Distinct() with lambda?

Right, so I have an enumerable and wish to get distinct values from it.

Using System.Linq , there's of course an extension method called Distinct . In the simple case, it can be used with no parameters, like:

var distinctValues = myStringList.Distinct();

Well and good, but if I have an enumerable of objects for which I need to specify equality, the only available overload is:

var distinctValues = myCustomerList.Distinct(someEqualityComparer);

The equality comparer argument must be an instance of IEqualityComparer<T> . I can do this, of course, but it's somewhat verbose and, well, cludgy.

What I would have expected is an overload that would take a lambda, say a Func<T, T, bool>:

var distinctValues
    = myCustomerList.Distinct((c1, c2) => c1.CustomerId == c2.CustomerId);

Anyone know if some such extension exists, or some equivalent workaround? Or am I missing something?

Alternatively, is there a way of specifying an IEqualityComparer inline (embarass me)?

Update

I found a reply by Anders Hejlsberg to a post in an MSDN forum on this subject. He says:

The problem you're going to run into is that when two objects compare equal they must have the same GetHashCode return value (or else the hash table used internally by Distinct will not function correctly). We use IEqualityComparer because it packages compatible implementations of Equals and GetHashCode into a single interface.

I suppose that makes sense..

---

IEnumerable<Customer> filteredList = originalList
  .GroupBy(customer => customer.CustomerId)
  .Select(group => group.First());

---

It looks to me like you want DistinctBy from MoreLINQ. You can then write:

var distinctValues = myCustomerList.DistinctBy(c => c.CustomerId);

Here's a cut-down version of DistinctBy (no nullity checking and no option to specify your own key comparer):

public static IEnumerable<TSource> DistinctBy<TSource, TKey>
     (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
    HashSet<TKey> knownKeys = new HashSet<TKey>();
    foreach (TSource element in source)
    {
        if (knownKeys.Add(keySelector(element)))
        {
            yield return element;
        }
    }
}

---

No there is no such extension method overload for this. I've found this frustrating myself in the past and as such I usually write a helper class to deal with this problem. The goal is to convert a Func<T,T,bool> to IEqualityComparer<T,T> .

Example

public class EqualityFactory {
  private sealed class Impl<T> : IEqualityComparer<T,T> {
    private Func<T,T,bool> m_del;
    private IEqualityComparer<T> m_comp;
    public Impl(Func<T,T,bool> del) { 
      m_del = del;
      m_comp = EqualityComparer<T>.Default;
    }
    public bool Equals(T left, T right) {
      return m_del(left, right);
    } 
    public int GetHashCode(T value) {
      return m_comp.GetHashCode(value);
    }
  }
  public static IEqualityComparer<T,T> Create<T>(Func<T,T,bool> del) {
    return new Impl<T>(del);
  }
}

This allows you to write the following

var distinctValues = myCustomerList
  .Distinct(EqualityFactory.Create((c1, c2) => c1.CustomerId == c2.CustomerId));

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