diff --git a/src/Volo.Abp.ApplicationContracts/Volo/Abp/Application/Services/Dtos/EntityDto.cs b/src/Volo.Abp.ApplicationContracts/Volo/Abp/Application/Services/Dtos/EntityDto.cs
index 7fab20c981..f9b8664a2b 100644
--- a/src/Volo.Abp.ApplicationContracts/Volo/Abp/Application/Services/Dtos/EntityDto.cs
+++ b/src/Volo.Abp.ApplicationContracts/Volo/Abp/Application/Services/Dtos/EntityDto.cs
@@ -6,7 +6,7 @@ namespace Volo.Abp.Application.Services.Dtos
     /// Implements common properties for entity based DTOs.
     /// </summary>
     /// <typeparam name="TPrimaryKey">Type of the primary key</typeparam>
-    public class EntityDto<TPrimaryKey>
+    public class EntityDto<TPrimaryKey> : IEntityDto<TPrimaryKey>
     {
         /// <summary>
         /// Id of the entity.
diff --git a/src/Volo.Abp.ApplicationContracts/Volo/Abp/Application/Services/Dtos/IEntityDto.cs b/src/Volo.Abp.ApplicationContracts/Volo/Abp/Application/Services/Dtos/IEntityDto.cs
new file mode 100644
index 0000000000..250c40c677
--- /dev/null
+++ b/src/Volo.Abp.ApplicationContracts/Volo/Abp/Application/Services/Dtos/IEntityDto.cs
@@ -0,0 +1,22 @@
+namespace Volo.Abp.Application.Services.Dtos
+{
+    ///// <summary>
+    ///// A shortcut of <see cref="IEntityDto{TPrimaryKey}"/> for most used primary key type (<see cref="int"/>).
+    ///// </summary>
+    //public interface IEntityDto : IEntityDto<int>
+    //{
+
+    //}
+
+    /// <summary>
+    /// Defines common properties for entity based DTOs.
+    /// </summary>
+    /// <typeparam name="TPrimaryKey"></typeparam>
+    public interface IEntityDto<TPrimaryKey>
+    {
+        /// <summary>
+        /// Id of the entity.
+        /// </summary>
+        TPrimaryKey Id { get; set; }
+    }
+}
\ No newline at end of file
diff --git a/src/Volo.Abp.EntityFrameworkCore/Microsoft/Extensions/DependencyInjection/AbpEfCoreServiceCollectionExtensions.cs b/src/Volo.Abp.EntityFrameworkCore/Microsoft/Extensions/DependencyInjection/AbpEfCoreServiceCollectionExtensions.cs
index 1d55e5344c..b75694ccea 100644
--- a/src/Volo.Abp.EntityFrameworkCore/Microsoft/Extensions/DependencyInjection/AbpEfCoreServiceCollectionExtensions.cs
+++ b/src/Volo.Abp.EntityFrameworkCore/Microsoft/Extensions/DependencyInjection/AbpEfCoreServiceCollectionExtensions.cs
@@ -25,7 +25,7 @@ namespace Microsoft.Extensions.DependencyInjection
             services.TryAddTransient<TDbContext>();
             services.TryAddSingleton(serviceProvider =>
             {
-                const string moduleName = "";
+                const string moduleName = ""; //TODO: Use AbpModuleDescriptor instead of module name?
 
                 var connInfoResolver = serviceProvider.GetRequiredService<IConnectionStringResolver>();
 
diff --git a/src/Volo.Abp/Volo/Abp/Domain/Entities/AggregateRoot.cs b/src/Volo.Abp/Volo/Abp/Domain/Entities/AggregateRoot.cs
index 02bcda67c4..24b9a5c6f5 100644
--- a/src/Volo.Abp/Volo/Abp/Domain/Entities/AggregateRoot.cs
+++ b/src/Volo.Abp/Volo/Abp/Domain/Entities/AggregateRoot.cs
@@ -2,5 +2,6 @@
 {
     public class AggregateRoot<TPrimaryKey> : Entity<TPrimaryKey>, IAggregateRoot<TPrimaryKey>
     {
+
     }
 }
\ No newline at end of file
diff --git a/src/Volo.Abp/Volo/Abp/Domain/Entities/Entity.cs b/src/Volo.Abp/Volo/Abp/Domain/Entities/Entity.cs
index 0273ba57b5..c7c8f29458 100644
--- a/src/Volo.Abp/Volo/Abp/Domain/Entities/Entity.cs
+++ b/src/Volo.Abp/Volo/Abp/Domain/Entities/Entity.cs
@@ -79,19 +79,6 @@ namespace Volo.Abp.Domain.Entities
                 return false;
             }
 
