Use a delegate for the equality comparer for LINQ's Distinct()

Question

I have a LINQ Distinct() statement that uses my own custom comparer, like this:

class MyComparer : IEqualityComparer where T : MyType
{
            


        

Answer 1

Distinct takes an IEqualityComparer as the second argument, so you will need an IEqualityComparer. It's not too hard to make a generic one that will take a delegate, though. Of course, this has probably already been implemented in some places, such as MoreLINQ suggested in one of the other answers.

You could implement it something like this:

public static class Compare
{
    public static IEnumerable DistinctBy(this IEnumerable source, Func identitySelector)
    {
        return source.Distinct(Compare.By(identitySelector));
    }

    public static IEqualityComparer By(Func identitySelector)
    {
        return new DelegateComparer(identitySelector);
    }

    private class DelegateComparer : IEqualityComparer
    {
        private readonly Func identitySelector;

        public DelegateComparer(Func identitySelector)
        {
            this.identitySelector = identitySelector;
        }

        public bool Equals(T x, T y)
        {
            return Equals(identitySelector(x), identitySelector(y));
        }

        public int GetHashCode(T obj)
        {
            return identitySelector(obj).GetHashCode();
        }
    }
}

Which gives you the syntax:

source.DistinctBy(a => a.Id);

Or, if you feel it's clearer this way:

source.Distinct(Compare.By(a => a.Id));