Added Entity and ValueObject classes.

src/Volo.Abp/Volo/Abp/Domain/Entities/Entity.cs

@@ -0,0 +1,127 @@
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+
+namespace Volo.Abp.Domain.Entities
+{
+    //TODO: Think on Entity class without PK
+    ///// <summary>
+    ///// A shortcut of <see cref="Entity{TPrimaryKey}"/> for most used primary key type (<see cref="int"/>).
+    ///// </summary>
+    //public abstract class Entity : Entity<string>, IEntity<>
+    //{
+
+    //}
+
+    /// <summary>
+    /// Basic implementation of IEntity interface.
+    /// An entity can inherit this class of directly implement to IEntity interface.
+    /// </summary>
+    /// <typeparam name="TPrimaryKey">Type of the primary key of the entity</typeparam>
+    public abstract class Entity<TPrimaryKey> : IEntity<TPrimaryKey>
+    {
+        /// <summary>
+        /// Unique identifier for this entity.
+        /// </summary>
+        public virtual TPrimaryKey Id { get; set; }
+
+        /// <summary>
+        /// Checks if this entity is transient (it has not an Id).
+        /// </summary>
+        /// <returns>True, if this entity is transient</returns>
+        public virtual bool IsTransient()
+        {
+            if (EqualityComparer<TPrimaryKey>.Default.Equals(Id, default(TPrimaryKey)))
+            {
+                return true;
+            }
+
+            //Workaround for EF Core since it sets int/long to min value when attaching to dbcontext
+            if (typeof(TPrimaryKey) == typeof(int))
+            {
+                return Convert.ToInt32(Id) <= 0;
+            }
+
+            if (typeof(TPrimaryKey) == typeof(long))
+            {
+                return Convert.ToInt64(Id) <= 0;
+            }
+
+            return false;
+        }
+
+        /// <inheritdoc/>
+        public override bool Equals(object obj)
+        {
+            if (obj == null || !(obj is Entity<TPrimaryKey>))
+            {
+                return false;
+            }
+
+            //Same instances must be considered as equal
+            if (ReferenceEquals(this, obj))
+            {
+                return true;
+            }
+
+            //Transient objects are not considered as equal
+            var other = (Entity<TPrimaryKey>)obj;
+            if (IsTransient() && other.IsTransient())
+            {
+                return false;
+            }
+
+            //Must have a IS-A relation of types or must be same type
+            var typeOfThis = GetType().GetTypeInfo();
+            var typeOfOther = other.GetType().GetTypeInfo();
+            if (!typeOfThis.IsAssignableFrom(typeOfOther) && !typeOfOther.IsAssignableFrom(typeOfThis))
+            {
+                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);
+        }
+
+        /// <inheritdoc/>
+        public override int GetHashCode()
+        {
+            return Id.GetHashCode();
+        }
+
+        /// <inheritdoc/>
+        public static bool operator ==(Entity<TPrimaryKey> left, Entity<TPrimaryKey> right)
+        {
+            if (Equals(left, null))
+            {
+                return Equals(right, null);
+            }
+
+            return left.Equals(right);
+        }
+
+        /// <inheritdoc/>
+        public static bool operator !=(Entity<TPrimaryKey> left, Entity<TPrimaryKey> right)
+        {
+            return !(left == right);
+        }
+
+        /// <inheritdoc/>
+        public override string ToString()
+        {
+            return $"[{GetType().Name} {Id}]";
+        }
+    }
+}

src/Volo.Abp/Volo/Abp/Domain/Entities/IEntity.cs

@@ -0,0 +1,29 @@
+namespace Volo.Abp.Domain.Entities
+{
+    //TODO: Think on Entity class without PK
+    ///// <summary>
+    ///// A shortcut of <see cref="IEntity{TPrimaryKey}"/> for most used primary key type (<see cref="int"/>).
+    ///// </summary>
+    //public interface IEntity : IEntity<int>
+    //{
+
+    //}
+
+    /// <summary>
+    /// Defines interface for base entity type. All entities in the system must implement this interface.
+    /// </summary>
+    /// <typeparam name="TPrimaryKey">Type of the primary key of the entity</typeparam>
+    public interface IEntity<TPrimaryKey>
+    {
+        /// <summary>
+        /// Unique identifier for this entity.
+        /// </summary>
+        TPrimaryKey Id { get; set; }
+
+        /// <summary>
+        /// Checks if this entity is transient (not persisted to database and it has not an <see cref="Id"/>).
+        /// </summary>
+        /// <returns>True, if this entity is transient</returns>
+        bool IsTransient();
+    }
+}

src/Volo.Abp/Volo/Abp/Domain/Values/ValueObject.cs

@@ -0,0 +1,97 @@
+using System;
+using System.Linq;
+using System.Reflection;
+
+namespace Volo.Abp.Domain.Values
+{
+    //Inspired from https://blogs.msdn.microsoft.com/cesardelatorre/2011/06/06/implementing-a-value-object-base-class-supertype-patternddd-patterns-related/
+    
+    /// <summary>
+    /// Base class for value objects.
+    /// </summary>
+    /// <typeparam name="TValueObject">The type of the value object.</typeparam>
+    public abstract class ValueObject<TValueObject> : IEquatable<TValueObject>
+        where TValueObject : ValueObject<TValueObject>
+    {
+        public bool Equals(TValueObject other)
+        {
+            if ((object)other == null)
+            {
+                return false;
+            }
+
+            var publicProperties = GetType().GetTypeInfo().GetProperties();
+            if (!publicProperties.Any())
+            {
+                return true;
+            }
+
+            return publicProperties.All(property => Equals(property.GetValue(this, null), property.GetValue(other, null)));
+        }
+
+        public override bool Equals(object obj)
+        {
+            if (obj == null)
+            {
+                return false;
+            }
+
+            var item = obj as ValueObject<TValueObject>;
+            return (object)item != null && Equals((TValueObject)item);
+
+        }
+
+        public override int GetHashCode()
+        {
+            const int index = 1;
+            const int initialHasCode = 31;
+
+            var publicProperties = GetType().GetTypeInfo().GetProperties();
+
+            if (!publicProperties.Any())
+            {
+                return initialHasCode;
+            }
+
+            var hashCode = initialHasCode;
+            var changeMultiplier = false;
+
+            foreach (var property in publicProperties)
+            {
+                var value = property.GetValue(this, null);
+
+                if (value == null)
+                {
+                    //support {"a",null,null,"a"} != {null,"a","a",null}
+                    hashCode = hashCode ^ (index * 13);
+                    continue;
+                }
+
+                hashCode = hashCode * (changeMultiplier ? 59 : 114) + value.GetHashCode();
+                changeMultiplier = !changeMultiplier;
+            }
+
+            return hashCode;
+        }
+
+        public static bool operator ==(ValueObject<TValueObject> x, ValueObject<TValueObject> y)
+        {
+            if (ReferenceEquals(x, y))
+            {
+                return true;
+            }
+
+            if (((object)x == null) || ((object)y == null))
+            {
+                return false;
+            }
+
+            return x.Equals(y);
+        }
+
+        public static bool operator !=(ValueObject<TValueObject> x, ValueObject<TValueObject> y)
+        {
+            return !(x == y);
+        }
+    }
+}