rename IEntityData into IEntityStruct, it's not necessary to add confusion with more names

fix MoveEntityView function
fix Preallocate function

Svelto.ECS/DataStructures/TypeSafeDictionary.cs

@@ -32,7 +32,7 @@ namespace Svelto.ECS.Internal
         int Count { get; }
     }
 
-    class TypeSafeDictionary<TValue> : FasterDictionary<int, TValue>, ITypeSafeDictionary where TValue : IEntityData
+    class TypeSafeDictionary<TValue> : FasterDictionary<int, TValue>, ITypeSafeDictionary where TValue : IEntityStruct
     {
         public TypeSafeDictionary(int size):base(size)
         {}
@@ -109,11 +109,6 @@ namespace Svelto.ECS.Internal
             }
         }
 
-        public void AddCapacity(int size)
-        {
-            throw new NotImplementedException();
-        }
-
         public ITypeSafeDictionary Create()
         {
             return new TypeSafeDictionary<TValue>();

Svelto.ECS/EnginesRoot.Entities.cs

@@ -61,44 +61,48 @@ namespace Svelto.ECS
 
         ///--------------------------------------------
 
-        /// <summary>
-        /// This function is experimental and untested. I never used it in production
-        /// it may not be necessary.
-        /// TODO: understand if this method is useful in a performance critical
-        /// scenario
-        /// </summary>
         void Preallocate<T>(int groupID, int size) where T : class, IEntityDescriptor, new()
         {
             var entityViewsToBuild = EntityDescriptorTemplate<T>.Info.entityViewsToBuild;
             var count              = entityViewsToBuild.Length;
 
+            //reserve space in the database
+            Dictionary<Type, ITypeSafeDictionary> @group;
+            if (_groupEntityDB.TryGetValue(groupID, out group) == false)
+                group = _groupEntityDB[groupID] = new Dictionary<Type, ITypeSafeDictionary>();
+
+            //reserve space in building buffer
+            Dictionary<Type, ITypeSafeDictionary> @groupBuffer;
+            if (_groupedEntityToAdd.current.TryGetValue(groupID, out @groupBuffer) == false)
+                @groupBuffer = _groupedEntityToAdd.current[groupID] = new Dictionary<Type, ITypeSafeDictionary>();
+
+            ITypeSafeDictionary dbList;
+
             for (var index = 0; index < count; index++)
             {
                 var entityViewBuilder = entityViewsToBuild[index];
                 var entityViewType    = entityViewBuilder.GetEntityType();
 
-                //reserve space for the global pool
-                ITypeSafeDictionary dbList;
-
-                //reserve space for the single group
-                Dictionary<Type, ITypeSafeDictionary> @group;
-                if (_groupEntityDB.TryGetValue(groupID, out group) == false)
-                    group = _groupEntityDB[groupID] = new Dictionary<Type, ITypeSafeDictionary>();
-                
                 if (group.TryGetValue(entityViewType, out dbList) == false)
                     group[entityViewType] = entityViewBuilder.Preallocate(ref dbList, size);
                 else
                     dbList.AddCapacity(size);
                 
-                if (_groupedEntityToAdd.current.TryGetValue(groupID, out group) == false)
-                    group = _groupEntityDB[groupID] = new Dictionary<Type, ITypeSafeDictionary>();
-                
-                //reserve space to the temporary buffer
-                if (group.TryGetValue(entityViewType, out dbList) == false)
-                    group[entityViewType] = entityViewBuilder.Preallocate(ref dbList, size);
+                if (@groupBuffer.TryGetValue(entityViewType, out dbList) == false)
+                    @groupBuffer[entityViewType] = entityViewBuilder.Preallocate(ref dbList, size);
                 else
                     dbList.AddCapacity(size);
             }
+
+            if (group.TryGetValue(_typeEntityInfoView, out dbList) == false)
+                group[_typeEntityInfoView] = EntityViewBuilder<EntityInfoView>.Preallocate(ref dbList, size);
+            else
+                dbList.AddCapacity(size);
+
+            if (@groupBuffer.TryGetValue(_typeEntityInfoView, out dbList) == false)
+                @groupBuffer[_typeEntityInfoView] = EntityViewBuilder<EntityInfoView>.Preallocate(ref dbList, size);
+            else
+                dbList.AddCapacity(size);
         }
         
         ///--------------------------------------------
@@ -161,26 +165,24 @@ namespace Svelto.ECS
                 _groupEntityDB.Add(toGroupID, groupedEntityViewsTyped);
             }
             
