Custom object using Except failing to use IEqualityComparer

Question

here is the object code:

public class DlpItem : IEqualityComparer<DlpItem>
{   
    public string Text { get; set; }
    public int Id { get; set; }

    public DlpItem(int pId)
    {
        Text = string.Empty;
        Id = pId;
    }

    public override bool Equals(object obj)
    {
        return Id == (obj as DlpItem).Id;
    }

    public bool Equals(DlpItem a, DlpItem b)
    {
        return a.Id == b.Id;
    }

    public int GetHashCode(DlpItem item)
    {
        return Id.GetHashCode();
    }
}

And I have two lists as follows:

var list1 = new List<DlpItem>();
list1.Add(new DlpItem(1));
list1.Add(new DlpItem(2));


var list2 = new List<DlpItem>();
list2.Add(new DlpItem(1));
list2.Add(new DlpItem(2));

var delItems = list1.Except(list2).ToList<DlpItem>();

delItems always has both items in it. What am I missing here?

EDIT: Code now implements IEquatable

public class DlpItem : IEqualityComparer<DlpItem>, IEquatable<DlpItem>
{   
    public string Text { get; set; }
    public int Id { get; set; }


    public override bool Equals(object obj)
    {
        return Id - (obj as DlpItem).Id == 0;
    }

    public bool Equals(DlpItem a, DlpItem b)
    {
        return a.Id == b.Id;
    }

    public bool Equals(DlpItem item)
    {
        return item != null && Id == item.Id;
    }

    public int GetHashCode(DlpItem item)
    {
        return Id.GetHashCode();
    }
}

Marc Gravell · Accepted Answer · 2011-03-25T19:53:45.887

4

In your example, you don't actually add anything to list2... a simple enough mistake, but there is a more significant issue:

It needs to be IEquatable<T> not an IEqualityComparer<T>; also, you might want to ensure the hashcode can't change; most simply by making Id read-only:

public class DlpItem : IEquatable<DlpItem>
{
    public string Text { get; set; }
    private readonly int id;
    public int Id { get { return id; } }

    public DlpItem(int id)
    {
        Text = "";
        this.id = id;
    }

    public override bool Equals(object obj)
    {
        return Equals(obj as DlpItem);
    }

    public bool Equals(DlpItem other)
    {
        return other != null && this.Id == other.Id;
    }
    public override int GetHashCode()
    {
        return Id.GetHashCode();
    }
}

edited Mar 25 '11 at 19:53

answered Mar 25 '11 at 19:46

Marc Gravell

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I tried using this interface as well as per the MSDN indicated. The result was the same. I also corrected the typo in my example. – Matthew Cox Mar 25 '11 at 20:23
I also just tried to add back in assuming that maybe I had made a mistake, still no good. I'll post the code. – Matthew Cox Mar 25 '11 at 20:23
I didn't override GetHashCode() ... I just had public int GetHashCode() ... therefore it was failing. subtle correction but looking at the MSDN then back at your example helped me spot the issue! – Matthew Cox Mar 25 '11 at 21:19

Custom object using Except failing to use IEqualityComparer

1 Answers1