>90</ID>\r\n              <To>NIDAN\\ostepanova;NIDAN\\kvinogradov;NIDAN\\skechutkina</To>\r\n              <RealTo>NIDAN\\ostepanova;NIDAN\\kvinogradov;NIDAN\\skechutkina</RealTo>\r\n              <Title>Департамент качества</Title>\r\n              <Description>Согласование документа Департамент качества</Description>\r\n              <StartDate>2012-04-03T15:30:08.5907302+04:00</StartDate>\r\n              <Duration>24</Duration>\r\n            </WorkflowsWorkflowStepTask>\r\n          </Task>\r\n          <ID>90</ID>\r\n        </WorkflowsWorkflowStep>\r\n      </Step>\r\n      <Name>Согласование</Name>\r\n    </Workflows
Workflow>\r\n  </Items>\r\n</Workflows>";

XDocument xd = XDocument.Parse(WHXMLHistory);
                var arrwf = from n in xd.Root.Descendants("WorkflowsWorkflow")
                         orderby n.Element("StartDate").Value
                         select new
                         {
                             WorkflowID = n.Element("WorklfowID").Value,
                             StartUser = n.Element("StartUser").Value,
                             StartDate = n.Element("StartDate").Value
                         };
                var wf = arrwf.FirstOrDefault();

The last line throws NullReferenceException. I wonder why and how to avoid it?

Answer 1

Here is little bit modified source, and it worked for me

var arrwf = from n in xd.Root.Descendants("WorkflowsWorkflow")
                    orderby n.Element("Step").Element("WorkflowsWorkflowStep").Element("Task").Element("WorkflowsWorkflowStepTask").Element("StartDate").Value
                    select new
                    {
                        WorkflowID = n.Element("WorklfowID") == null ? null : n.Element("WorklfowID").Value,
                        StartUser = n.Element("StartUser") == null ? null : n.Element("StartUser").Value,
                        StartDate = n.Element("StartDate") == null ? null : n.Element("StartDate").Value
                    };
        var wf = arrwf.FirstOrDefault();

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