using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    // test class

    public class StudentTestData {

        public string AccessionNumber { get;set; }

        public string LoginId { get; set; }

    }

    // Custom comparer taking generic input parameter and a delegate function to do matching

    public class CustomComparer<T> : IEqualityComparer<T> {

        private readonly Func<T, object> _match;

        public CustomComparer(Func<T, object> match) {

            _match = match;

        }

        // tries to match both argument its return values against eachother

        public bool Equals(T data1, T data2) {

            return object.Equals(_match(data1), _match(data2));

        }

        // overly simplistic implementation

        public int GetHashCode(T data) {

            var matchValue = _match(data);

            if (matchValue == null) {

                return 42.GetHashCode();

            }

            return matchValue.GetHashCode();

        }

    }

    public static void Main()

    {

        var data = new List<StudentTestData>() {

            new StudentTestData() {

                AccessionNumber = "A1",

                LoginId = "L1"

            },

            new StudentTestData() {

                AccessionNumber = "A1",

                LoginId = "L2"

            },

            new StudentTestData() {

                AccessionNumber = "A2",

                LoginId = "L1"

            },

            new StudentTestData() {

                AccessionNumber = "A3",

                LoginId = "L1"

            },

            new StudentTestData() {

                AccessionNumber = "A1",

                LoginId = "L3"

            }

        };

        // compare by access number

        var accessComparer = new CustomComparer<StudentTestData>(d => d.AccessionNumber );

        // compare by login id

        var loginComparer = new CustomComparer<StudentTestData>(d => d.LoginId );

        Console.WriteLine("Per accession number");

        // add IEqualityComparer to distinct call

        foreach (var d in data.Distinct( accessComparer )) {

            Console.WriteLine( "{0}, {1}", d.AccessionNumber, d.LoginId);

        }

        Console.WriteLine("Per login number");

        // add IEqualityComparer to distinct call

        foreach (var d in data.Distinct( loginComparer )) {

            Console.WriteLine( "{0}, {1}", d.AccessionNumber, d.LoginId);

        }

    }

}