Entities are one of the core concepts of DDD (Domain Driven Design). Eric Evans describe it as "An object that is not fundamentally defined by its attributes, but rather by a thread of continuity and identity". So, entities have Id's and stored in a database. An entity is generally mapped to a table in the database.

Entity Classes

In ASP.NET Boilerplate, Entities are derived from Entity class. See the sample below:

public class Person : Entity
{
    public virtual string Name { get; set; }

    public virtual DateTime CreationTime { get; set; }

    public Person()
    {
        CreationTime = DateTime.Now;
    }
}

Person class is defined as an entity. It has two properties. Also, Entity class defines an Id property. It's primary key of the Entity. So, name of primary keys of all Entities are same, it's Id.

Type of Id (primary key) can be changed. It's int (Int32) by default. If you want to define another type as Id, you should explicitly declare it as shown below:

public class Person : Entity<long>
{
    public virtual string Name { get; set; }

    public virtual DateTime CreationTime { get; set; }

    public Person()
    {
        CreationTime = DateTime.Now;
    }
}

Also, you can set it as string, Guid or something else.

Entity class overrides equality operator (==) to easily check if two entities are equal (their Id is equal). It also defines the IsTransient() method to check if it has an Id or not.

AggregateRoot Class

"Aggregate is a pattern in Domain-Driven Design. A DDD aggregate is a cluster of domain objects that can be treated as a single unit. An example may be an order and its line-items, these will be separate objects, but it's useful to treat the order (together with its line items) as a single aggregate." (Martin Fowler - see full description)

While ABP does not enforce you to use aggregates, you may want to create aggregates and aggregate roots in your application. ABP defines AggregateRoot class that extends Entity to create aggregate root entities for an aggregate.

Domain Events

AggregateRoot defines DomainEvents collection to generate domain events by the aggregate root class. These events are automatically triggered just before the current unit of work is completed. Actually, any entity can generate domain events by implementing IGeneratesDomainEvents interface, but it's common (best practice) to generate domain events in aggregate roots. That's why it's default for AggregateRoot but not for Entity class.

Conventional Interfaces

In many application, similar entity properties (and database table fields) are used like CreationTime indicates that when this entity is created. ASP.NET Boilerplate provides some useful interfaces to make this common properties explicit and expressive. Also, this provides a way of coding common code for Entities which implement these interfaces.

Auditing

IHasCreationTime makes it possible to use a common property for 'creation time' information of an entity. ASP.NET Boilerplate automatically sets CreationTime to current time when an Entity is inserted into database which implements this interface.

public interface IHasCreationTime
{
    DateTime CreationTime { get; set; }
}

Person class can be re-written as shown below by implementing IHasCreationTime interface:

public class Person : Entity<long>, IHasCreationTime
{
    public virtual string Name { get; set; }

    public virtual DateTime CreationTime { get; set; }

    public Person()
    {
        CreationTime = DateTime.Now;
    }
}

ICreationAudited extens IHasCreationTime by adding  CreatorUserId:

public interface ICreationAudited : IHasCreationTime
{
    long? CreatorUserId { get; set; }
}

ASP.NET Boilerplate automatically sets CreatorUserId to current user's id when saving a new entity. You can also implement ICreationAudited easily by deriving your entity from CreationAuditedEntity class. It has also a generic version for different type of Id properties.

There is also similar interfaces for modifications:

public interface IHasModificationTime
{
    DateTime? LastModificationTime { get; set; }
}

public interface IModificationAudited : IHasModificationTime
{
    long? LastModifierUserId { get; set; }
}

ASP.NET Boilerplate also automatically sets these properties when updating an entity. You just define them for your entity.

If you want to implement all of audit properties, you can direcly implement IAudited interface:

public interface IAudited : ICreationAudited, IModificationAudited
{

}

As a shortcut, you can derive from AuditedEntity class instead of direcly implementing IAudited. AuditedEntity class has also a generic version for different type of Id properties.

Note: ASP.NET Boilerplate gets current user's Id from ABP Session.

Soft Delete

Soft delete is a commonly used pattern to mark an Entity as deleted instead of actually deleting it from database. For instace, you may not want to hard delete a User from database since it has many releations to other tables. ISoftDelete interface is used for this purpose:

public interface ISoftDelete
{
    bool IsDeleted { get; set; }
}

ASP.NET Boilerplate implements soft delete pattern out-of-the-box. When a soft-delete entity is being deleted, ASP.NET Boilerplate detects this, prevents deleting, sets IsDeleted as true and updates entity in the database. Also, it does not retrive (select) soft deleted entities from database, automatically filters them.

If you use soft delete, you may also want to store information when an entity is deleted and who deleted it. You can implement IDeletionAudited interface that is shown below:

public interface IDeletionAudited : ISoftDelete
{
    long? DeleterUserId { get; set; }

    DateTime? DeletionTime { get; set; }
}
			

IDeletionAudited extends ISoftDelete as you noticed. ASP.NET Boilerplate automatically sets these properties when an entity is deleted.

If you want to implement all audit interfaces (creation, modification and deletion) for an entity, you can directly implement IFullAudited since it inherits all:

public interface IFullAudited : IAudited, IDeletionAudited
{

}

 As a shortcut, you can derive your entity from FullAuditedEntity class that implements all.

NOTE: All audit interfaces and classes have a generic version for defining navigation property to your User entity (like ICreationAudited<TUser> and FullAuditedEntity<TPrimaryKey, TUser>). 

Active/Passive Entities

Some entities need to be marked as Active or Passive. Then you may take action upon active/passive state of the entity. You can implement IPassivable interface that is created for this reason. It defines IsActive property.

If your entity will be active on first creation, you can set IsActive to true in the constructor.

This is different than soft delete (IsDeleted). If an entity is soft deleted, it can not be retrieved from database (ABP prevents it as default). But, for active/passive entities, it's completely up to you to control getting entities.

Entity Change Events

ASP.NET Boilerplate automatically triggers certain events when an entity is inserted, updated or deleted. Thus, you can register to these events and perform any logic you need. See Predefined Events section in event bus documentation for more information.

IEntity Interfaces

Actually, Entity class implements IEntity interface (and Entity<TPrimaryKey> implements IEntity<TPrimaryKey>). If you do not want to derive from Entity class, you can implement these interfaces directly. There are also corresponding interfaces for other entity classes. But this is not the suggested way, unless you have a good reason to do not derive from Entity classes.