Question

在ASP.NET Core中，您可以使用Microsoft的依赖注入框架is bind "open generics"（未绑定到具体类型的泛型类型）执行以下操作之一，如下所示：

public void ConfigureServices(IServiceCollection services) {
    services.AddSingleton(typeof(IRepository<>), typeof(Repository<>))
}

您也可以使用the factory pattern to hydrate dependencies。这是一个人为的例子：

public interface IFactory<out T> {
    T Provide();
}

public void ConfigureServices(IServiceCollection services) {
    services.AddTransient(typeof(IFactory<>), typeof(Factory<>));

    services.AddSingleton(
        typeof(IRepository<Foo>), 
        p => p.GetRequiredService<IFactory<IRepository<Foo>>().Provide()
    ); 
}

然而，我无法弄清楚如何将这两个概念结合在一起。看起来它会从这样的东西开始，但我需要用于水合IRepository<>实例的具体类型。

public void ConfigureServices(IServiceCollection services) {
    services.AddTransient(typeof(IFactory<>), typeof(Factory<>));

    services.AddSingleton(
        typeof(IRepository<>), 
        provider => {
            // Say the IServiceProvider is trying to hydrate 
            // IRepository<Foo> when this lambda is invoked. 
            // In that case, I need access to a System.Type 
            // object which is IRepository<Foo>. 
            // i.e.: repositoryType = typeof(IRepository<Foo>);

            // If I had that, I could snag the generic argument
            // from IRepository<Foo> and hydrate the factory, like so:

            var modelType = repositoryType.GetGenericArguments()[0];
            var factoryType = typeof(IFactory<IRepository<>>).MakeGenericType(modelType);
            var factory = (IFactory<object>)p.GetRequiredService(factoryType);

            return factory.Provide();
        }           
    ); 
}

如果我尝试将Func<IServiceProvider, object>仿函数与开放式泛型一起使用，我会从dotnet CLI获取this ArgumentException消息Open generic service type 'IRepository<T>' requires registering an open generic implementation type.。它甚至没有达到lambda。

这种类型的绑定是否可以与Microsoft的依赖注入框架一起使用？

Answer 1

net.core依赖项不允许您在注册开放泛型类型时提供工厂方法，但您可以通过提供将实现所请求接口的类型来解决此问题，但在内部它将充当工厂。伪装的工厂：

services.AddSingleton(typeof(IMongoCollection<>), typeof(MongoCollectionFactory<>)); //this is the important part
services.AddSingleton(typeof(IRepository<>), typeof(Repository<>))


public class Repository : IRepository {
    private readonly IMongoCollection _collection;
    public Repository(IMongoCollection collection)
    {
        _collection = collection;
    }

    // .. rest of the implementation
}

//and this is important as well
public class MongoCollectionFactory<T> : IMongoCollection<T> {
    private readonly _collection;

    public RepositoryFactoryAdapter(IMongoDatabase database) {
        // do the factory work here
        _collection = database.GetCollection<T>(typeof(T).Name.ToLowerInvariant())
    }

    public T Find(string id) 
    {
        return collection.Find(id);
    }   
    // ... etc. all the remaining members of the IMongoCollection<T>, 
    // you can generate this easily with ReSharper, by running 
    // delegate implementation to a new field refactoring
}

当容器解析MongoCollectionFactory时，ti将知道T是什么类型，并将正确创建集合。然后我们将创建的集合保存在内部，并将所有调用委托给它。（我们模仿c this=factory.Create()，这在csharp中是不允许的。：））

更新：正如Kristian Hellang所指出的，ASP.NET Logging使用了相同的模式

public class Logger<T> : ILogger<T>
{
    private readonly ILogger _logger;

    public Logger(ILoggerFactory factory)
    {
        _logger = factory.CreateLogger(TypeNameHelper.GetTypeDisplayName(typeof(T)));
    }

    void ILogger.Log<TState>(...)
    {
        _logger.Log(logLevel, eventId, state, exception, formatter);
    }
}

https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Extensions.Logging.Abstractions/LoggerOfT.cs#L29

原创讨论：

https://twitter.com/khellang/status/839120286222012416

Answer 2

我也不明白你的lambda表达的意思，所以我会向你解释我的做法。

我想您希望达到您分享的文章中所解释的内容

这允许我在向ASP.NET Core依赖注入系统提供依赖项之前检查传入的请求

我需要检查HTTP请求中的自定义标头，以确定哪个客户正在请求我的API。然后我可以稍后在管道中决定我的IDatabaseRepository（链接到SQL数据库的文件系统或实体框架）的哪个实现来提供这个唯一的请求。

所以我首先编写一个中间件

public class ContextSettingsMiddleware
{
    private readonly RequestDelegate _next;

    public ContextSettingsMiddleware(RequestDelegate next, IServiceProvider serviceProvider)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context, IServiceProvider serviceProvider, IHostingEnvironment env, IContextSettings contextSettings)
    {
        var customerName = context.Request.Headers["customer"];
        var customer = SettingsProvider.Instance.Settings.Customers.FirstOrDefault(c => c.Name == customerName);
        contextSettings.SetCurrentCustomer(customer);

        await _next.Invoke(context);
    }
}

我的SettingsProvider只是一个为我提供相应客户对象的单身人士。

要让我们的中间件访问此ContextSettings，我们首先需要在Startup.cs中的ConfigureServices中注册它

var contextSettings = new ContextSettings();
services.AddSingleton<IContextSettings>(contextSettings);