-            ITypeSafeDictionary toSafeList;
+            ITypeSafeDictionary toSafeDic;
 
             for (var i = 0; i < entityViewBuildersCount; i++)
             {
                 var entityViewBuilder = entityViewBuilders[i];
                 var entityViewType    = entityViewBuilder.GetEntityType();
 
-                var fromSafeList = groupedEntities[entityViewType];
-                if (groupedEntityViewsTyped.TryGetValue(entityViewType, out toSafeList) == false)
-                    groupedEntityViewsTyped[entityViewType] = toSafeList = fromSafeList.Create();
+                var fromSafeDic = groupedEntities[entityViewType];
+                if (groupedEntityViewsTyped.TryGetValue(entityViewType, out toSafeDic) == false)
+                    groupedEntityViewsTyped[entityViewType] = toSafeDic = fromSafeDic.Create();
 
-                entityViewBuilder.MoveEntityView(entityegid, toGroupID, fromSafeList, toSafeList);
-                fromSafeList.Remove(entityegid.entityID);
+                entityViewBuilder.MoveEntityView(entityegid, toGroupID, fromSafeDic, toSafeDic);
             }
             
-            if (groupedEntityViewsTyped.TryGetValue(_typeEntityInfoView, out toSafeList) == false)
-                groupedEntityViewsTyped[_typeEntityInfoView] = toSafeList = entityInfoViewDictionary.Create();
+            if (groupedEntityViewsTyped.TryGetValue(_typeEntityInfoView, out toSafeDic) == false)
+                groupedEntityViewsTyped[_typeEntityInfoView] = toSafeDic = entityInfoViewDictionary.Create();
 
-            EntityViewBuilder<EntityInfoView>.MoveEntityView(entityegid, toGroupID, entityInfoViewDictionary, toSafeList);
-            entityInfoViewDictionary.Remove(entityegid.entityID);
+            EntityViewBuilder<EntityInfoView>.MoveEntityView(entityegid, toGroupID, entityInfoViewDictionary, toSafeDic);
         }
         
         EGID SwapFirstEntityGroup(int fromGroupID, int toGroupId)
@@ -208,7 +210,7 @@ namespace Svelto.ECS
             _id = id;
         }
 
-        public void Init<T>(ref T initializer) where T: struct, IEntityData
+        public void Init<T>(ref T initializer) where T: struct, IEntityStruct
         {
             var typeSafeDictionary = (TypeSafeDictionary<T>) _current[typeof(T)];
 

Svelto.ECS/EntityViewBuilder.cs

@@ -9,13 +9,13 @@ using Svelto.Utilities;
 
 namespace Svelto.ECS
 {
-    public class EntityViewBuilder<T> : IEntityViewBuilder where T : IEntityData, new()
+    public class EntityViewBuilder<T> : IEntityViewBuilder where T : IEntityStruct, new()
     {
         public EntityViewBuilder()
         {
             _initializer = default(T);
-            
-#if DEBUG && !PROFILER           
+
+#if DEBUG && !PROFILER
             if (needsReflection == false && typeof(T) != typeof(EntityInfoView))
             {
                 var type = typeof(T);
@@ -30,10 +30,10 @@ namespace Svelto.ECS
                     if (field.FieldType.IsPrimitive == true || field.FieldType.IsValueType == true)
                         continue;
                     
-                    throw new EntityStructException();
+                    throw new EntityStructException(field.FieldType);
                 }
             }
-#endif            
+#endif
             if (needsReflection == true)
             {
                 EntityView<T>.InitCache();
@@ -69,7 +69,12 @@ namespace Svelto.ECS
             }
         }
 
-        public ITypeSafeDictionary Preallocate(ref ITypeSafeDictionary dictionary, int size)
+        ITypeSafeDictionary IEntityViewBuilder.Preallocate(ref ITypeSafeDictionary dictionary, int size)
+        {
+            return Preallocate(ref dictionary, size);
+        }
+
+        public static ITypeSafeDictionary Preallocate(ref ITypeSafeDictionary dictionary, int size)
         {
             if (dictionary == null)
                 dictionary = new TypeSafeDictionary<T>(size);
@@ -95,9 +100,9 @@ namespace Svelto.ECS
             var toCastedDic = toSafeDic as TypeSafeDictionary<T>;
 
             var entity = fromCastedDic[entityID.entityID];
+            fromCastedDic.Remove(entityID.entityID);
             entity.ID = new EGID(entityID.entityID, toGroupID);
             toCastedDic.Add(entityID.entityID, entity);
-            fromCastedDic.Remove(entityID.entityID);
         }
 
         static FasterList<KeyValuePair<Type, ActionCast<T>>> entityViewBlazingFastReflection
@@ -114,7 +119,7 @@ namespace Svelto.ECS
 
     public class EntityStructException : Exception
     {
-        public EntityStructException():base("EntityStruct must contains only value types!")
+        public EntityStructException(Type fieldType):base("EntityStruct must contains only value types! " + fieldType.ToString())
         {}
     }
 }

