using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var fields = new List<Field>{new Field(1), new Field(1), new Field(2), new Field(3), new Field(3)};

        var artifacts = new List<Artifact>{new Artifact(1), new Artifact(2), new Artifact(2), new Artifact(3), new Artifact(3)};

        var duplicateFields = GetDuplicates(fields, new FieldComparer());

        var duplicateArtifacts = GetDuplicates(artifacts, new ArtifactComparer());

        Console.WriteLine("Dup Fields: {0}", string.Join(", ", duplicateFields.Select(x => x.ArtifactID)));

        Console.WriteLine("Dup Artifacts: {0}", string.Join(", ", duplicateArtifacts.Select(x => x.ArtifactID)));

    }

​

    // Added comparer parameter

    private static List<T> GetDuplicates<T>(IEnumerable<T> collection, IEqualityComparer<T> comparer)

    {

        List<T> collectionList = collection.ToList();

        // comparer used here

        IEnumerable<T> uniqueItems = collectionList.Distinct(comparer);

        if (uniqueItems.Count() == collectionList.Count)

        {

            return new List<T>();

        }

​

        // comparer used here too

        List<T> duplicateItems = collectionList.GroupBy(i => i, comparer).Where(group => group.Count() > 1).Select(group => group.Key).ToList();

        return duplicateItems;

    }

}

​

public class Field

{

    public int ArtifactID

    {

        get;

        set;

    }

​

    public Field(int id)

    {

        ArtifactID = id;

    }

}

​

public class Artifact

{

    public int ArtifactID

    {

        get;

        set;

    }

​

    public Artifact(int id)

    {

        ArtifactID = id;

    }

}

​

public class FieldComparer : IEqualityComparer<Field>

{

    public bool Equals(Field a, Field b)

    {

        return a.ArtifactID == b.ArtifactID;

    }

​

    public int GetHashCode(Field field)

    {

        return field.ArtifactID.GetHashCode();

    }

}

​

public class ArtifactComparer : IEqualityComparer<Artifact>

{

    public bool Equals(Artifact a, Artifact b)

    {

        return a.ArtifactID == b.ArtifactID;

    }

​

    public int GetHashCode(Artifact artifact)

    {

        return artifact.ArtifactID.GetHashCode();

    }

}