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如何将查找的远程信息,即i lookup<tkey,derived>转换为ilookup<tkey,base>?

linq c#

Kasper van den Berg · 技术社区 · 6 年前

我有一个 ILookup<int, Derived> 我想还一个 ILookup<int, Base> 哪里 Derived 实现或扩展 Base 是的。

目前我使用 SelectMany() 然后 ToLookup() 首先提取 ILookup<int,派生> 住进公寓 IEnumerable 然后创建一个新的 ILookup<int,Base> :

class Base { }

class Derived: Base { }

class Test
{
    ILookup<int, Base> CastLookup(ILookup<int, Derived> existing)
    {
        IEnumerable<KeyValuePair<int, Base>> flattened = existing.SelectMany(
            (x) => x,
            (gr, v) => new KeyValuePair<int, Base>(gr.Key, (Base)v));    // I know the explicit cast can be implicit here; it is just to demonstrate where the up casting is happening.
        ILookup<int, Base> result = flattened.ToLookup(
            (x) => x.Key,
            (x) => x.Value);
        return result;
    }
}

如何转换ilookup而不迭代其条目并重新打包它们?

_{注:一个相关的问题是

Shouldn't ILookup<TKey, TElement> be (declared) covariant in TElement?

由

bigge

是的。

Ryszard DÅ¼egan

answers

主要是因为历史原因:

ILookup<TKey, TElement>

在具有协方差的泛型之前发展起来的。

Herzmeister

问

something similar

为了一个

Dictionary<TKey, TValue>

是的。

Mehrdad Afshari

answers

对于可变字典,协方差是不安全的。

确实,如果

Ilookup<TKey, TElement>

协变的

TElement

,我不会遇到

iLookup<tKey,远程通讯>

提出问题;但事实并非如此,所以我仍在继续寻找更好的方法。}

_{注意:我当然可以编写一个扩展方法来完成它,但这并不妨碍迭代和重新打包所需的计算工作。}

2 回复 | 直到 6 年前

xanatos 6 年前

可以创建代理:

public static ILookup<TKey, TValueBase> ToLookupBase<TKey, TValue, TValueBase>(this ILookup<TKey, TValue> lookup)
    where TValue : class, TValueBase
{
    return new LookupProxy<TKey, TValue, TValueBase>(lookup);
}

public class LookupProxy<TKey, TValue, TValueBase> : ILookup<TKey, TValueBase>
    where TValue : class, TValueBase
{
    private readonly ILookup<TKey, TValue> lookup;

    public LookupProxy(ILookup<TKey, TValue> lookup)
    {
        this.lookup = lookup;
    }

    public IEnumerable<TValueBase> this[TKey key] => lookup[key];

    public int Count => lookup.Count;

    public bool Contains(TKey key) => lookup.Contains(key);

    public IEnumerator<IGrouping<TKey, TValueBase>> GetEnumerator() => lookup.GetEnumerator();

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

请注意,您必须:

var base = existing.ToLookupBase<int, Derived, Base>();

所以显式地告诉所有的泛型参数。如果你想支持协方差 TKey 它稍微复杂一些,需要一个单独的支持类和一个单独的方法:

public static ILookup<TKeyBase, TValueBase> ToLookupBase2<TKey, TValue, TKeyBase, TValueBase>(ILookup<TKey, TValue> lookup)
    where TKey : class, TKeyBase
    where TValue : class, TValueBase
{
    return new LookupProxy2<TKey, TValue, TKeyBase, TValueBase>(lookup);
}

public class LookupProxy2<TKey, TValue, TKeyBase, TValueBase> : ILookup<TKeyBase, TValueBase>
    where TKey : class, TKeyBase
    where TValue : class, TValueBase
{
    private readonly ILookup<TKey, TValue> lookup;

    public LookupProxy2(ILookup<TKey, TValue> lookup)
    {
        this.lookup = lookup;
    }

    public IEnumerable<TValueBase> this[TKeyBase key] => key is TKey ? lookup[(TKey)key] : Enumerable.Empty<TValueBase>();

    public int Count => lookup.Count;

    public bool Contains(TKeyBase key) => key is TKey ? lookup.Contains((TKey)key) : false;

    public IEnumerator<IGrouping<TKeyBase, TValueBase>> GetEnumerator() => lookup.GetEnumerator();

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

这是因为你需要添加 where TKey : class, TKeyBase (这将不支持键的值类型,如您的示例中所示)。

Paul Suart Wes 6 年前

创建自己的类来实现 ILookup<TKey, TBase> ,就像这样:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            var lookup1 = new List<Derived> { new Derived() { Key = 1, Prop1 = "A", Prop2 = "B"} } .ToLookup(x => x.Key, x => x);

            var baseLookup = new BaseLookup<int, Base, Derived>(lookup1);

            Console.WriteLine(baseLookup[1]);

        }        
    }

    public class BaseLookup<TKey, TBase, TDerived> : ILookup<TKey, TBase> where TDerived : TBase
    {
        private readonly ILookup<TKey, TDerived> _inner;

        public BaseLookup(ILookup<TKey, TDerived> inner)
        {
            _inner = inner;
        }

        IEnumerator<IGrouping<TKey, TBase>> IEnumerable<IGrouping<TKey, TBase>>.GetEnumerator()
        {
            return (IEnumerator<IGrouping<TKey, TBase>>) _inner.GetEnumerator();
        }

        public IEnumerator GetEnumerator()
        {
            return ((IEnumerable) _inner).GetEnumerator();
        }

        public bool Contains(TKey key)
        {
            return _inner.Contains(key);
        }

        public int Count => _inner.Count;

        public IEnumerable<TBase> this[TKey key] => _inner[key].Cast<TBase>();
    }

    public class Base
    {
        public int Key { get; set; }

        public string Prop1 { get; set; }
    }

    public class Derived : Base
    {
        public string Prop2 { get; set; }
    }
}