字典中Type的自定义相等比较器 [英] Custom equality comparer for Type in dictionary

问题描述

由于Int32 是 Object，我希望此打印"True"

Since Int32 is a Object, I want this to print "True"

    Dictionary<Type, string> dict = new Dictionary<Type, string>(new MyComparer());
    dict[typeof(object)] = "Hello";

    Console.WriteLine(dict.ContainsKey(typeof(int))); // currently prints false :(

这是我尝试过的比较器:

Here's the comparer I tried:

    public class MyComparer : IEqualityComparer<Type>
    {
        public bool Equals(Type x, Type y)
        {
            return y.IsAssignableFrom(x);
        }

        public int GetHashCode(Type obj)
        {
            return obj.GetHashCode();
        }
    }

但是它不起作用.我不太确定要在GetHashCode中返回什么-我知道这是错误的原因，在调试时我什至没有达到Equals-知道如何正确编写此代码吗?谢谢.

But it's not working. I'm not quite sure what to return in GetHashCode - I know it's wrong cause when debugging I'm not even reaching Equals - Any idea how to write this correctly? Thanks.

推荐答案

那根本不是字典的有效比较器，并且结果定义不明确.平等比较应该是可交换的，尤其是a eq b 当且仅当 b eq a时.这不适用于您的情况.同样，有效的哈希码实现指出:

That simply is not a valid comparer for a dictionary, and the result is not well-defined. Equality comparisons should be commutative, specifically a eq b if and only if b eq a. That does not apply in your case. Likewise, a valid hash-code implementation states that:

• 如果两个散列不相等，则两个值不相等
• 两个相等的值必须具有相同的哈希码

那也失败了.

基本上，这是行不通的.

Basically, that isn't going to work.

具体来说，来自MSDN的:

Specifically, from MSDN:

实施者注意事项

要求执行以确保如果Equals方法为两个对象x和y返回 true ，则GetHashCode方法为x返回的值必须等于为y返回的值.

Implementations are required to ensure that if the Equals method returns true for two objects x and y, then the value returned by the GetHashCode method for x must equal the value returned for y.

等于方法是自反的，对称的和可传递的.也就是说，如果用于将对象与其自身进行比较，则返回 true ；否则，它将返回 true .如果两个对象x和y为 true ，则为 true ；和 true (如果两个对象x和z为 true 和y和z为 true )，则为 true .

The Equals method is reflexive, symmetric, and transitive. That is, it returns true if used to compare an object with itself; true for two objects x and y if it is true for y and x; and true for two objects x and z if it is true for x and y and also true for y and z.

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