-            //TODO: How to handle this?
-            //if (this is IMayHaveTenant && other is IMayHaveTenant &&
-            //    this.As<IMayHaveTenant>().TenantId != other.As<IMayHaveTenant>().TenantId)
-            //{
-            //    return false;
-            //}
-
-            //if (this is IMustHaveTenant && other is IMustHaveTenant &&
-            //    this.As<IMustHaveTenant>().TenantId != other.As<IMustHaveTenant>().TenantId)
-            //{
-            //    return false;
-            //}
-
             return Id.Equals(other.Id);
         }
 
diff --git a/src/Volo.Abp/Volo/Abp/Domain/Entities/EntityHelper.cs b/src/Volo.Abp/Volo/Abp/Domain/Entities/EntityHelper.cs
index b1140df9b3..68e1887029 100644
--- a/src/Volo.Abp/Volo/Abp/Domain/Entities/EntityHelper.cs
+++ b/src/Volo.Abp/Volo/Abp/Domain/Entities/EntityHelper.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Reflection;
+using JetBrains.Annotations;
 using Volo.Abp.Reflection;
 
 namespace Volo.Abp.Domain.Entities
@@ -9,7 +10,7 @@ namespace Volo.Abp.Domain.Entities
     /// </summary>
     public static class EntityHelper
     {
-        public static bool IsEntity(Type type)
+        public static bool IsEntity([NotNull] Type type)
         {
             return ReflectionHelper.IsAssignableToGenericType(type, typeof (IEntity<>));
         }
@@ -22,7 +23,7 @@ namespace Volo.Abp.Domain.Entities
         /// <summary>
         /// Gets primary key type of given entity type
         /// </summary>
-        public static Type GetPrimaryKeyType(Type entityType)
+        public static Type GetPrimaryKeyType([NotNull] Type entityType)
         {
             foreach (var interfaceType in entityType.GetTypeInfo().GetInterfaces())
             {
diff --git a/src/Volo.Abp/Volo/Abp/Domain/Entities/EntityNotFoundException.cs b/src/Volo.Abp/Volo/Abp/Domain/Entities/EntityNotFoundException.cs
index 762b86876c..51ca026ff0 100644
--- a/src/Volo.Abp/Volo/Abp/Domain/Entities/EntityNotFoundException.cs
+++ b/src/Volo.Abp/Volo/Abp/Domain/Entities/EntityNotFoundException.cs
@@ -5,7 +5,7 @@ namespace Volo.Abp.Domain.Entities
     /// <summary>
     /// This exception is thrown if an entity excepted to be found but not found.
     /// </summary>
-    //[Serializable]
+    //TODO: [Serializable]
     public class EntityNotFoundException : AbpException
     {
         /// <summary>
diff --git a/src/Volo.Abp/Volo/Abp/Domain/Repositories/IQueryableRepository.cs b/src/Volo.Abp/Volo/Abp/Domain/Repositories/IQueryableRepository.cs
index 80f713208e..d3450fd690 100644
--- a/src/Volo.Abp/Volo/Abp/Domain/Repositories/IQueryableRepository.cs
+++ b/src/Volo.Abp/Volo/Abp/Domain/Repositories/IQueryableRepository.cs
@@ -2,6 +2,7 @@
 using System.Linq;
 using System.Linq.Expressions;
 using System.Threading.Tasks;
+using JetBrains.Annotations;
 using Volo.Abp.Domain.Entities;
 
 namespace Volo.Abp.Domain.Repositories
@@ -16,7 +17,7 @@ namespace Volo.Abp.Domain.Repositories
         /// given predicate.
         /// </summary>
         /// <param name="predicate">A condition to filter entities</param>
-        void Delete(Expression<Func<TEntity, bool>> predicate);
+        void Delete([NotNull] Expression<Func<TEntity, bool>> predicate);
 
         /// <summary>
         /// Deletes many entities by function.
@@ -25,6 +26,6 @@ namespace Volo.Abp.Domain.Repositories
         /// given predicate.
         /// </summary>
         /// <param name="predicate">A condition to filter entities</param>
-        Task DeleteAsync(Expression<Func<TEntity, bool>> predicate);
+        Task DeleteAsync([NotNull] Expression<Func<TEntity, bool>> predicate);
     }
 }
\ No newline at end of file
diff --git a/src/Volo.Abp/Volo/Abp/Domain/Repositories/IRepository.cs b/src/Volo.Abp/Volo/Abp/Domain/Repositories/IRepository.cs
index d77fcdd931..d08fd83894 100644
--- a/src/Volo.Abp/Volo/Abp/Domain/Repositories/IRepository.cs
+++ b/src/Volo.Abp/Volo/Abp/Domain/Repositories/IRepository.cs
@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Threading.Tasks;
+using JetBrains.Annotations;
 using Volo.Abp.Domain.Entities;
 using Volo.DependencyInjection;
 
