MediatR 12处理器未注册求助：控制器映射正常但处理器失效

问题详情 #

配置MediatR 12后，控制器能正常注入IMediator，但发送GetUserByIdQuery请求时触发以下错误：

System.InvalidOperationException: No service for type
 'MediatR.IRequestHandler`2[ACVOB.Application.Features.User.Queries.GetUserByIdQuery,ACVOB.Domain.User.Queries.UserResponse]'
 has been registered.

相关代码如下：

Startup配置

public void ConfigureServices(IServiceCollection services)
{
    AddSwagger(services);
    services.AddRazorPages();
    services.AddScoped<UsuarioRepository>();
    services.AddScoped<CategoriaRepository>();
    services.AddControllers();
    services.AddDbContext<ACVOBContext>(options=> options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
    services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(Assembly.GetExecutingAssembly()));
}

控制器代码

[SwaggerOperation(Summary = "Get user by ID")]
[HttpGet("{userId}")]
public async Task<IActionResult> GetUsuarioById(long userId)
{
    var request = new GetUserByIdQuery() { UserID = userId };
    var response  = await _mediator.Send(request);
    return this.Ok(response);
}

处理器代码

public class GetUserByIdQueryHandler : IRequestHandler<GetUserByIdQuery, UserResponse>
{
    private readonly IUserService _userService;

    public GetUserByIdQueryHandler(IUserService userService)
    {
        _userService = userService;
    }

    public async Task<UserResponse> Handle(GetUserByIdQuery request, CancellationToken cancellationToken)
    {
        return await _userService.GetUserById(request.UserID);
    }
}

解决办法 #

1. 检查程序集扫描范围
   Assembly.GetExecutingAssembly()只会扫描Startup所在的程序集。如果你的GetUserByIdQueryHandler或GetUserByIdQuery位于其他程序集（比如ACVOB.Application），需要指定对应的程序集：

   // 改用处理器所在的程序集注册
   services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(typeof(GetUserByIdQueryHandler).Assembly));
   

   若需扫描多个程序集，可链式添加：

   services.AddMediatR(cfg => 
   {
       cfg.RegisterServicesFromAssembly(Assembly.GetExecutingAssembly());
       cfg.RegisterServicesFromAssembly(typeof(GetUserByIdQueryHandler).Assembly);
   });
   

2. 确认依赖包安装正确
   确保已安装MediatR.Extensions.Microsoft.DependencyInjection——这是MediatR与ASP.NET Core DI集成的官方包，MediatR 12必须依赖它才能自动注册处理器：

   Install-Package MediatR.Extensions.Microsoft.DependencyInjection
   

3. 验证Query的接口实现
   检查GetUserByIdQuery是否正确继承了IRequest<UserResponse>接口，这是MediatR关联请求与处理器的核心：

   public class GetUserByIdQuery : IRequest<UserResponse>
   {
       public long UserID { get; set; }
   }
   

4. 检查处理器类的访问修饰符
   确保GetUserByIdQueryHandler是public修饰的，私有/内部类无法被MediatR的程序集扫描机制识别。

5. 清理并重新生成解决方案
   编译器缓存可能导致程序集扫描不完整，尝试清理解决方案、删除bin/obj目录后重新生成项目。

内容的提问来源于stack exchange，提问作者Mariano Ezequiel Carioni