tag-on-injection-site.md

Tag on injection site

Sometimes you need to determine which binding will be used to inject explicitly. To do this, use a special tag created by calling the Tag.On() method. Tag on injection site is specified in a special format: Tag.On("<namespace>.<type>.<member>[:argument]"). The argument is specified only for the constructor and methods. For example, for namespace MyNamespace and type Class1:

• `Tag.On("MyNamespace.Class1.Class1:state1") - the tag corresponds to the constructor argument named state of type MyNamespace.Class1
• Tag.On("MyNamespace.Class1.DoSomething:myArg") - the tag corresponds to the myArg argument of the DoSomething method
• Tag.On("MyNamespace.Class1.MyData") - the tag corresponds to property or field MyData

The wildcards * and ? are supported. All names are case-sensitive. The global namespace prefix global:: must be omitted. You can also combine multiple tags in a single Tag.On("...", "...") call.

For generic types, the type name also contains the number of type parameters, e.g., for the myDep constructor argument of the Consumer<T> class, the tag on the injection site would be MyNamespace.Consumer`1.Consumer:myDep:

using Shouldly;
using Pure.DI;

DI.Setup(nameof(Composition))
    .Bind(
        Tag.On("*UserService.UserService:localRepo"),
        // Tag on injection site for generic type
        Tag.On("*UserFetcher`1.UserFetcher:repo"))
        .To<SqlUserRepository>()
    .Bind(
        // Combined tag
        Tag.On(
            "*UserService.UserService:cloudRepo",
            "*UserService:BackupRepository"))
        .To<ApiUserRepository>()
    .Bind<IUserService>().To<UserService>()

    // Specifies to create the composition root named "Root"
    .Root<IUserService>("Ui");

var composition = new Composition();
var userService = composition.Ui;
userService.LocalRepository.ShouldBeOfType<SqlUserRepository>();
userService.CloudRepository.ShouldBeOfType<ApiUserRepository>();
userService.BackupRepository.ShouldBeOfType<ApiUserRepository>();
userService.FetcherRepository.ShouldBeOfType<SqlUserRepository>();

interface IUserRepository;

class SqlUserRepository : IUserRepository;

class ApiUserRepository : IUserRepository;

class UserFetcher<T>(IUserRepository repo)
{
    public IUserRepository Repository { get; } = repo;
}

interface IUserService
{
    IUserRepository LocalRepository { get; }

    IUserRepository CloudRepository { get; }

    IUserRepository BackupRepository { get; }

    IUserRepository FetcherRepository { get; }
}

class UserService(
    IUserRepository localRepo,
    IUserRepository cloudRepo,
    UserFetcher<string> fetcher)
    : IUserService
{
    public IUserRepository LocalRepository { get; } = localRepo;

    public IUserRepository CloudRepository { get; } = cloudRepo;

    public required IUserRepository BackupRepository { init; get; }

    public IUserRepository FetcherRepository => fetcher.Repository;
}

Running this code sample locally

• Make sure you have the .NET SDK 10.0 or later installed

dotnet --list-sdk

• Create a net10.0 (or later) console application

dotnet new console -n Sample

• Add references to the NuGet packages
  • Pure.DI
  • Shouldly

dotnet add package Pure.DI
dotnet add package Shouldly

• Copy the example code into the Program.cs file

You are ready to run the example 🚀

dotnet run

Warning

Each potentially injectable argument, property, or field contains an additional tag. This tag can be used to specify what can be injected there. This will only work if the binding type and the tag match. So while this approach can be useful for specifying what to enter, it can be more expensive to maintain and less reliable, so it is recommended to use attributes like [Tag(...)] instead.

The following partial class will be generated:

partial class Composition
{
  public IUserService Ui
  {
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    get
    {
      return new UserService(new SqlUserRepository(), new ApiUserRepository(), new UserFetcher<string>(new SqlUserRepository()))
      {
        BackupRepository = new ApiUserRepository()
      };
    }
  }
}

Class diagram:

---
 config:
  maxTextSize: 2147483647
  maxEdges: 2147483647
  class:
   hideEmptyMembersBox: true
---
classDiagram
	SqlUserRepository --|> IUserRepository
	SqlUserRepository --|> IUserRepository
	ApiUserRepository --|> IUserRepository
	UserService --|> IUserService
	Composition ..> UserService : IUserService Ui
	UserService *--  SqlUserRepository : IUserRepository
	UserService *-- "2 " ApiUserRepository : IUserRepository
	UserService *--  UserFetcherᐸStringᐳ : UserFetcherᐸStringᐳ
	UserFetcherᐸStringᐳ *--  SqlUserRepository : IUserRepository
	namespace Pure.DI.UsageTests.Advanced.TagOnInjectionSiteScenario {
		class ApiUserRepository {
				<<class>>
			+ApiUserRepository()
		}
		class Composition {
		<<partial>>
		+IUserService Ui
		}
		class IUserRepository {
			<<interface>>
		}
		class IUserService {
			<<interface>>
		}
		class SqlUserRepository {
				<<class>>
			+SqlUserRepository()
		}
		class UserFetcherᐸStringᐳ {
				<<class>>
			+UserFetcher(IUserRepository repo)
		}
		class UserService {
				<<class>>
			+UserService(IUserRepository localRepo, IUserRepository cloudRepo, UserFetcherᐸStringᐳ fetcher)
			+IUserRepository BackupRepository
		}
	}

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