Svelto.ECS/EntityViewsDB.cs

@@ -12,12 +12,12 @@ namespace Svelto.ECS.Internal
             _groupEntityViewsDB = groupEntityViewsDB;
         }
 
-        public ReadOnlyCollectionStruct<T> QueryEntityViews<T>() where T:class, IEntityData
+        public ReadOnlyCollectionStruct<T> QueryEntityViews<T>() where T:class, IEntityStruct
         {
             return QueryEntityViews<T>(ExclusiveGroups.StandardEntity);
         }
 
-        public ReadOnlyCollectionStruct<T> QueryEntityViews<T>(int @group) where T:class, IEntityData
+        public ReadOnlyCollectionStruct<T> QueryEntityViews<T>(int @group) where T:class, IEntityStruct
         {
             Dictionary<Type, ITypeSafeDictionary> entitiesInGroupPerType;
 
@@ -31,12 +31,12 @@ namespace Svelto.ECS.Internal
             return (outList as TypeSafeDictionary<T>).FasterValues;
         }
 
-        public T[] QueryEntities<T>(out int count) where T : IEntityData
+        public T[] QueryEntities<T>(out int count) where T : IEntityStruct
         {
             return QueryEntities<T>(ExclusiveGroups.StandardEntity, out count);
         }
         
-        public T[] QueryEntities<T>(int @group, out int count) where T : IEntityData
+        public T[] QueryEntities<T>(int @group, out int count) where T : IEntityStruct
         {
             count = 0;
             
@@ -52,7 +52,7 @@ namespace Svelto.ECS.Internal
             return ((TypeSafeDictionary<T>)typeSafeDictionary).GetFasterValuesBuffer(out count);
         }
 
-        public T[] QueryEntities<T>(EGID entityGID, out uint index) where T : IEntityData
+        public T[] QueryEntities<T>(EGID entityGID, out uint index) where T : IEntityStruct
         {
             TypeSafeDictionary<T> casted;
             if (!FindSafeDictionary(entityGID, out casted))
@@ -74,7 +74,7 @@ namespace Svelto.ECS.Internal
             return QueryEntities<T>(out count);
         }
 
-        public T QueryEntityView<T>(EGID entityGID) where T : class, IEntityData
+        public T QueryEntityView<T>(EGID entityGID) where T : class, IEntityStruct
         {
             T entityView;
 
@@ -83,7 +83,7 @@ namespace Svelto.ECS.Internal
             return entityView;
         }
 
-        public void ExecuteOnEntity<T, W>(EGID entityGID, ref W value, ActionRef<T, W> action) where T : IEntityData
+        public void ExecuteOnEntity<T, W>(EGID entityGID, ref W value, ActionRef<T, W> action) where T : IEntityStruct
         {
             TypeSafeDictionary<T> casted;
             if (!FindSafeDictionary(entityGID, out casted)) return;
@@ -92,7 +92,7 @@ namespace Svelto.ECS.Internal
                 casted.ExecuteOnEntityView(entityGID.entityID, ref value, action);
         }
         
-        public void ExecuteOnEntity<T>(EGID entityGID, ActionRef<T> action) where T : IEntityData
+        public void ExecuteOnEntity<T>(EGID entityGID, ActionRef<T> action) where T : IEntityStruct
         {
             TypeSafeDictionary<T> casted;
             if (!FindSafeDictionary(entityGID, out casted)) return;
@@ -101,7 +101,7 @@ namespace Svelto.ECS.Internal
                 casted.ExecuteOnEntityView(entityGID.entityID, action);
         }
 
-        public bool Exists<T>(EGID entityGID) where T : IEntityData
+        public bool Exists<T>(EGID entityGID) where T : IEntityStruct
         {
             TypeSafeDictionary<T> casted;
             if (!FindSafeDictionary(entityGID, out casted)) return false;
@@ -115,7 +115,7 @@ namespace Svelto.ECS.Internal
             return false;
         }
 
