At least one one object must implement Icomparable

Question

I am attempting to get unique values in a list of similar value distinguished only by a one element in a pipe delimited string... I keep getting at least one object must implement Icomparable. I don't understand why I keep getting that. I am able to groupBy that value... Why can't I find the max... I guess it is looking for something to compare it with. If I get the integer version will it stop yelling at me? This is the last time I am going to try using LINQ...

    var queryResults = PatientList.GroupBy(x => x.Value.Split('|')[1]).Select(x => x.Max());

I know I can get the unique values some other way. I am just having a hard time figuring it out. In that List I know that the string with the highest value amongst its similar brethren is the one that I want to add to the list. How can I do that? I am totally drawing a blank because I have been trying to get this to work in linq for the last few days with no luck...

    foreach (XmlNode node in nodeList)
            {
                XmlDocument xDoc = new XmlDocument();
                xDoc.LoadXml(node.OuterXml);
                string popPatInfo = xDoc.SelectSingleNode("./template/elements/element[@name=\"FirstName\"]").Attributes["value"].Value + ", " + xDoc.SelectSingleNode("./template/elements/element[@name=\"LastName\"]").Attributes["value"].Value + " | " + DateTime.Parse(xDoc.SelectSingleNode("./template/elements/element[@name=\"DateOfBirth\"]").Attributes["value"].Value.Split('T')[0]).ToString("dd-MMM-yyyy");
                string patientInfo = xDoc.SelectSingleNode("./template/elements/element[@name=\"PatientId\"]").Attributes["value"].Value + "|" + xDoc.SelectSingleNode("./template/elements/element[@name=\"PopulationPatientID\"]").Attributes["enc"].Value;// +"|" + xDoc.SelectSingleNode("./template/elements/element[@name=\"AdminDate\"]").Attributes["value"].Value;
                int enc = Int32.Parse(patientInfo.Split('|')[1]);
                if (enc > temp)
                {
                    lastEncounter.Add(enc, patientInfo);
                    temp = enc;
                }
                //lastEncounter.Add(Int32.Parse(patientInfo.Split('|')[1]));

                PatientList.Add( new SelectListItem { Text = popPatInfo, Value = patientInfo });
            }

I was thinking about using some kind of temp variable to find out what is the highest value and then add that string to the List. I am totally drawing a blank however...

Solution 1

Here I get the IDs in an anonymous type to make it readable.

var patientEncounters= from patient in PatientList
                       let PatientID=Int32.Parse(patient.Value.Split('|')[0])
                       let EncounterID=Int32.Parse(patient.Value.Split('|')[1])
                       select new { PatientID, EncounterID };

Then we group by UserID and get the last encounter

var lastEncounterForEachUser=from pe in patientEncounters
                             group pe by pe.PatientID into grouped
                             select new 
                                    { 
                                        PatientID=grouped.Key,
                                        LastEncounterID=grouped.Max(g=>g.EncounterID)
                                    };

Solution 2

Linq doesn't know how to compare 2 Patient objects, so it can't determine which one is the "greatest". You need to make the Patient class implement IComparable<Patient>, to define how Patient objects are compared.

// Compare objets by Id value
public int CompareTo(Patient other)
{
    return this.Id.CompareTo(other.Id);
}

Another option is to use the MaxBy extension method available in Jon Skeet's MoreLinq project:

var queryResults = PatientList.GroupBy(x => x.Value.Split('|')[1])
                              .Select(x => x.MaxBy(p => p.Id));

EDIT: I assumed there was a Patient class, but reading your code again, I realize it's not the case. PatientList is actually a collection of SelectListItem, so you need to implement IComparable in that class.