Question

I have no idea what this exception is. Just when I thought I was getting the hang of linq something like this happens. I am getting this exception:

Unable to cast object of type 'WhereSelectEnumerableIterator`2[System.Xml.Linq.XElement,TestImplementationIdeas.PatientClass]' to type 'TestImplementationIdeas.PatientClass'.

Now, I have know idea why this exception is being throw.

PatientClass template = (PatientClass)(from templates in xDocument.Descendants("template").Where
                                                       (templates => ((templates.Descendants("element").Attributes("name").ToString() == "EncounterId") && templates.Descendants("element").Attributes("value").ToString() == Enc.ToString())) //elem.XPathSelectElements(string.Format("//templates/template[./elements/element[@name=\"PopulationPatientID\"and @value='{0}' and @enc='{1}']]",PopulationPatID, Enc))
                                                       select new PatientClass
                                                       {
                                                           PatientId = Convert.ToInt32("0" + templates.XPathSelectElement("elements/element[@name='PatientId']").Attribute("value").Value),
                                                           EMPIID = Convert.ToInt32("0" + templates.XPathSelectElement("elements/element[@name='EMPIID']").Attribute("value").Value),
                                                           //public int PopulationPatientID { get; set; }
                                                           FirstName = templates.XPathSelectElement("elements/element[@name='FirstName']").Attribute("value").Value,
                                                           LastName = templates.XPathSelectElement("elements/element[@name='LastName']").Attribute("value").Value,

                                                       });
            return template != null ? template : null;

Answer 1

In general, LINQ queries always return an IEnumerable<> object, in your case, IEnumerable<PatientClass>. You need to perform additional opertions to filter a collection to a single object.

//  will return the first `PatientClass` in the collection.  
//   And will throw an exception if the collection is empty.
return template.First(); 

//  will return the first `PatientClass` in the collection or return the default value       
//   (null in this case) if the collection is empty.
return template.FirstOrDefault(); 

// will return the only `PatientClass` in the collection if the collection only has 
//  one object and will throw an exception if it is empty or has more than 1 object.
return template.Single(); 

// will return the only `PatientClass` in the collection if the collection only has 
//  one object, the default value (null in this case) if it is empty and will throw an    
//  exception if it has more than 1 object.
return template.SingleOrDefault(); 

And instead of the OrDefault() methods, if you want to control what value is returned when it is empty, there is a DefaultIfEmpty<>() extension method that you can specify a value if the collection is empty.

// will return the first element in the collection, but if it is empty, 
//  it will return a new Patient class object created with the default constructor instead of a null
return template.DefaultIfEmpty(new PatientClass()).First();