@@ -17,12 +18,14 @@ namespace Volo.Abp.Domain.Repositories
         /// Used to get all entities.
         /// </summary>
         /// <returns>List of all entities</returns>
+        [NotNull]
         List<TEntity> GetList();
 
         /// <summary>
         /// Used to get all entities.
         /// </summary>
         /// <returns>List of all entities</returns>
+        [NotNull]
         Task<List<TEntity>> GetListAsync();
 
         /// <summary>
@@ -30,6 +33,7 @@ namespace Volo.Abp.Domain.Repositories
         /// </summary>
         /// <param name="id">Primary key of the entity to get</param>
         /// <returns>Entity</returns>
+        [NotNull]
         TEntity Get(TPrimaryKey id);
 
         /// <summary>
@@ -37,6 +41,7 @@ namespace Volo.Abp.Domain.Repositories
         /// </summary>
         /// <param name="id">Primary key of the entity to get</param>
         /// <returns>Entity</returns>
+        [NotNull]
         Task<TEntity> GetAsync(TPrimaryKey id);
 
         /// <summary>
@@ -44,6 +49,7 @@ namespace Volo.Abp.Domain.Repositories
         /// </summary>
         /// <param name="id">Primary key of the entity to get</param>
         /// <returns>Entity or null</returns>
+        [CanBeNull]
         TEntity Find(TPrimaryKey id);
 
         /// <summary>
@@ -51,19 +57,22 @@ namespace Volo.Abp.Domain.Repositories
         /// </summary>
         /// <param name="id">Primary key of the entity to get</param>
         /// <returns>Entity or null</returns>
+        [CanBeNull]
         Task<TEntity> FindAsync(TPrimaryKey id);
 
         /// <summary>
         /// Inserts a new entity.
         /// </summary>
         /// <param name="entity">Inserted entity</param>
-        TEntity Insert(TEntity entity);
+        [NotNull]
+        TEntity Insert([NotNull] TEntity entity);
 
         /// <summary>
         /// Inserts a new entity.
         /// </summary>
         /// <param name="entity">Inserted entity</param>
-        Task<TEntity> InsertAsync(TEntity entity);
+        [NotNull]
+        Task<TEntity> InsertAsync([NotNull] TEntity entity);
 
         /// <summary>
         /// Inserts a new entity and gets it's Id.
@@ -72,7 +81,7 @@ namespace Volo.Abp.Domain.Repositories
         /// </summary>
         /// <param name="entity">Entity</param>
         /// <returns>Id of the entity</returns>
-        TPrimaryKey InsertAndGetId(TEntity entity);
+        TPrimaryKey InsertAndGetId([NotNull] TEntity entity);
 
         /// <summary>
         /// Inserts a new entity and gets it's Id.
@@ -81,31 +90,33 @@ namespace Volo.Abp.Domain.Repositories
         /// </summary>
         /// <param name="entity">Entity</param>
         /// <returns>Id of the entity</returns>
-        Task<TPrimaryKey> InsertAndGetIdAsync(TEntity entity);
+        Task<TPrimaryKey> InsertAndGetIdAsync([NotNull] TEntity entity);
 
         /// <summary>
         /// Updates an existing entity.
         /// </summary>
         /// <param name="entity">Entity</param>
-        TEntity Update(TEntity entity);
+        [NotNull]
+        TEntity Update([NotNull] TEntity entity);
 
         /// <summary>
         /// Updates an existing entity. 
         /// </summary>
         /// <param name="entity">Entity</param>
-        Task<TEntity> UpdateAsync(TEntity entity);
+        [NotNull]
+        Task<TEntity> UpdateAsync([NotNull] TEntity entity);
 
         /// <summary>
         /// Deletes an entity.
         /// </summary>
         /// <param name="entity">Entity to be deleted</param>
-        void Delete(TEntity entity);
+        void Delete([NotNull] TEntity entity);
 
         /// <summary>
         /// Deletes an entity.
         /// </summary>
         /// <param name="entity">Entity to be deleted</param>
-        Task DeleteAsync(TEntity entity);
+        Task DeleteAsync([NotNull] TEntity entity);
 
         /// <summary>
         /// Deletes an entity by primary key.