.cs

//Interface:
public interface IProjectService<T, K>: IService<T, K> where T: Project where K: ProjectRes{}

//Injected implementation:

 public class ProjectService<T, K> : Service<T, K>, IProjectService<T, K> where T : Project where K : ProjectRes, new()
    {
        private readonly IRepo<T, K> repo;
        private readonly IUnitOfWork uOfWork;
        private readonly IMapper mapper;

        public ProjectService(IProjectRepo<T, K> repo, IUnitOfWork uOfWork, IMapper mapper) : base(repo, uOfWork, mapper)
        {
            
            this.repo = repo;
            this.uOfWork = uOfWork;
            this.mapper = mapper;
        }
        public async override Task<IQueryable<K>> GetAll(){
             var projects = this.repo.GetAll();
             return projects.MapToProjectResource() as IQueryable<K>;
             //var projecResourse = 
        }

    }
    //DI container in StartUp.cs: 
    //compiler allow me implement only such explicit declaration with real classes types in generic params
   
   services.AddScoped(typeof(IProjectService<Project, ProjectRes>), typeof(ProjectService<Project, ProjectRes>));
    
  // actually i curios if there is better solution than this one.  

@Poltuu try creating a wrapper class for DI class OnlyForDI<T> : SpecificImplementationService<T,TSpecificImplementation>) and then register services.AddScoped(typeof(IService<>), typeof(OnlyForDI<>));.