Speed up large lists

Author: TylerCarlson1

src/AutoMapper.Collection.Tests/MapCollectionWithEquality.cs

@@ -50,6 +50,15 @@ public void Should_Update_Existing_Item()
             Mapper.Map(dtos, items.ToList()).Should().HaveElementAt(0, items.First());
         }
 
+        public void Should_Be_Fast_With_Large_Lists()
+        {
+            var dtos = new object[100000].Select((_, i) => new ThingDto {ID = i}).ToList();
+
+            var items = new object[100000].Select((_, i) => new Thing { ID = i }).ToList();
+
+            Mapper.Map(dtos, items.ToList()).Should().HaveElementAt(0, items.First());
+        }
+
         public void Should_Work_With_Null_Destination()
         {
             var dtos = new List<ThingDto>

src/AutoMapper.Collection/Mappers/EquivalentExpressionAddRemoveCollectionMapper.cs

@@ -21,7 +21,13 @@ public static TDestination Map<TSource, TSourceItem, TDestination, TDestinationI
             if (source == null || destination == null)
                 return destination;
 
-            var compareSourceToDestination = source.ToDictionary(s => s, s => destination.FirstOrDefault(d => EquivalencyExpression.IsEquivalent(s, d)));
+            var destList = destination.ToList();
+            var compareSourceToDestination = source.ToDictionary(s => s, s =>
+            {
+                var match = destList.FirstOrDefault(d => EquivalencyExpression.IsEquivalent(s, d));
+                destList.Remove(match);
+                return match;
+            });
 
             foreach (var removedItem in destination.Except(compareSourceToDestination.Values).ToList())
                 destination.Remove(removedItem);