-        bool FindSafeDictionary<T>(EGID entityGID, out TypeSafeDictionary<T> casted) where T : IEntityData
+        bool FindSafeDictionary<T>(EGID entityGID, out TypeSafeDictionary<T> casted) where T : IEntityStruct
         {
             var type = typeof(T);
 
@@ -133,26 +133,26 @@ namespace Svelto.ECS.Internal
             return true;
         }
 
-        public bool HasAny<T>() where T : IEntityData
+        public bool HasAny<T>() where T : IEntityStruct
         {
             int count;
             QueryEntities<T>(out count);
             return count > 0;
         }
 
-        public bool HasAny<T>(int @group) where T : IEntityData
+        public bool HasAny<T>(int @group) where T : IEntityStruct
         {
             int count;
             QueryEntities<T>(group, out count);
             return count > 0;
         }
 
-        public bool TryQueryEntityView<T>(EGID entityegid, out T entityView) where T : class, IEntityData
+        public bool TryQueryEntityView<T>(EGID entityegid, out T entityView) where T : class, IEntityStruct
         {
             return TryQueryEntityViewInGroup(entityegid, out entityView);
         }
 
-        bool TryQueryEntityViewInGroup<T>(EGID entityGID, out T entityView) where T:IEntityData
+        bool TryQueryEntityViewInGroup<T>(EGID entityGID, out T entityView) where T:IEntityStruct
         {
             TypeSafeDictionary<T> casted;
             if (!FindSafeDictionary(entityGID, out casted))

Svelto.ECS/GenericEntityDescriptor.cs

@@ -1,6 +1,6 @@
 namespace Svelto.ECS
 {
-    public abstract class GenericEntityDescriptor<T>:IEntityDescriptor where T : IEntityData, new()
+    public abstract class GenericEntityDescriptor<T>:IEntityDescriptor where T : IEntityStruct, new()
     {
         static GenericEntityDescriptor()
         {
@@ -15,8 +15,8 @@
         static readonly IEntityViewBuilder[] entityViewBuilders;
     }
 
-    public abstract class GenericEntityDescriptor<T, U> : IEntityDescriptor     where T : IEntityData, new() 
-                                                                       where U : IEntityData, new()
+    public abstract class GenericEntityDescriptor<T, U> : IEntityDescriptor     where T : IEntityStruct, new() 
+                                                                       where U : IEntityStruct, new()
     {
         static GenericEntityDescriptor()
         {
@@ -31,9 +31,9 @@
         static readonly IEntityViewBuilder[] entityViewBuilders;
     }
 
-    public abstract class GenericEntityDescriptor<T, U, V> : IEntityDescriptor     where T : IEntityData, new()
-                                                                          where U : IEntityData, new()
-                                                                          where V : IEntityData, new()
+    public abstract class GenericEntityDescriptor<T, U, V> : IEntityDescriptor     where T : IEntityStruct, new()
+                                                                          where U : IEntityStruct, new()
+                                                                          where V : IEntityStruct, new()
     {
         static GenericEntityDescriptor()
         {
@@ -48,10 +48,10 @@
         static readonly IEntityViewBuilder[] entityViewBuilders;
     }
 
-    public abstract class GenericEntityDescriptor<T, U, V, W> : IEntityDescriptor     where T : IEntityData, new()
-                                                                             where U : IEntityData, new()
-                                                                             where V : IEntityData, new()
-                                                                             where W : IEntityData, new()
+    public abstract class GenericEntityDescriptor<T, U, V, W> : IEntityDescriptor     where T : IEntityStruct, new()
+                                                                             where U : IEntityStruct, new()
+                                                                             where V : IEntityStruct, new()
+                                                                             where W : IEntityStruct, new()
     {
         static GenericEntityDescriptor()
         {
@@ -66,11 +66,11 @@
         static readonly IEntityViewBuilder[] entityViewBuilders;
     }
 
-    public abstract class GenericEntityDescriptor<T, U, V, W, X> : IEntityDescriptor     where T : IEntityData, new()
-                                                                                where U : IEntityData, new()
-                                                                                where V : IEntityData, new()
-                                                                                where W : IEntityData, new()
-                                                                                where X : IEntityData, new()
+    public abstract class GenericEntityDescriptor<T, U, V, W, X> : IEntityDescriptor     where T : IEntityStruct, new()
+                                                                                where U : IEntityStruct, new()
+                                                                                where V : IEntityStruct, new()
+                                                                                where W : IEntityStruct, new()
+                                                                                where X : IEntityStruct, new()
     {
         static GenericEntityDescriptor()
         {
@@ -85,12 +85,12 @@
         static readonly IEntityViewBuilder[] entityViewBuilders;
     }
 
