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 fully 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 get and return DTOs instead of entities. ABP does not force this rule. However, exposing entities to the presentation layer (or to remote clients) has significant problems and is 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.

Object to object mapping provides abstractions and it is implemented by the AutoMapper library by default.

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:

public class BookDto
{
    public Guid Id { get; set; }

    public string Name { get; set; }

    public BookType Type { get; set; }

    public float? Price { get; set; }
}

AutoMapper requires to create a mapping profile class. Example:

public class MyProfile : Profile
{
    public MyProfile()
    {
        CreateMap<Book, BookDto>();
    }
}

You should then register profiles using the AbpAutoMapperOptions:

[DependsOn(typeof(AbpAutoMapperModule))]
public class MyModule : AbpModule
{
    public override void ConfigureServices(ServiceConfigurationContext context)
    {
        Configure<AbpAutoMapperOptions>(options =>
        {
            //Add all mappings defined in the assembly of the MyModule class
            options.AddMaps<MyModule>();
        });
    }
}

AddMaps registers all profile classes defined in the assembly of the given class, typically your module class. It also registers for the attribute mapping.

Then you can implement the GetAsync method as shown below:

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

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 a 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 an interface for an application service is good practice, but not required by the ABP. 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:

public class BookDto : AuditedEntityDto<Guid>
{
    public string Name { get; set; }

    public BookType Type { get; set; }

    public float Price { get; set; }
}

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; }
}

Profile class of DTO class.

public class MyProfile : Profile
{
    public MyProfile()
    {
        CreateMap<Book, BookDto>();
        CreateMap<CreateUpdateBookDto, Book>();
    }
}

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

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

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

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

    public string Name { get; set; }
}

Authorization (for CRUD App Services)

There are two ways of authorizing the base application service methods;

1. You can set the policy properties (xxxPolicyName) in the constructor of your service. Example:

public class MyPeopleAppService : CrudAppService<Person, PersonDto, Guid>
{
    public MyPeopleAppService(IRepository<Person, Guid> repository) 
        : base(repository)
    {
        GetPolicyName = "...";
        GetListPolicyName = "...";
        CreatePolicyName = "...";
        UpdatePolicyName = "...";
        DeletePolicyName = "...";
    }
}

CreatePolicyName is checked by the CreateAsync method and so on... You should specify a policy (permission) name defined in your application.

2. You can override the check methods (CheckXxxPolicyAsync) in your service. Example:

public class MyPeopleAppService : CrudAppService<Person, PersonDto, Guid>
{
    public MyPeopleAppService(IRepository<Person, Guid> repository) 
        : base(repository)
    {
    }

    protected async override Task CheckDeletePolicyAsync()
    {
        await AuthorizationService.CheckAsync("...");
    }
}

You can perform any logic in the CheckDeletePolicyAsync method. It is expected to throw an AbpAuthorizationException in any unauthorized case, like AuthorizationService.CheckAsync already does.

Base Properties & Methods

CRUD application service base class provides many useful base methods that you can override to customize it based on your requirements.

CRUD Methods

These are the essential CRUD methods. You can override any of them to completely customize the operation. Here, the definitions of the methods:

Task<TGetOutputDto> GetAsync(TKey id);
Task<PagedResultDto<TGetListOutputDto>> GetListAsync(TGetListInput input);
Task<TGetOutputDto> CreateAsync(TCreateInput input);
Task<TGetOutputDto> UpdateAsync(TKey id, TUpdateInput input);
Task DeleteAsync(TKey id);

Querying

These methods are low level methods that can control how to query entities from the database.

• CreateFilteredQuery can be overridden to create an IQueryable<TEntity> that is filtered by the given input. If your TGetListInput class contains any filter, it is proper to override this method and filter the query. It returns the (unfiltered) repository (which is already IQueryable<TEntity>) by default.
• ApplyPaging is used to make paging on the query. If your TGetListInput already implements IPagedResultRequest, you don't need to override this since the ABP automatically understands it and performs the paging.
• ApplySorting is used to sort (order by...) the query. If your TGetListInput already implements the ISortedResultRequest, ABP automatically sorts the query. If not, it fallbacks to the ApplyDefaultSorting which tries to sort by creation time, if your entity implements the standard IHasCreationTime interface.
• GetEntityByIdAsync is used to get an entity by id, which calls Repository.GetAsync(id) by default.
• DeleteByIdAsync is used to delete an entity by id, which calls Repository.DeleteAsync(id) by default.

