C# covariant/contravariant 类型利用字典泛型

Question

我需要帮助解决我的代码中的协变情况。

让我举例说明情况：

abstract class BaseSource { }
abstract class BaseTarget { }

class SourceEntityA : BaseSource { }
class TargetEntityB : BaseTarget { }

interface IEntityMapper { }
interface IEntityMapper<in TSource, out TTarget>: IEntityMapper
{
    TTarget Map(TSource entity);
}


abstract class BaseEntityMap<TSource, TTarget> : IEntityMapper<TSource, TTarget>, IEntityMapper
{
    public abstract TTarget Map(TSource entity);
}

class OrderEntityMapper : BaseEntityMap<SourceEntityA, TargetEntityB>
{
    public override TargetEntityB Map(SourceEntityA entity)
    {
        //do some map stuff
        return null;
    }
}

class GoodsEntityMapper : BaseEntityMap<SourceEntityC, TargetEntityD>
{
    public override TargetEntityD Map(SourceEntityC entity)
    {
        //do some map stuff
        return null;
    }
}

class Workflow
{
    IDictionary<Type, IEntityMapper> MappersMap = new Dictionary<Type, IEntityMapper>();
    public MappingArchestrator()
    {
        MappersMap.Add(typeof(SourceEntityA), new OrderEntityMapper());
        MappersMap.Add(typeof(SourceEntityC), new GoodsEntityMapper());
        //....more mappers
    }
    private IEnumerable<BaseSource> GetSourceEntities()
    {

        //return source data
        return null;
    }
    public void Execute()
    {
        foreach (var m in MappersMap.Keys)
        {
            var mapper = MappersMap[m];

            var entities = GetSourceEntities();

            foreach (var entity in entities)
            {
                //Need to handle this situations with correct covariations.
                var target = (IEntityMapper<BaseSource, BaseTarget>mapper).Map(entity);
                //.... code continues
            }
        }
    }
}

不久我需要不同类型映射器的字典，并在公共代码中使用它们的 Execute 方法来处理工作流。

你能指导我使用更好的代码或修复现有的解决方案吗？有可能吗？

或者我必须忘记这里的泛型并在 Map 方法中使用 BaseClasses 并将它们强制转换为我需要的类型。

像这样

abstract class BaseEntityMap : IEntityMapper
{
    public abstract BaseTarget Map(BaseSource entity);
}

谢谢

Answer 1

您可以将 BaseEntityMap<TSource, TTarget> class 中的通用约束应用于您的 BaseSource 和 BaseTarget classes（或在 IEntityMapper<in TSource, out TTarget> 中应用约束接口并摆脱基础抽象 class)

abstract class BaseEntityMap<TSource, TTarget> : IEntityMapper<TSource, TTarget>
    where TSource : BaseSource
    where TTarget : BaseTarget
{
    public abstract TTarget Map(TSource entity);
}

然后使用 Func<BaseSource, BaseTarget> 委托声明映射器字典，如下所示。它应该没问题，因为你已经约束了你的 BaseEntityMap<TSource, TTarget> 和 Func<in T, out TResult> 声明支持输入参数的逆变和结果的协方差

IDictionary<Type, Func<BaseSource, BaseTarget>> MappersMap = new Dictionary<Type, Func<BaseSource, BaseTarget>>();

public Workflow()
{
    var orderMapper = new OrderEntityMapper();
    MappersMap.Add(typeof(SourceEntityA), source => orderMapper.Map((SourceEntityA)source));
    var goodsMapper = new GoodsEntityMapper();
    MappersMap.Add(typeof(SourceEntityC), source => goodsMapper.Map((SourceEntityC)source));
    //....more mappers
}

使用示例

foreach (var entity in entities)
{
    var target = mapper(entity);
    //.... code continues
}

C# covariant/contravariant 类型利用字典泛型

C# covariant/contravariant types utilizing generics with Dictionary

.net

c#

generics

covariance