使用AddKeyedScoped注册服务及实现时出现循环依赖错误排查

代码展示 #

接口定义 #

public interface IService
{
  public Task<string> GetString();
}

DAL类实现 #

public class SampleDAL : IService
{
  public async Task<string> GetString()
  { 
    //do something 
  }
}

BL类实现 #

public class SampleBl([FromKeyedServices("SampleBl")]IService service) : IService
{
  public Task<string> GetString()
  {
   var something = service.GetString();
   return something;
  }
}

控制器代码 #

[Route("[controller]")]
[ApiController]
public class SampleController([FromKeyedServices("SampleBl")] IService serviceBl) : Controller
{
  [HttpGet("GetSomeString")]
  public async Task<string> GetString()
  {
    var somethingout = serviceBl.GetString();
    return somethingout;
  }
}

服务注册代码 #

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddKeyedScoped<IService, SampleDal>("SampleDal");
builder.Services.AddKeyedScoped<IService, SampleBl>("SampleBl");

错误信息 #

Some services are not able to be constructed (Error while validating the service descriptor 'ServiceType: Lifetime: Scoped ServiceKey: KeyedImplementationType: ': A circular dependency was detected for the service of type ''.

错误原因 #

SampleBl类构造函数使用[FromKeyedServices("SampleBl")]注入IService，这会让DI容器尝试注入SampleBl自身，形成循环依赖：SampleBl依赖SampleBl，容器无法解析这种引用关系。

解决方案 #

SampleBl作为业务逻辑层，应该依赖DAL层的实现，也就是Key为"SampleDal"的IService，修改构造函数的注入标记即可：

修改后的SampleBl代码：

public class SampleBl([FromKeyedServices("SampleDal")]IService service) : IService
{
  public async Task<string> GetString()
  {
   var something = await service.GetString();
   return something;
  }
}

另外注意：service.GetString()是异步方法，需要加上await关键字，否则会返回未完成的Task，可能引发后续执行异常。

内容的提问来源于stack exchange，提问作者user876120