-    public abstract class GenericEntityDescriptor<T, U, V, W, X, Y> : IEntityDescriptor     where T : IEntityData, new()
-                                                                                   where U : IEntityData, new()
-                                                                                   where V : IEntityData, new()
-                                                                                   where W : IEntityData, new()
-                                                                                   where X : IEntityData, new()
-                                                                                   where Y : IEntityData, new()
+    public abstract class GenericEntityDescriptor<T, U, V, W, X, Y> : IEntityDescriptor     where T : IEntityStruct, new()
+                                                                                   where U : IEntityStruct, new()
+                                                                                   where V : IEntityStruct, new()
+                                                                                   where W : IEntityStruct, new()
+                                                                                   where X : IEntityStruct, new()
+                                                                                   where Y : IEntityStruct, new()
     {
         static GenericEntityDescriptor()
         {

Svelto.ECS/IEntityView.cs

@@ -5,13 +5,15 @@ using Svelto.DataStructures;
 using Svelto.Utilities;
 
 namespace Svelto.ECS
-{   
-    public interface IEntityData
+{
+    ///<summary>EntityStruct MUST implement IEntiyStruct</summary>
+    public interface IEntityStruct
     {
         EGID ID { get; set; }
     }
-    
-    public interface IEntityView:IEntityData
+
+    ///<summary>EntityViews and EntityViewStructs MUST implement IEntityView</summary>
+    public interface IEntityView:IEntityStruct
     {}
     
     public class EntityView : IEntityView
@@ -25,14 +27,14 @@ namespace Svelto.ECS
         EGID _ID;
     }
 
-    public struct EntityInfoView : IEntityData
+    public struct EntityInfoView : IEntityStruct
     {
         public EGID ID { get; set; }
         
         public IEntityViewBuilder[] entityToBuild;
     }
 
-    public static class EntityView<T> where T: IEntityData, new()
+    public static class EntityView<T> where T: IEntityStruct, new()
     {
         internal static readonly FasterList<KeyValuePair<Type, ActionCast<T>>> cachedFields;
 

Svelto.ECS/IEntityViewsDB.cs

@@ -7,25 +7,25 @@ namespace Svelto.ECS
     public interface IEntityViewsDB
     {
         //to use with EntityViews
-        ReadOnlyCollectionStruct<T> QueryEntityViews<T>() where T : class, IEntityData;
-        ReadOnlyCollectionStruct<T> QueryEntityViews<T>(int group) where T : class, IEntityData;
+        ReadOnlyCollectionStruct<T> QueryEntityViews<T>() where T : class, IEntityStruct;
+        ReadOnlyCollectionStruct<T> QueryEntityViews<T>(int group) where T : class, IEntityStruct;
 
         //to use with EntityViews, EntityStructs and EntityViewStructs
-        T[] QueryEntities<T>(out int    count) where T : IEntityData;
-        T[] QueryEntities<T>(int group, out int count) where T : IEntityData;
-        T[] QueryEntities<T>(EGID entityGID, out uint index) where T : IEntityData;
+        T[] QueryEntities<T>(out int    count) where T : IEntityStruct;
+        T[] QueryEntities<T>(int group, out int count) where T : IEntityStruct;
+        T[] QueryEntities<T>(EGID entityGID, out uint index) where T : IEntityStruct;
 
         //to use with EntityViews
-        bool TryQueryEntityView<T>(EGID egid, out T entityView) where T : class, IEntityData;
-        T    QueryEntityView<T>(EGID egid) where T : class, IEntityData;
+        bool TryQueryEntityView<T>(EGID egid, out T entityView) where T : class, IEntityStruct;
+        T    QueryEntityView<T>(EGID egid) where T : class, IEntityStruct;
         
         //to use with EntityViews, EntityStructs and EntityViewStructs
-        void ExecuteOnEntity<T, W>(EGID egid, ref W value, ActionRef<T, W> action) where T : IEntityData;
-        void ExecuteOnEntity<T>(EGID egid, ActionRef<T> action) where T : IEntityData;
+        void ExecuteOnEntity<T, W>(EGID egid, ref W value, ActionRef<T, W> action) where T : IEntityStruct;
+        void ExecuteOnEntity<T>(EGID egid, ActionRef<T> action) where T : IEntityStruct;
         
