Application Services

Application services are used to implement the use cases of an application. They are used to expose domain logic to the presentation layer.

An Application Service is called from the presentation layer (optionally) with a DTO (Data Transfer Object) as the parameter. It uses domain objects to perform some specific business logic and (optionally) returns a DTO back to the presentation layer. Thus, the presentation layer is completely isolated from domain layer.

Example

Book Entity

Assume that you have a Book entity (actually, an aggregate root) defined as shown below:

public class Book : AggregateRoot<Guid>
{
    public const int MaxNameLength = 128;

    public virtual string Name { get; protected set; }

    public virtual BookType Type { get; set; }

    public virtual float? Price { get; set; }

    protected Book()
    {
        
    }

    public Book(Guid id, [NotNull] string name, BookType type, float? price = 0)
    {
        Id = id;
        Name = CheckName(name);
        Type = type;
        Price = price;
    }

    public virtual void ChangeName([NotNull] string name)
    {
        Name = CheckName(name);
    }

    private static string CheckName(string name)
    {
        if (string.IsNullOrWhiteSpace(name))
        {
            throw new ArgumentException($"name can not be empty or white space!");
        }

        if (name.Length > MaxNameLength)
        {
            throw new ArgumentException($"name can not be longer than {MaxNameLength} chars!");
        }

        return name;
    }
}

• Book entity has a MaxNameLength that defines the maximum length of the Name property.
• Book constructor and ChangeName method to ensure that the Name is always a valid value. Notice that Name's setter is not public.

ABP does not force you to design your entities like that. It just can have public get/set for all properties. It's your decision to full implement DDD practices.

IBookAppService Interface

In ABP, an application service should implement the IApplicationService interface. It's good to create an interface for each application service:

public interface IBookAppService : IApplicationService
{
    Task CreateAsync(CreateBookDto input);
}

A Create method will be implemented as the example. CreateBookDto is defined like that:

public class CreateBookDto
{
    [Required]
    [StringLength(Book.MaxNameLength)]
    public string Name { get; set; }

    public BookType Type { get; set; }

    public float? Price { get; set; }
}

See data transfer objects document for more about DTOs.

BookAppService (Implementation)

public class BookAppService : ApplicationService, IBookAppService
{
    private readonly IRepository<Book, Guid> _bookRepository;

    public BookAppService(IRepository<Book, Guid> bookRepository)
    {
        _bookRepository = bookRepository;
    }

    public async Task CreateAsync(CreateBookDto input)
    {
        var book = new Book(
            GuidGenerator.Create(),
            input.Name,
            input.Type,
            input.Price
        );

        await _bookRepository.InsertAsync(book);
    }
}

• BookAppService inherits from the ApplicationService base class. It's not required, but the ApplicationService class provides helpful properties for common application service requirements like GuidGenerator used in this service. If we didn't inherit from it, we would need to inject the IGuidGenerator service manually (see guid generation document).
• BookAppService implements the IBookAppService as expected.
• BookAppService injects IRepository<Book, Guid> (see repositories) and uses it inside the CreateAsync method to insert a new entity to the database.
• CreateAsync uses the constructor of the Book entity to create a new book from the properties of given input.

Data Transfer Objects

Application services gets and returns DTOs instead of entities. ABP does not force this rule. However, exposing entities to presentation layer (or to remote clients) have significant problems and not suggested.

See the DTO documentation for more.

Object to Object Mapping

The CreateAsync method above manually creates a Book entity from given CreateBookDto object. Because the Book entity enforces it (we designed it like that).

However, in many cases, it's very practical to use auto object mapping to set properties of an object from a similar object. ABP provides an object to object mapping infrastructure to make this even easier.

Let's create another method to get a book. First, define the method in the IBookAppService interface:

public interface IBookAppService : IApplicationService
{
    Task CreateAsync(CreateBookDto input);

    Task<BookDto> GetAsync(Guid id); //New method
}

BookDto is a simple DTO class defined as below:

[AbpAutoMapFrom(typeof(Book))] //Defines the mapping
public class BookDto
{
    public Guid Id { get; set; }

    public string Name { get; set; }

    public BookType Type { get; set; }

    public float? Price { get; set; }
}

• BookDto defines [AbpAutoMapFrom(typeof(Book))] attribute to create the object mapping from Book to BookDto.

Then you can implement the GetAsync method as shown below:

public async Task<BookDto> GetAsync(Guid id)
{
    var book = await _bookRepository.GetAsync(id);
    return book.MapTo<BookDto>();
}

MapTo extension method converts Book object to BookDto object by copying all properties with the same naming.

