(There are two solutions here - see the end for the second one):
My MiscUtil library has a ProjectionEqualityComparer
class (and two supporting classes to make use of type inference).
Here's an example of using it:
EqualityComparer<GalleryImage> comparer =
ProjectionEqualityComparer<GalleryImage>.Create(x => x.id);
Here's the code (comments removed)
// Helper class for construction
public static class ProjectionEqualityComparer
{
public static ProjectionEqualityComparer<TSource, TKey>
Create<TSource, TKey>(Func<TSource, TKey> projection)
{
return new ProjectionEqualityComparer<TSource, TKey>(projection);
}
public static ProjectionEqualityComparer<TSource, TKey>
Create<TSource, TKey> (TSource ignored,
Func<TSource, TKey> projection)
{
return new ProjectionEqualityComparer<TSource, TKey>(projection);
}
}
public static class ProjectionEqualityComparer<TSource>
{
public static ProjectionEqualityComparer<TSource, TKey>
Create<TKey>(Func<TSource, TKey> projection)
{
return new ProjectionEqualityComparer<TSource, TKey>(projection);
}
}
public class ProjectionEqualityComparer<TSource, TKey>
: IEqualityComparer<TSource>
{
readonly Func<TSource, TKey> projection;
readonly IEqualityComparer<TKey> comparer;
public ProjectionEqualityComparer(Func<TSource, TKey> projection)
: this(projection, null)
{
}
public ProjectionEqualityComparer(
Func<TSource, TKey> projection,
IEqualityComparer<TKey> comparer)
{
projection.ThrowIfNull("projection");
this.comparer = comparer ?? EqualityComparer<TKey>.Default;
this.projection = projection;
}
public bool Equals(TSource x, TSource y)
{
if (x == null && y == null)
{
return true;
}
if (x == null || y == null)
{
return false;
}
return comparer.Equals(projection(x), projection(y));
}
public int GetHashCode(TSource obj)
{
if (obj == null)
{
throw new ArgumentNullException("obj");
}
return comparer.GetHashCode(projection(obj));
}
}
Second solution
To do this just for Distinct, you can use the DistinctBy
extension in MoreLINQ:
public static IEnumerable<TSource> DistinctBy<TSource, TKey>
(this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector)
{
return source.DistinctBy(keySelector, null);
}
public static IEnumerable<TSource> DistinctBy<TSource, TKey>
(this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IEqualityComparer<TKey> comparer)
{
source.ThrowIfNull("source");
keySelector.ThrowIfNull("keySelector");
return DistinctByImpl(source, keySelector, comparer);
}
private static IEnumerable<TSource> DistinctByImpl<TSource, TKey>
(IEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IEqualityComparer<TKey> comparer)
{
HashSet<TKey> knownKeys = new HashSet<TKey>(comparer);
foreach (TSource element in source)
{
if (knownKeys.Add(keySelector(element)))
{
yield return element;
}
}
}
In both cases, ThrowIfNull
looks like this:
public static void ThrowIfNull<T>(this T data, string name) where T : class
{
if (data == null)
{
throw new ArgumentNullException(name);
}
}