添加实现类型的泛型类型列表，并将其连接到具有相同接口值的字典 [英] Add list of generic type that implements and interface to a dictionary that has a value of the same interface

问题描述

我有这段代码：

private Dictionary<Type, List<ICacheable>> cachedEntities;
 
public ICacheable GetFromCache<T>(Expression<Func<T, bool>> predicate) where T : ICacheable
{
    var type = typeof(T);
 
    if (!cachedEntities.ContainsKey(type))
        cachedEntities.Add(type, new List<T>());
 
    var entitiesList = cachedEntities[type].AsQueryable();
    var result = entitiesList.Where(predicate);
 
    return result;
}

我在尝试添加新列表< T>时遇到错误字典声称它不能从T转换为ICacheable，但我强迫T实现，所以我不太明白为什么我得到它。

I'm getting an error when trying to add the new List<T> to the dictionary claiming that it can't convert from T to ICacheable, but I force T to implement, so I'm not really understanding why I'm getting it.

我是entityList.Where（谓词）中也有一个错误，声称它无法转换为Expression< Func< T，bool>>表达式< Func< ICacheable，int，bool>>。

I'm also getting an error in the entitiesList.Where(predicate) claiming it can't convert from Expression<Func<T,bool>> to Expression<Func<ICacheable, int, bool>>.

推荐答案

我猜这是一个协方差/逆变问题。但是......因为Dictionary被声明为只保存ICacheable的列表，该方法的那部分实际上是否需要使用泛型类型？

I would guess this is a covariance/contravariance problem. But...since the Dictionary is declared as only holding lists of ICacheable, does that part of the method actually need to use the generic type at all?

例如。你不能这样做：

 private Dictionary<Type, List<ICacheable>> cachedEntities;

public IQueryable<ICacheable> GetFromCache<T>(Expression<Func<ICacheable, bool>> predicate)
{
    var type = typeof(T);

    if (!cachedEntities.ContainsKey(type))
        cachedEntities.Add(type, new List<ICacheable>());

    var entitiesList = cachedEntities[type].AsQueryable();
    var result = entitiesList.Where(predicate);

    return result;
}

（不得不将方法的返回类型更改为IQueryable，因为那就是Where （）方法返回）。

(Had to change the return type of the method to IQueryable as well since that's what the Where() method returns).

这篇关于添加实现类型的泛型类型列表，并将其连接到具有相同接口值的字典的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！