类不能用作泛型类型或方法中的类型参数，没有隐式引用转换

Question

我正在为我的 MVC 4 项目构建（尝试构建）通用存储库、任务和控制器，并使用它来学习泛型和接口。我已经走了很远，但坚持这个错误。 CustomerContactsListViewModel 确实继承自 IViewModelList，这就是我感到困惑的原因。

错误：

The type 'OTIS.AppServ.OrderMgmt.ViewModels.CustomerContactsListViewModel' cannot be used as type parameter 'TViewModelList' in the generic type or method 'OTIS.AppServ.BaseAppServGrid<TRepository,TViewModelSingle,TViewModelList>'. There is no implicit reference conversion from 'OTIS.AppServ.OrderMgmt.ViewModels.CustomerContactsListViewModel' to 'OTIS.AppServ.IViewModelList<OTIS.domain.OrderMgmt.Customer,OTIS.AppServ.OrderMgmt.ViewModels.CustomerContactsListViewModel>'.

界面：

public interface IViewModelList<TClass, TViewModelList>
    {
        IEnumerable<TClass> ConvertViewModelToClass(IEnumerable<TViewModelList> entityList);
        IEnumerable<TViewModelList> ConvertClassToViewModel(IEnumerable<TClass> entityList);
    }

继承接口的视图模型：

    public class CustomerContactsListViewModel : IViewModelList<CustomerContact, CustomerContactsListViewModel>, IEntity

继承的通用任务（应用程序服务）类：

public class BaseAppServGrid<TRepository, TViewModelSingle, TViewModelList> : BaseAppServ<TRepository, TViewModelSingle>
            where TRepository : class, IEntity, IAuditStamps, new()
            where TViewModelSingle : class, IViewModelSingle<TRepository, TViewModelSingle>, new()
            where TViewModelList : class, IEntity, IViewModelList<TRepository, TViewModelList>, new()

尝试继承Base的特定Task/App服务类，抛出错误：

public class ManageCustomersAppServ : BaseAppServGrid<Customer, CustomerViewModel, CustomerContactsListViewModel>

Answer 1

在CustomerContactsListViewModel 中，您已将IViewModelList 的第一个参数实现为CustomerContact 类型。

但是，ManageCustomersAppServ 类已声明为 Customer 类型。

所以

IViewModelList<Customer, CustomerContactsListViewModel>

不能分配给

IViewModelList<CustomerContact, CustomerContactsListViewModel>

您需要：

1. 更改BaseAppServGrid 以采用另一个通用参数，如下所示：

   public class BaseAppServGrid<TModel, TRepository, TViewModelSingle, TViewModelList> :
       BaseAppServ<TRepository, TViewModelSingle>
       where TRepository : class, IEntity, IAuditStamps, new()
       where TViewModelSingle : class, IViewModelSingle<TRepository, TViewModelSingle>, new()
       where TViewModelList : class, IEntity, IViewModelList<TModel, TViewModelList>, new()
   
   public class ManageCustomersAppServ : BaseAppServGrid<CustomerContact, Customer, CustomerViewModel, CustomerContactsListViewModel>
   

2. 查看 Customer 和 CustomerContact 是否可以从某个 IRepository 接口/类派生，如果这是您将 Customer 传递给名为“TRepository”的通用参数时的意图。