Use static ReferenceComparer for reference comparisons

src/NHibernate/Collection/AbstractPersistentCollection.cs

@@ -741,7 +741,7 @@ protected virtual ICollection GetOrphans(ICollection oldElements, ICollection cu
 				return oldElements;
 			}
 
-			if (currentElements.Count == oldElements.Count && currentElements.Cast<object>().SequenceEqual(oldElements.Cast<object>(), new IdentityEqualityComparer()))
+			if (currentElements.Count == oldElements.Count && currentElements.Cast<object>().SequenceEqual(oldElements.Cast<object>(), ReferenceComparer<object>.Instance))
 				return Array.Empty<object>();
 
 			var persister = session.Factory.GetEntityPersister(entityName);

src/NHibernate/IdentityEqualityComparer.cs

@@ -5,6 +5,8 @@
 
 namespace NHibernate
 {
+	// Since v5.3 
+	[Obsolete("This class has no more usages and will be removed in a future version")]
 	[Serializable]
 	public class IdentityEqualityComparer : IEqualityComparer, IEqualityComparer<object>
 	{

src/NHibernate/Proxy/Poco/BasicLazyInitializer.cs

@@ -14,8 +14,6 @@ namespace NHibernate.Proxy.Poco
 	[Obsolete("DynamicProxy has been obsoleted, use static proxies instead (see StaticProxyFactory)")]
 	public abstract class BasicLazyInitializer : AbstractLazyInitializer
 	{
-		private static readonly IEqualityComparer IdentityEqualityComparer = new IdentityEqualityComparer();
-
 		internal System.Type persistentClass;
 		protected internal MethodInfo getIdentifierMethod;
 		protected internal MethodInfo setIdentifierMethod;
@@ -73,7 +71,7 @@ public virtual object Invoke(MethodInfo method, object[] args, object proxy)
 			{
 				if (!overridesEquals && methodName == "GetHashCode")
 				{
-					return IdentityEqualityComparer.GetHashCode(proxy);
+					return ReferenceComparer<object>.Instance.GetHashCode(proxy);
 				}
 				else if (IsEqualToIdentifierMethod(method))
 				{
@@ -92,7 +90,7 @@ public virtual object Invoke(MethodInfo method, object[] args, object proxy)
 			{
 				if (!overridesEquals && methodName == "Equals")
 				{
-					return IdentityEqualityComparer.Equals(args[0], proxy);
+					return ReferenceComparer<object>.Instance.Equals(args[0], proxy);
 				}
 				else if (setIdentifierMethod!=null&&method.Equals(setIdentifierMethod))
 				{

src/NHibernate/Transform/DistinctRootEntityResultTransformer.cs

@@ -3,6 +3,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using NHibernate.Util;
 
 namespace NHibernate.Transform
 {
@@ -12,19 +13,6 @@ public class DistinctRootEntityResultTransformer : IResultTransformer, ITupleSub
 		private static readonly INHibernateLogger log = NHibernateLogger.For(typeof(DistinctRootEntityResultTransformer));
 		internal static readonly DistinctRootEntityResultTransformer Instance = new DistinctRootEntityResultTransformer();
 
-		sealed class IdentityComparer<T> : IEqualityComparer<T>
-		{
-			public bool Equals(T x, T y)
-			{
-				return ReferenceEquals(x, y);
-			}
-
-			public int GetHashCode(T obj)
-			{
-				return RuntimeHelpers.GetHashCode(obj);
-			}
-		}
-
 		public object TransformTuple(object[] tuple, string[] aliases)
 		{
 			return tuple[tuple.Length - 1];
@@ -36,7 +24,7 @@ public IList TransformList(IList list)
 				return list;
 
 			IList result = (IList) Activator.CreateInstance(list.GetType());
-			var distinct = new HashSet<object>(new IdentityComparer<object>());
+			var distinct = new HashSet<object>(ReferenceComparer<object>.Instance);
 
 			for (int i = 0; i < list.Count; i++)
 			{
@@ -54,9 +42,9 @@ public IList TransformList(IList list)
 			return result;
 		}
 
-		internal static List<T> TransformList<T>(IEnumerable<T> list)
+		internal static List<T> TransformList<T>(IEnumerable<T> list) where T: class
 		{
-			var result = list.Distinct(new IdentityComparer<T>()).ToList();
+			var result = list.Distinct(ReferenceComparer<T>.Instance).ToList();
 			if (log.IsDebugEnabled())
 			{
 				log.Debug("transformed: {0} rows to: {1} distinct results", list.Count(), result.Count);

src/NHibernate/Util/IdentityMap.cs

@@ -39,7 +39,7 @@ public sealed class IdentityMap : IDictionary, IDeserializationCallback
 		/// <returns>A new IdentityMap based on a Hashtable.</returns>
 		public static IDictionary Instantiate(int size)
 		{
-			return new IdentityMap(new Hashtable(size, new IdentityEqualityComparer()));
+			return new IdentityMap(new Hashtable(size, ReferenceComparer<object>.Instance));
 		}
 
 		/// <summary>
@@ -50,7 +50,7 @@ public static IDictionary Instantiate(int size)
 		/// <returns>A new IdentityMap based on ListDictionary.</returns>
 		public static IDictionary InstantiateSequenced(int size)
 		{
-			return new IdentityMap(new SequencedHashMap(size, new IdentityEqualityComparer()));
+			return new IdentityMap(new SequencedHashMap(size, ReferenceComparer<object>.Instance));
 		}
 
 		/// <summary>