Optimize DistinctRootEntityResultTransformer

Author: bahusoid

src/NHibernate/Transform/DistinctRootEntityResultTransformer.cs

@@ -12,24 +12,16 @@ public class DistinctRootEntityResultTransformer : IResultTransformer, ITupleSub
 		private static readonly INHibernateLogger log = NHibernateLogger.For(typeof(DistinctRootEntityResultTransformer));
 		private static readonly object Hasher = new object();
 
-		internal sealed class Identity
+		internal sealed class IdentityComparer : IEqualityComparer<object>
 		{
-			internal readonly object entity;
-
-			internal Identity(object entity)
-			{
-				this.entity = entity;
-			}
-
-			public override bool Equals(object other)
+			public new bool Equals(object x, object y)
 			{
-				Identity that = (Identity) other;
-				return ReferenceEquals(entity, that.entity);
+				return ReferenceEquals(x, y);
 			}
 
-			public override int GetHashCode()
+			public int GetHashCode(object obj)
 			{
-				return RuntimeHelpers.GetHashCode(entity);
+				return RuntimeHelpers.GetHashCode(obj);
 			}
 		}
 
@@ -40,13 +32,16 @@ public object TransformTuple(object[] tuple, string[] aliases)
 
 		public IList TransformList(IList list)
 		{
-			IList result = (IList)Activator.CreateInstance(list.GetType());
-			var distinct = new HashSet<Identity>();
+			if (list.Count < 2)
+				return list;
+
+			IList result = (IList) Activator.CreateInstance(list.GetType());
+			var distinct = new HashSet<object>(new IdentityComparer());
 
 			for (int i = 0; i < list.Count; i++)
 			{
 				object entity = list[i];
-				if (distinct.Add(new Identity(entity)))
+				if (distinct.Add(entity))
 				{
 					result.Add(entity);
 				}
@@ -77,13 +72,10 @@ public bool IsTransformedValueATupleElement(String[] aliases, int tupleLength)
 
 		public override bool Equals(object obj)
 		{
-			if (obj == null || obj.GetHashCode() != Hasher.GetHashCode())
-			{
-				return false;
-			}
-			// NH-3957: do not rely on hashcode alone.
-			// Must be the exact same type
-			return obj.GetType() == typeof(DistinctRootEntityResultTransformer);
+			if (ReferenceEquals(obj, this))
+				return true;
+
+			return obj is DistinctRootEntityResultTransformer;
 		}
 
 		public override int GetHashCode()