在 CS0535 中隐藏具体类型结果后面的泛型类型参数

Question

我有一个相当学术的问题：为什么技术上不可能使用以下代码？

// abstractions
public record MappingRequest<TContract, TEntity>(TContract Contract, TEntity Entity);

public interface IMapper<TResult, TContract, TEntity>
{
    TResult Map(MappingRequest<TContract, TEntity> request);
}

// more readable abstraction
public interface ILocationMapper : IMapper<LocationResult, LocationContract, LocationEntity>
{
}

// concrete
public record LocationResult;
public record LocationContract;
public record LocationEntity;

public record LocationRequest(LocationContract Contract, LocationEntity Entity) : MappingRequest<LocationContract, LocationEntity>(Contract, Entity);

public class LocationMapper : ILocationMapper
{
    public LocationResult Map(LocationRequest request)
    {
        return new LocationResult();
    }
}

这给了我以下编译器错误：

Error CS0535 : 'LocationMapper' does not implement interface member 'IMapper<LocationResult, LocationContract, LocationEntity>.Map(MappingRequest<LocationContract, LocationEntity>)'

为什么编译器不能“推断”（不完全确定这是否是正确的术语）

LocationRequest

继承自

MappingRequest

？

谢谢你的解释🙏🏻

Answer 1

您的

LocationMapper.Map

方法当前接受

LocationRequest

，但

IMapper<LocationResult, LocationContract, LocationEntity>

的任何实现都必须实现接受

any

Map（而不仅仅是

MappingRequest<TContract, TEntity>

）的

LocationRequest

方法。

你的代码很简单，但想象一下

LocationRequest

有一个属性

Foo

:

public record LocationRequest(LocationContract Contract, LocationEntity Entity)
    : MappingRequest<LocationContract, LocationEntity>(Contract, Entity)
{
    public string Foo { get; }
}

你的方法尝试使用该属性：

public class LocationMapper : ILocationMapper
{
    public LocationResult Map(LocationRequest request)
    {
        Console.WriteLine(request.Foo);
        return new LocationResult();
    }
}

现在，如果您尝试将其称为

IMapper<LocationResult, LocationContract, LocationEntity>

，并使用另一个

MappingRequest<TContract, TEntity>

，我们就会遇到问题，因为没有 foo 属性。

例如

var request = new MappingRequest<TContract, TEntity>(default, default);
IMapper<LocationResult, LocationContract, LocationEntity> mapper = new LocationMapper();
mapper.Map(request); // We don't have any Foo to use !!

---

问题在于

LocationRequest

不是

MappingRequest<LocationContract, LocationEntity>

的简化别名，它是完全不同的类型。

如果冗长对您来说是个问题，那么您可以像这样为您的类型添加别名：

using LocationRequest = MappingRequest<LocationContract, LocationEntity>;