Today I was writing a LINQ query and I needed to select distinct values based on a comparison criteria.
Fortunately, LINQ’s Distinct method allows an equality comparer to be supplied, but, unfortunately, sometimes, this means having to write custom equality comparer.
Because I was going to need more than one equality comparer for this set of tools I was building, I decided to build a generic equality comparer that would just take a custom predicate. Something like this:
public class PredicateEqualityComparer<T> : EqualityComparer<T>
{
private Func<T, T, bool> predicate;
public PredicateEqualityComparer(Func<T, T, bool> predicate)
: base()
{
this.predicate = predicate;
}
public override bool Equals(T x, T y)
{
if (x != null)
{
return ((y != null) && this.predicate(x, y));
}
if (y != null)
{
return false;
}
return true;
}
public override int GetHashCode(T obj)
{
// Always return the same value to force the call to IEqualityComparer<T>.Equals
return 0;
}
}
Now I can write code like this:
.Distinct(new PredicateEqualityComparer<Item>((x, y) => x.Field == y.Field))
But I felt that I’d lost all conciseness and expressiveness of LINQ and it doesn’t support anonymous types. So I came up with another Distinct extension method:
public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource, bool> predicate)
{
return source.Distinct(new PredicateEqualityComparer<TSource>(predicate));
}
And the query is now written like this:
.Distinct((x, y) => x.Field == y.Field)
Looks a lot better, doesn’t it? And it works wit anonymous types.
Update: I, accidently, had published the wrong version of the IEqualityComparer<T>.Equals method,