有效地获取IReadOnlyDictionary< int，Animals>摘自Dictionary< int，Fleas> [英] Efficiently Obtain IReadOnlyDictionary<int, Animals> from Dictionary<int, Fleas>

问题描述

public class Flea : Animals {...}

var fleas = new Dictionary<int, Flea>();

public IReadOnlyDictionary<string, Animal> Animals => fleas.ToDictionary(pair => pair.Key, pair => (Animal)pair.Value);

Q 是否有更有效的方法获取动物来自跳蚤？

Q Is there a more efficient way to obtain Animals from fleas?

推荐答案

.NET在接口，委托，泛型类型和数组中支持协方差。接口或类型必须使用 out 关键字来指定其协变。

.NET supports covariance in interfaces, delegates, generic types and arrays. The interface or type has to specify it's covariant though with the out keyword.

您可以编写

IEnumerable<Animal> animals=new List<Flea>();

或

var dict=new Dictionary<int,Flea>{
    [1]=new Flea()
};
IEnumerable<Animal> animals=dict.Values;

之所以有效，是因为 Dictionary.Values 返回一个 IEnumerable< Flea> 和 IEnumerable 是协变的-其定义为 IEnumerable< out T> 。

This works because Dictionary.Values returns an IEnumerable<Flea> and IEnumerable is covariant - its definition is IEnumerable<out T>.

KeyValuePair 虽然不是协变的，这意味着使用它的类如 IDictionary< TKey，TValue> 和 IReadOnlyDictionary< TKey，TValue> 也不是。

KeyValuePair though isn't covariant which means that the classes that use it like IDictionary<TKey,TValue> and IReadOnlyDictionary<TKey,TValue> aren't either. This was intentional.

由于您只需要从该词典中读取，因此可以使用委托创建访问器方法，或者在C＃7中然后是局部函数您可以将该函数传递给期望 Func< TKey，TValue> 的方法，并使用它从字典中读取值。

Since you only need to read from that dictionary, you can create an accessor method using a delegate or, in C# 7 and later, a local function. You can pass that function to methods that expect a Func<TKey,TValue> and use it to read values from the dictionary.

如果您有一种方法需要基于密钥的访问，请说：

If you have a method that needs key-based access, let's say :

void Process(Func<int,Animal> reader)
{
    var value=reader(1);
}

在C＃7中，您可以编写：

In C# 7 you can write :

var dict =...

Animal get(int key)=>dict[key];

Process(get);

通过使用变量捕获来访问字典，这有点作弊。

This cheats a bit, by using variable capture to access the dictionary.

在C＃7之前，您将使用委托：

Before C# 7 you'd use a delegate :

Func<int,Animal> get= key=>dict[key];
Process(get);

这似乎很奇怪，但是 LINQ 本身就是这样工作的谓词和委托，而不是接口和包装。

This may seem strange, but that's how LINQ itself works, by using predicates and delegates instead of interfaces and wrappers.

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