在Configure方法中，我们注册了我们的中间件

app.UseMiddleware<ContextSettingsMiddleware>();

既然我们的客户可以从其他地方访问，那就来写我们的工厂。

public class DatabaseRepositoryFactory
{
    private IHostingEnvironment _env { get; set; }

    public Func<IServiceProvider, IDatabaseRepository> DatabaseRepository { get; private set; }

    public DatabaseRepositoryFactory(IHostingEnvironment env)
    {
        _env = env;
        DatabaseRepository = GetDatabaseRepository;
    }

    private IDatabaseRepository GetDatabaseRepository(IServiceProvider serviceProvider)
    {
        var contextSettings = serviceProvider.GetService<IContextSettings>();
        var currentCustomer = contextSettings.GetCurrentCustomer();

        if(SOME CHECK)
        {
            var currentDatabase = currentCustomer.CurrentDatabase as FileSystemDatabase;
            var databaseRepository = new FileSystemDatabaseRepository(currentDatabase.Path);
            return databaseRepository;
        }
        else
        {
            var currentDatabase = currentCustomer.CurrentDatabase as EntityDatabase;
            var dbContext = new CustomDbContext(currentDatabase.ConnectionString, _env.EnvironmentName);
            var databaseRepository = new EntityFrameworkDatabaseRepository(dbContext);
            return databaseRepository;
        }
    }
}

要使用serviceProvider.GetService<>()方法，您需要在CS文件中包含以下内容

using Microsoft.Extensions.DependencyInjection;

最后，我们可以在ConfigureServices方法

中使用我们的工厂

var databaseRepositoryFactory = new DatabaseRepositoryFactory(_env);
services.AddScoped<IDatabaseRepository>(databaseRepositoryFactory.DatabaseRepository);

因此，DatabaseRepository的每个HTTP请求都可能因多个参数而异。我可以使用文件系统或SQL数据库，我可以获得与我的客户相对应的正确数据库。（是的我每个客户有多个数据库，不要试图理解为什么）

我尽可能简化它，我的代码实际上更复杂，但你明白了（我希望）。现在您可以修改它以满足您的需求。

Answer 3

我也不满意现有的解决方案。

这是使用内置容器的完整解决方案，它支持我们需要的所有内容：

简单的依赖项。
复杂的依存关系（需要解决IServiceProvider。
配置数据（例如连接字符串）。

我们将注册一个我们真正想要使用的类型的代理。代理只是从预期的类型继承而来，但是通过单独注册的Options类型来获取“困难”部分（复杂的依赖关系和配置）。

由于Options类型是非泛型的，因此很容易照常进行自定义。

public static class RepositoryExtensions
{
    /// <summary>
    /// A proxy that injects data based on a registered Options type.
    /// As long as we register the Options with exactly what we need, we are good to go.
    /// That's easy, since the Options are non-generic!
    /// </summary>
    private class ProxyRepository<T> : Repository<T>
    {
        public ProxyRepository(Options options, ISubdependency simpleDependency)
            : base(
                // A simple dependency is injected to us automatically - we only need to register it
                simpleDependency,
                // A complex dependency comes through the non-generic, carefully registered Options type
                options?.ComplexSubdependency ?? throw new ArgumentNullException(nameof(options)),
                // Configuration data comes through the Options type as well
                options.ConnectionString)
        {
        }
    }

    public static IServiceCollection AddRepositories(this ServiceCollection services, string connectionString)
    {
        // Register simple subdependencies (to be automatically resolved)
        services.AddSingleton<ISubdependency, Subdependency>();

        // Put all regular configuration on the Options instance
        var optionObject = new Options(services)
        {
            ConnectionString = connectionString ?? throw new ArgumentNullException(nameof(connectionString))
        };

        // Register the Options instance
        // On resolution, last-minute, add the complex subdependency to the options as well (with access to the service provider)
        services.AddSingleton(serviceProvider => optionObject.WithSubdependency(ResolveSubdependency(serviceProvider)));

        // Register the open generic type
        // All dependencies will be resolved automatically: the simple dependency, and the Options (holding everything else)
        services.AddSingleton(typeof(IRepository<>), typeof(ProxyRepository<>));

        return services;

        // Local function that resolves the subdependency according to complex logic ;-)
        ISubdependency ResolveSubdependency(IServiceProvider serviceProvider)
        {
            return new Subdependency();
        }
    }

    internal sealed class Options
    {
        internal IServiceCollection Services { get; }

        internal ISubdependency ComplexSubdependency { get; set; }
        internal string ConnectionString { get; set; }

        internal Options(IServiceCollection services)
        {
            this.Services = services ?? throw new ArgumentNullException(nameof(services));
        }

        /// <summary>
        /// Fluently sets the given subdependency, allowing to options object to be mutated and returned as a single expression.
        /// </summary>
        internal Options WithSubdependency(ISubdependency subdependency)
        {
            this.ComplexSubdependency = subdependency ?? throw new ArgumentNullException(nameof(subdependency));
            return this;
        }
    }
}

Answer 4

请参见此issue on the dotnet (5) runtime git。这将增加对通过工厂注册开放通用名称的支持。

工厂模式与开放的泛型

4 个答案: