如何在Scoped服务生命周期中使多个接口共享同一实例

问题场景 #

现有接口与实现类定义如下：

public interface IOperation
{
  string OperationId { get; }
}

public interface IScopedOperation1 : IOperation { }

public interface IScopedOperation2 : IOperation { }

public class DefaultOperation : IScopedOperation1, IScopedOperation2
{
   public string OperationId { get; } = Guid.NewGuid().ToString()[^4..];
}

public class OperationLogger
{
  private readonly IScopedOperation1 _scopedOperation1;
  private readonly IScopedOperation2 _scopedOperation2;

  public OperationLogger(
      IScopedOperation1 op1, IScopedOperation2 op2) =>
      (_scopedOperation1, _scopedOperation2) = (op1, op2);

  public void LogOperations()
  {
     Console.WriteLine($"Operation1: [ {_scopedOperation1.OperationId} ]");
     Console.WriteLine($"Operation2: [ {_scopedOperation2.OperationId} ]");
  }
}

按常规方式注册服务后：

using IHost host = Host.CreateDefaultBuilder(args)
    .ConfigureServices((_, services) =>
        services
         .AddScoped<IScopedOperation1, DefaultOperation>()
         .AddScoped<IScopedOperation2, DefaultOperation>()
         .AddTransient<OperationLogger>())
    .Build();

host.Services.GetService<OperationLogger>().LogOperations();

会发现IScopedOperation1和IScopedOperation2对应的实例OperationId不一致——这是因为DI容器会为每个接口的Scoped注册单独创建实例。

解决方案 #

方案1：先注册实现类，再映射所有接口到该实例 #

先将实现类本身注册为Scoped服务，随后让所有相关接口都解析到这个已注册的实例：

using IHost host = Host.CreateDefaultBuilder(args)
    .ConfigureServices((_, services) =>
        services
         .AddScoped<DefaultOperation>()
         .AddScoped<IScopedOperation1>(sp => sp.GetRequiredService<DefaultOperation>())
         .AddScoped<IScopedOperation2>(sp => sp.GetRequiredService<DefaultOperation>())
         .AddTransient<OperationLogger>())
    .Build();

host.Services.GetService<OperationLogger>().LogOperations();

方案2：复用已注册接口的实例 #

可以直接让其中一个接口的实例作为另一个接口的解析来源，减少重复注册逻辑：

using IHost host = Host.CreateDefaultBuilder(args)
    .ConfigureServices((_, services) =>
        services
         .AddScoped<IScopedOperation1, DefaultOperation>()
         .AddScoped<IScopedOperation2>(sp => sp.GetRequiredService<IScopedOperation1>())
         .AddTransient<OperationLogger>())
    .Build();

host.Services.GetService<OperationLogger>().LogOperations();

原理 #

默认情况下，每个AddScoped<TInterface, TImplementation>会让DI容器为该接口维护独立的Scoped实例缓存。通过让多个接口指向同一个实例（无论是直接引用实现类的注册，还是复用其他接口的实例），就能保证同一作用域内所有接口请求都拿到同一个对象。

内容的提问来源于stack exchange，提问作者drazen