-        bool Exists<T>(EGID egid) where T : IEntityData;
+        bool Exists<T>(EGID egid) where T : IEntityStruct;
         
-        bool HasAny<T>() where T:IEntityData;
-        bool HasAny<T>(int group) where T:IEntityData;
+        bool HasAny<T>() where T:IEntityStruct;
+        bool HasAny<T>(int group) where T:IEntityStruct;
     }
 }

Svelto.ECS/MultiEntitiesEngine.cs

@@ -1,7 +1,7 @@
 namespace Svelto.ECS
 {
     public abstract class MultiEntitiesEngine<T, U> : SingleEntityEngine<T>, IHandleEntityStructEngine<U>
-        where U : IEntityData where T : IEntityData
+        where U : IEntityStruct where T : IEntityStruct
     {
         public void AddInternal(ref U entityView)
         { Add(ref entityView); }
@@ -13,7 +13,7 @@ namespace Svelto.ECS
     }
     
     public abstract class MultiEntitiesEngine<T, U, V> : MultiEntitiesEngine<T, U>, IHandleEntityStructEngine<V>
-        where V :  IEntityData where U :  IEntityData where T :  IEntityData
+        where V :  IEntityStruct where U :  IEntityStruct where T :  IEntityStruct
     {
         public void AddInternal(ref V entityView)
         { Add(ref entityView); }
@@ -30,7 +30,7 @@ namespace Svelto.ECS
     ///     already too many responsabilities.
     /// </summary>
     public abstract class MultiEntitiesEngine<T, U, V, W> : MultiEntitiesEngine<T, U, V>, IHandleEntityStructEngine<W>
-        where W :  IEntityData where V :  IEntityData where U :  IEntityData where T : IEntityData
+        where W :  IEntityStruct where V :  IEntityStruct where U :  IEntityStruct where T : IEntityStruct
     {
         public void AddInternal(ref W entityView)
         { Add(ref entityView); }

Svelto.ECS/MultiEntityViewsEngine.cs

@@ -1,7 +1,7 @@
 namespace Svelto.ECS
 {
     public abstract class MultiEntityViewsEngine<T, U> : SingleEntityViewEngine<T>, IHandleEntityStructEngine<U>
-        where U : class, IEntityData where T : class, IEntityData
+        where U : class, IEntityStruct where T : class, IEntityStruct
     {
         public void AddInternal(ref    U entityView)
         { Add(entityView); }
@@ -13,7 +13,7 @@ namespace Svelto.ECS
     }
 
     public abstract class MultiEntityViewsEngine<T, U, V> : MultiEntityViewsEngine<T, U>, IHandleEntityStructEngine<V>
-        where V :  class, IEntityData where U :  class, IEntityData where T :  class, IEntityData
+        where V :  class, IEntityStruct where U :  class, IEntityStruct where T :  class, IEntityStruct
     {
         public void AddInternal(ref    V entityView)
         { Add(entityView); }
@@ -30,7 +30,7 @@ namespace Svelto.ECS
     ///     already too many responsabilities.
     /// </summary>
     public abstract class MultiEntityViewsEngine<T, U, V, W> : MultiEntityViewsEngine<T, U, V>, IHandleEntityStructEngine<W>
-        where W :  class, IEntityData where V : class, IEntityData where U :  class, IEntityData where T : class, IEntityData
+        where W :  class, IEntityStruct where V : class, IEntityStruct where U :  class, IEntityStruct where T : class, IEntityStruct
     {
         public void AddInternal(ref    W entityView)
         { Add(entityView); }

Svelto.ECS/SingleEntityEngine.cs

@@ -1,6 +1,6 @@
 namespace Svelto.ECS
 {
-    public abstract class SingleEntityEngine<T> : IHandleEntityStructEngine<T> where T : IEntityData
+    public abstract class SingleEntityEngine<T> : IHandleEntityStructEngine<T> where T : IEntityStruct
     {
         public void AddInternal(ref T entityView)
         { Add(ref entityView); }

Svelto.ECS/SingleEntityViewEngine.cs

@@ -1,6 +1,6 @@
 namespace Svelto.ECS
 {
-    public abstract class SingleEntityViewEngine<T> : IHandleEntityStructEngine<T> where T : class, IEntityData
+    public abstract class SingleEntityViewEngine<T> : IHandleEntityStructEngine<T> where T : class, IEntityStruct
     {
         public void AddInternal(ref T entityView)
         { Add(entityView); }