IAuthorAppService section completed.

docs/en/Tutorials/Part-8.md

@@ -52,6 +52,125 @@ This tutorials has multiple versions based on your **UI** and **Database** prefe
 
 This part explains to create an application layer for the `Author` entity created before.
 
-## Application Service
+## IAuthorAppService
+
+We will first create the [application service](../Application-Services.md) interface and the related [DTO](../Data-Transfer-Objects.md)s. Create a new interface, named `IAuthorAppService`, in the `Authors` namespace (folder) of the `Acme.BookStore.Application.Contracts` project:
+
+````csharp
+using System;
+using System.Threading.Tasks;
+using Volo.Abp.Application.Dtos;
+using Volo.Abp.Application.Services;
+
+namespace Acme.BookStore.Authors
+{
+    public interface IAuthorAppService : IApplicationService
+    {
+        Task<AuthorDto> GetAsync(Guid id);
+
+        Task<PagedResultDto<AuthorDto>> GetListAsync(GetAuthorListDto input);
+
+        Task<AuthorDto> CreateAsync(CreateAuthorDto input);
+
+        Task UpdateAsync(Guid id, UpdateAuthorDto input);
+
+        Task DeleteAsync(Guid id);
+    }
+}
+````
+
+* `IApplicationService` is a conventional interface that is inherited by all the application services, so the ABP Framework can identify the service.
+* Defined standard methods to perform CRUD operations on the `Author` entity.
+* `PagedResultDto` is a pre-defined DTO class in the ABP Framework. It has an `Items` collection and a `TotalCount` property to return a paged result.
+* Preferred to return an `AuthorDto` (for the newly created author) from the `CreateAsync` method, while it is not used by this application - just to show a different usage.
+
+This interface is using the DTOs defined below.
+
+### AuthorDto
+
+````csharp
+using System;
+using Volo.Abp.Application.Dtos;
+
+namespace Acme.BookStore.Authors
+{
+    public class AuthorDto : EntityDto<Guid>
+    {
+        public string Name { get; set; }
+
+        public DateTime BirthDate { get; set; }
+
+        public string ShortBio { get; set; }
+    }
+}
+````
+
+* `EntityDto<T>` simply has an `Id` property with the given generic argument. You could create an `Id` property yourself instead of inheriting the `EntityDto<T>`.
+
+### GetAuthorListDto
+
+````csharp
+using Volo.Abp.Application.Dtos;
+
+namespace Acme.BookStore.Authors
+{
+    public class GetAuthorListDto : PagedAndSortedResultRequestDto
+    {
+        public string Filter { get; set; }
+    }
+}
+````
+
+* `Filter` is used to search authors. It can be `null` (or empty string) to get all the authors.
+* `PagedAndSortedResultRequestDto` has the standard paging and sorting properties: `int MaxResultCount`, `int SkipCount` and `string Sorting`.
+
+> ABP Framework has such base DTO classes to simplify and standardize your DTOs. See the [DTO documentation](../Data-Transfer-Objects.md) for all.
+
+### CreateAuthorDto
+
+````csharp
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Acme.BookStore.Authors
+{
+    public class CreateAuthorDto
+    {
+        [Required]
+        [StringLength(AuthorConsts.MaxNameLength)]
+        public string Name { get; set; }
+
+        [Required]
+        public DateTime BirthDate { get; set; }
+        
+        public string ShortBio { get; set; }
+    }
+}
+````
+
+Data annotation attributes can be used to validate the DTO. See the [validation document](../Validation.md) for details.
+
+### UpdateAuthorDto
+
+````csharp
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Acme.BookStore.Authors
+{
+    public class UpdateAuthorDto
+    {
+        [Required]
+        [StringLength(AuthorConsts.MaxNameLength)]
+        public string Name { get; set; }
+
+        [Required]
+        public DateTime BirthDate { get; set; }
+        
+        public string ShortBio { get; set; }
+    }
+}
+````
+
+We could share (re-use) the same DTO among the create and the update operations. While you can do it, we prefer to create different DTOs for these operations since we see they generally be different by the time. So, code duplication is reasonable here compared to a tightly coupled design.
 
-TODO