// See http://stackoverflow.com/questions/33641530/filtering-duplicates-from-a-type-collection-using-linq#33641530

​

using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public class SomeType

    {

        public string ExternalDisplayId

        {

            get;

            set;

        }

​

        public string ExternalID

        {

            get;

            set;

        }

​

        public string Application

        {

            get;

            set;

        }

​

        public DateTime CreateDate

        {

            get;

            set;

        }

    }

​

    // Custom comparer for the SomeType class

    class ApplicationExternalIDComparer : IEqualityComparer<SomeType>

    {

        // SomeTypes are equal if their fields are equal.

        public bool Equals(SomeType x, SomeType y)

        {

            //Check whether the compared objects reference the same data.

            if (Object.ReferenceEquals(x, y))

                return true;

            //Check whether the SomeTypes' properties are equal.

            return (x.Application == y.Application && x.ExternalID == y.ExternalID);

        }

​

        // If Equals() returns true for a pair of objects 

        // then GetHashCode() must return the same value for these objects.

        public int GetHashCode(SomeType sometype)

        {

            //Calculate the hash code for the SomeType.

            return new { sometype.Application, sometype.ExternalID}.GetHashCode();

        }

    }

​

    // Custom comparer for the SomeType class

    class ApplicationExternalDisplayIdComparer : IEqualityComparer<SomeType>

    {

        // SomeTypes are equal if their fields are equal.

        public bool Equals(SomeType x, SomeType y)

        {

            //Check whether the compared objects reference the same data.

            if (Object.ReferenceEquals(x, y))

                return true;

            //Check whether any of the compared objects is null.

            if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))

                return false;

            //Check whether the SomeTypes' properties are equal.

            return (x.Application == y.Application && x.ExternalDisplayId == y.ExternalDisplayId);

        }

​

        // If Equals() returns true for a pair of objects 

        // then GetHashCode() must return the same value for these objects.

        public int GetHashCode(SomeType sometype)

        {

            //Calculate the hash code for the product.

            return new { sometype.Application, sometype.ExternalDisplayId }.GetHashCode();

        }

    }

​

    public static void Main()

    {

        SomeType[] products =

        {

            new SomeType

            {

                Application = "Test", ExternalID = "1234", ExternalDisplayId = "111", CreateDate = new DateTime(2015,2,1) 

            }

            , new SomeType

            {

                Application = "Test", ExternalID = "1234", ExternalDisplayId = "5", CreateDate = new DateTime(2015,1,1) 

            }

            , new SomeType

            {

                Application = "Mono", ExternalID = "012", ExternalDisplayId = "90", CreateDate = new DateTime(2015,6,6) 

            }

            , new SomeType

            {

                Application = "Epic", ExternalID = "999", ExternalDisplayId = "78", CreateDate = new DateTime(2015,8,8)

            }

            , new SomeType

            {

                Application = "Epic", ExternalID = "333", ExternalDisplayId = "78", CreateDate = new DateTime(2015,8,12)

            }

            , new SomeType

            {

                Application = "Test", ExternalID = "345", ExternalDisplayId = "33", CreateDate = new DateTime(2015,2,1)

            }

            , new SomeType

            {

                Application = "Test", ExternalID = "666", ExternalDisplayId = "33", CreateDate = new DateTime(2015,1,1) 

            }

        };

​

        //Exclude duplicates.

        var noduplicates = products.GroupBy(x => new {x.Application, x.ExternalDisplayId})

                                   .Select(y => y.OrderByDescending(x => x.CreateDate).First())

                                   .ToList()

                                   .Distinct(new ApplicationExternalDisplayIdComparer())

                                   .GroupBy(x => new {x.Application, x.ExternalID})

                                   .Select(y => y.OrderByDescending(x => x.CreateDate).First())

                                   .ToList()

                                   .Distinct(new ApplicationExternalIDComparer());

        foreach (var product in noduplicates)

            Console.WriteLine("App:" + product.Application + " ExternalID:" + product.ExternalID + " ExternalDisplayID:" + product.ExternalDisplayId + " " + product.CreateDate.Date);

    }

}