[RESOLVED] List<T>.Except() not working for me

In a nutshell, I have two generic lists of objects that I am using the .Except() extended method to return a list of exceptions into a new list. I remember getting this to work when I first researched it, but now that I am plugging it into the rest of the code, it returns the entire list instead of the exceptions.

The list is of these records:

Code:

---

using System;
using System.Configuration;        // ADD REFERENCE: System.Configuration (StringValidator)

namespace OHS_RCIS.Report.OB_Adv
{
    public class DenialRecord
    {
        private string inv_no;
        private string cur_bal;
        private string vendor;
        private string adm_dt;
        private string dschrg_dt;
        private string crt_dt;
        private string visit_ID;

        // Exposed constructor
        public DenialRecord
        (
            string Inv_No,
            string Cur_Bal,
            string Vendor,
            string Adm_Dt,
            string Dschrg_Dt,
            string Crt_Dt,
            string Visit_ID
        )
        {
            this.inv_no = FieldValidate(Inv_No);
            this.cur_bal = FieldValidate(Cur_Bal);
            this.vendor = FieldValidate(Vendor);
            this.adm_dt = FieldValidate(Adm_Dt);
            this.dschrg_dt = FieldValidate(Dschrg_Dt);
            this.crt_dt = FieldValidate(Crt_Dt);
            this.visit_ID = FieldValidate(Visit_ID);
        }

        // Properties used by the recon files as fields
        public string Inv_No { get { return inv_no; } set { inv_no = FieldValidate(Inv_No); } }            // always a string of 7-4 - null if Mansfield/Marion
        public string Cur_Bal { get { return cur_bal; } set { cur_bal = FieldValidate(Cur_Bal); } }        // currency in spreadsheet
        public string Vendor { get { return vendor; } set { vendor = FieldValidate(Vendor); } }            // always a string
        public string Adm_Dt { get { return adm_dt; } set { adm_dt = FieldValidate(Adm_Dt); } }            // a date in spreadsheet
        public string Dschrg_Dt { get { return dschrg_dt; } set { dschrg_dt = FieldValidate(Dschrg_Dt); } } // a date in spreadsheet
        public string Crt_Dt { get { return dschrg_dt; } set { dschrg_dt = FieldValidate(Crt_Dt); } }      // a date in spreadsheet
        public string Visit_ID { get { return visit_ID; } set { visit_ID = FieldValidate(Visit_ID); } }    // a number in spreadsheet - null if O'Bleness

        // Current AdviCare files come in with the balance field containing $ - invoice has a hyphen - headers contain underscores - 
        // dates have a forward slash. No other special characters should be legal.
        private string FieldValidate(string field)
        {
            string testVal = field;
            StringValidator strValidator = new StringValidator(1, 40, "!@#%^&*()+={}[]|?:;");

            try
            {
                strValidator.Validate(field);
            }
            catch (ArgumentException e)
            {
                Console.WriteLine("Error: {0}", e.ToString());
            }
            return field;
        }

        // Built-in comparer for list to list exceptions finding. Uses defined field in each record to compare.
        public DenialRecord Except(DenialRecord other)
        {
            if (this.inv_no != other.inv_no)
            {
                return this;
            }
            else
            {
                return null;
            }
        }
    }
}

---

The declaration of the lists is as follows:

Code:

---

            // Declare lists to hold active inventory and to hold exceptions
            List<DenialRecord> hospList = new List<DenialRecord>();
            List<DenialRecord> vendList = new List<DenialRecord>();
            List<DenialRecord> hospExcpt = new List<DenialRecord>();
            List<DenialRecord> vendExcpt = new List<DenialRecord>();

---

The actual call to the Except method is here:

Code:

---

            // Compare the lists each way for denials not in the other source
            hospExcpt = hospList.Except(vendList).ToList();
            vendExcpt = vendList.Except(hospList).ToList();

---

The class that builds the list is here:

Code:

---

        // Load csv to List<T>
        public List<DenialRecord> CsvToList(out int returnValue)
        {
            //int errorCode = 0;
            using (TextFieldParser csvParser = new TextFieldParser(this.fullPath))
            {
                csvParser.TextFieldType            = FieldType.Delimited;
                csvParser.SetDelimiters(",");
                csvParser.HasFieldsEnclosedInQuotes = true;
                csvParser.TrimWhiteSpace            = true;
                returnValue                        = Error.ERROR_SUCCESS;
                while (!csvParser.EndOfData)
                {
                    try
                    {
                        string[] field = csvParser.ReadFields();
                        if (field.Length == columnCount)
                        {
                            DenialRecord dr = new DenialRecord(field[0], field[1], field[2], field[3], field[4], field[5], field[6]);
                            inputList.Add(dr);
                        }
                    }
                    catch (Exception e)
                    {
                        switch(e.HResult)
                        {
                            case 2:
                                returnValue = Error.ERROR_FILE_NOT_FOUND;
                                break;
                            case 3:
                                returnValue = Error.ERROR_PATH_NOT_FOUND;
                                break;
                            case 5:
                                returnValue = Error.ERROR_ACCESS_DENIED;
                                break;
                            case 11:
                                returnValue = Error.ERROR_FORMAT;
                                break;
                            case 167:
                                returnValue = Error.ERROR_MALFORMED_LINE;
                                break;
                            case unchecked((int)0x80131502):
                                returnValue = Error.ERROR_ARGUMENT_OUT_OF_RANGE;
                                break;
                        }
                        DenialRecord dr = new DenialRecord("bad data", "bad data", "bad data", "bad data", "bad data", "bad data", "bad data");
                        inputList.Add(dr);
                    }
                }
            }
            return inputList;
        }
    }
}

---

The field each list is using for the comparison is a string. So far, I have not seen a reason in looking at the List<T> class, its methods or the classes it inherits from. There has got to be a reason I am just not seeing. Has anyone thought of what I could have overlooked?

EDIT: I found the solution here on CodeGuru. Mutant_Fruit explained it much more clearly than the places I had been looking. Essentially, even though you roll your own .Except() method in your class, when comparing objects or non-numbers, you have to build a hash of the field and override the Equals() function to do the correct compare required by the Exclude() function. Below is the working code:

Code:

---

                // Built-in comparer for list to list exceptions finding. Uses defined field in each record to compare.
        public DenialRecord Except(DenialRecord other)
        {
            if (this.inv_no != other.inv_no)
            {
                return this;
            }
            else
            {
                return null;
            }
        }

        // Except() requires overriding the Equals() and GetHashCode() to know how to compare
        public override bool Equals(object other)
        {
            if (other is DenialRecord)
                return ((DenialRecord)other).inv_no == inv_no;
            else
                return ((DenialRecord)other).inv_no == inv_no;
        }

        public override int GetHashCode()
        {
            return inv_no.GetHashCode();
        }

---

Jim, took the liberty of marking the thread resolved since you indicated you found the solution.