Object to Object Mapping

These methods are used to convert Entities to DTOs and vice verse. They use the IObjectMapper by default.

• MapToGetOutputDtoAsync is used to map the entity to the DTO returned from the GetAsync, CreateAsync and UpdateAsync methods. Alternatively, you can override the MapToGetOutputDto if you don't need to perform any async operation.
• MapToGetListOutputDtosAsync is used to map a list of entities to a list of DTOs returned from the GetListAsync method. It uses the MapToGetListOutputDtoAsync to map each entity in the list. You can override one of them based on your case. Alternatively, you can override the MapToGetListOutputDto if you don't need to perform any async operation.
• MapToEntityAsync method has two overloads;
  • MapToEntityAsync(TCreateInput) is used to create an entity from TCreateInput.
  • MapToEntityAsync(TUpdateInput, TEntity) is used to update an existing entity from TUpdateInput.

Miscellaneous

Working with Streams

Stream object itself is not serializable. So, you may have problems if you directly use Stream as the parameter or the return value for your application service. ABP provides a special type, IRemoteStreamContent to be used to get or return streams in the application services.

Example: Application Service Interface that can be used to get and return streams

using System;
using System.Threading.Tasks;
using Volo.Abp.Application.Services;
using Volo.Abp.Content;

namespace MyProject.Test
{
    public interface ITestAppService : IApplicationService
    {
        Task Upload(Guid id, IRemoteStreamContent streamContent);
        Task<IRemoteStreamContent> Download(Guid id);

        Task CreateFile(CreateFileInput input);
        Task CreateMultipleFile(CreateMultipleFileInput input);
    }

    public class CreateFileInput
    {
        public Guid Id { get; set; }

        public IRemoteStreamContent Content { get; set; }
    }

    public class CreateMultipleFileInput
    {
        public Guid Id { get; set; }

        public IEnumerable<IRemoteStreamContent> Contents { get; set; }
    }
}

You need to configure AbpAspNetCoreMvcOptions to add DTO class to FormBodyBindingIgnoredTypes to use IRemoteStreamContent in DTO (Data Transfer Object)

Configure<AbpAspNetCoreMvcOptions>(options =>
{
    options.ConventionalControllers.FormBodyBindingIgnoredTypes.Add(typeof(CreateFileInput));
    options.ConventionalControllers.FormBodyBindingIgnoredTypes.Add(typeof(CreateMultipleFileInput));
});

Example: Application Service Implementation that can be used to get and return streams

using System;
using System.IO;
using System.Threading.Tasks;
using Volo.Abp;
using Volo.Abp.Application.Services;
using Volo.Abp.Content;

namespace MyProject.Test
{
    public class TestAppService : ApplicationService, ITestAppService
    {
        public Task<IRemoteStreamContent> Download(Guid id)
        {
            var fs = new FileStream("C:\\Temp\\" + id + ".blob", FileMode.OpenOrCreate);
            return Task.FromResult(
                (IRemoteStreamContent) new RemoteStreamContent(fs) {
                    ContentType = "application/octet-stream" 
                }
            );
        }

        public async Task Upload(Guid id, IRemoteStreamContent streamContent)
        {
            using (var fs = new FileStream("C:\\Temp\\" + id + ".blob", FileMode.Create))
            {
                await streamContent.GetStream().CopyToAsync(fs);
                await fs.FlushAsync();
            }
        }

        public async Task CreateFileAsync(CreateFileInput input)
        {
            using (var fs = new FileStream("C:\\Temp\\" + input.Id + ".blob", FileMode.Create))
            {
                await input.Content.GetStream().CopyToAsync(fs);
                await fs.FlushAsync();
            }
        }

        public async Task CreateMultipleFileAsync(CreateMultipleFileInput input)
        {
            using (var fs = new FileStream("C:\\Temp\\" + input.Id + ".blob", FileMode.Append))
            {
                foreach (var content in input.Contents)
                {
                    await content.GetStream().CopyToAsync(fs);
                }
                await fs.FlushAsync();
            }
        }
    }
}

IRemoteStreamContent is compatible with the Auto API Controller and Dynamic C# HTTP Proxy systems.

Lifetime

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

See Also

• Video tutorial Part-1
• Video tutorial Part-2