An alternative to the MapTo is using the IObjectMapper service:

public async Task<BookDto> GetAsync(Guid id)
{
    var book = await _bookRepository.GetAsync(id);
    return ObjectMapper.Map<Book, BookDto>(book);
}

While the second syntax is a bit harder to write, it better works if you write unit tests.

See the object to object mapping document for more.

Validation

Inputs of application service methods are automatically validated (like ASP.NET Core controller actions). You can use the standard data annotation attributes or custom validation method to perform the validation. ABP also ensures that the input is not null.

See the validation document for more.

Authorization

It's possible to use declarative and imperative authorization for application service methods.

See the authorization document for more.

CRUD Application Services

If you need to create a simple CRUD application service which has Create, Update, Delete and Get methods, you can use ABP's base classes to easily build your services. You can inherit from the CrudAppService.

Example

Create an IBookAppService interface inheriting from the ICrudAppService interface.

public interface IBookAppService : 
    ICrudAppService< //Defines CRUD methods
        BookDto, //Used to show books
        Guid, //Primary key of the book entity
        PagedAndSortedResultRequestDto, //Used for paging/sorting on getting a list of books
        CreateUpdateBookDto, //Used to create a new book
        CreateUpdateBookDto> //Used to update a book
{
}

ICrudAppService has generic arguments to get the primary key type of the entity and the DTO types for the CRUD operations (it does not get the entity type since the entity type is not exposed to the clients use this interface).

Creating interface for an application service is a good practice, but not required by the ABP Framework. You can skip the interface part.

ICrudAppService declares the following methods:

public interface ICrudAppService<
    TEntityDto,
    in TKey,
    in TGetListInput,
    in TCreateInput,
    in TUpdateInput>
    : IApplicationService
    where TEntityDto : IEntityDto<TKey>
{
    Task<TEntityDto> GetAsync(TKey id);

    Task<PagedResultDto<TEntityDto>> GetListAsync(TGetListInput input);

    Task<TEntityDto> CreateAsync(TCreateInput input);

    Task<TEntityDto> UpdateAsync(TKey id, TUpdateInput input);

    Task DeleteAsync(TKey id);
}

DTO classes used in this example are BookDto and CreateUpdateBookDto:

[AbpAutoMapFrom(typeof(Book))]
public class BookDto : AuditedEntityDto<Guid>
{
    public string Name { get; set; }

    public BookType Type { get; set; }

    public float Price { get; set; }
}

[AbpAutoMapTo(typeof(Book))]
public class CreateUpdateBookDto
{
    [Required]
    [StringLength(128)]
    public string Name { get; set; }

    [Required]
    public BookType Type { get; set; } = BookType.Undefined;

    [Required]
    public float Price { get; set; }
}

• CreateUpdateBookDto is shared by create and update operations, but you could use separated DTO classes as well.

And finally, the BookAppService implementation is very simple:

public class BookAppService : 
    CrudAppService<Book, BookDto, Guid, PagedAndSortedResultRequestDto,
                        CreateUpdateBookDto, CreateUpdateBookDto>,
    IBookAppService
{
    public BookAppService(IRepository<Book, Guid> repository) 
        : base(repository)
    {
    }
}

CrudAppService implements all methods declared in the ICrudAppService interface. You can then add your own custom methods or override and customize base methods.

CrudAppService has different versions gets different number of generic arguments. Use the one suitable for you.

AbstractKeyCrudAppService

CrudAppService requires to have an Id property as the primary key of your entity. If you are using composite keys then you can not utilize it.

AbstractKeyCrudAppService implements the same ICrudAppService interface, but this time without making assumption about your primary key.

Example

Assume that you have a District entity with CityId and Name as a composite primary key. Using AbstractKeyCrudAppService requires to implement DeleteByIdAsync and GetEntityByIdAsync methods yourself:

public class DistrictAppService
    : AbstractKeyCrudAppService<District, DistrictDto, DistrictKey>
{
    public DistrictAppService(IRepository<District> repository) 
        : base(repository)
    {
    }

    protected override async Task DeleteByIdAsync(DistrictKey id)
    {
        await Repository.DeleteAsync(d => d.CityId == id.CityId && d.Name == id.Name);
    }

    protected override async Task<District> GetEntityByIdAsync(DistrictKey id)
    {
        return await AsyncQueryableExecuter.FirstOrDefaultAsync(
            Repository.Where(d => d.CityId == id.CityId && d.Name == id.Name)
        );
    }
}

This implementation requires you to create a class represents your composite key:

public class DistrictKey
{
    public Guid CityId { get; set; }

    public string Name { get; set; }
}

Lifetime

Lifetime of application services are transient and they are automatically registered to the dependency injection system.