How to get Requirement ID filed value by workitem collection?
<p>i am trying to get the TFS task information through below code but when i am trying to fetch the value of Requirement ID column for a task  then it is throwing FieldDefinitionNotExistException exception.</p>

<pre><code>    if (teamProjectCollection == null)
        {
            teamProjectCollection = TfsTeamProjectCollectionFactory.GetTeamProjectCollection(new Uri(tfsServer));
        }

        var workItemStore = (WorkItemStore)teamProjectCollection.GetService(typeof(WorkItemStore));

        //var project = workItemStore.Projects["SmartPracticeAids"];

        string iterationPath = @"SmartPracticeAids\2013 SPA Lite";
        string querystring = @"Select [System.Id], [TRTA.Common.RequirementID], [System.Title], [System.AssignedTo], [Microsoft.VSTS.Scheduling.OriginalEstimate], [System.IterationPath], [System.AreaPath], [System.State], [TRTA.Common.AlternateAssignedTo] from WorkItems where [System.TeamProject] = 'SmartPracticeAids' and [System.IterationPath] under '" + iterationPath + "' order by [System.IterationPath], [System.AssignedTo], [System.AreaPath]";


        var wiCollection = workItemStore.Query(querystring);

        CreateExcelDoc excell_app = new CreateExcelDoc();
        CreateHearders(excell_app);

        int i = 2;
        foreach (WorkItem workItem in wiCollection)
        {
            foreach (Field item in workItem.Fields)
            {
                Console.WriteLine(item.Name);
                char c = 'A';

                excell_app.addData(i, 1, workItem.Id.ToString(), c.ToString() + i, c.ToString() + i, "");
                excell_app.addData(i, 2, workItem.Fields["Requirement ID"].OriginalValue.ToString(), c.ToString() + i, c.ToString() + i, "");
                excell_app.addData(i, 3, workItem.Title, c.ToString() + i, c.ToString() + i, "");
</code></pre>
