IEqualityComparer for anonymous type

Question

I have this

 var n = ItemList.Select(s => new { s.Vchr, s.Id, s.Ctr, s.Vendor, s.Description, s.Invoice }).ToList();
 n.AddRange(OtherList.Select(s => new { s.Vchr, s.Id, s.Ctr, s.Vendor, s.Description, s.Invoice }).ToList(););

I would like to do this if it where allowed

n = n.Distinct((x, y) => x.Vchr == y.Vchr)).ToList();

I tried using the generic LambdaComparer but since im using anonymous types there is no type associate it with.

"Help me Obi Wan Kenobi, you're my only hope"

Great question, I was just looking for the same thing. Unbelievable it's not in the standard library. — orip, Aug 06 '09 at 14:57
It could help [Wrap a delegate in an IEqualityComparer](http://stackoverflow.com/questions/98033/wrap-a-delegate-in-an-iequalitycomparer) — marbel82, Oct 07 '16 at 15:10

score 19 · Accepted Answer · edited Oct 17 '12 at 01:40

19

The trick is to create a comparer that only works on inferred types. For instance:

public class Comparer<T> : IComparer<T> {
  private Func<T,T,int> _func;
  public Comparer(Func<T,T,int> func) {
    _func = func;
  }
  public int Compare(T x,  T y ) {
    return _func(x,y);
  }
}

public static class Comparer {
  public static Comparer<T> Create<T>(Func<T,T,int> func){ 
    return new Comparer<T>(func);
  }
  public static Comparer<T> CreateComparerForElements<T>(this IEnumerable<T> enumerable, Func<T,T,int> func) {
    return new Comparer<T>(func);
  }
}

Now I can do the following ... hacky solution:

var comp = n.CreateComparerForElements((x, y) => x.Vchr == y.Vchr);

edited Oct 17 '12 at 01:40

Nathan Baulch

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answered Jul 01 '09 at 22:17

JaredPar

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Very slick. Im thinking though that in the interest of writing clean code I should create an interface to use as the T in an IEqualityComparer. – kjgilla Jul 02 '09 at 02:32
I couldn't get by the problem. See below. – Tormod Oct 05 '10 at 11:31

score 3 · Answer 2 · edited May 23 '17 at 12:00

3

Most of the time when you compare (for equality or sorting) you're interested in choosing the keys to compare by, not the equality or comparison method itself (this is the idea behind Python's list sort API).

There's an example key equality comparer here.

edited May 23 '17 at 12:00

Community

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answered Aug 06 '09 at 14:56

orip

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score 1 · Answer 3 · answered May 31 '11 at 16:19

I note that JaredPar's answer does not quite answer the question since the set methods like Distinct and Except require an IEqualityComparer<T> not an IComparer<T>. The following assumes that an IEquatable will have a suitable GetHashCode, and it certainly has a suitable Equals method.

public class GeneralComparer<T, TEquatable> : IEqualityComparer<T>
{
    private readonly Func<T, IEquatable<TEquatable>> equatableSelector;

    public GeneralComparer(Func<T, IEquatable<TEquatable>> equatableSelector)
    {
        this.equatableSelector = equatableSelector;
    }

    public bool Equals(T x, T y)
    {
        return equatableSelector.Invoke(x).Equals(equatableSelector.Invoke(y));
    }

    public int GetHashCode(T x)
    {
        return equatableSelector(x).GetHashCode();
    }
}

public static class GeneralComparer
{
    public static GeneralComparer<T, TEquatable> Create<T, TEquatable>(Func<T, TEquatable> equatableSelector)
    {
        return new GeneralComparer<T, TEquatable>(equatableSelector);
    }
}

Where the same inference from a static class trick is used as in JaredPar's answer.

To be more general you could provide two Funcs: a Func<T, T, bool> to check equality and Func<T, T, int> to select a hash code.

Though JaredPar's answer is about comparer, it gives the idea how to go abt it. — nawfal, Apr 15 '13 at 07:19
In this case, would be nice to show how it's used in methods like `Distinct()` — PandaWood, Jan 28 '20 at 00:23

IEqualityComparer for anonymous type

3 Answers3

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