为什么很重要重写GetHash code等于方法被重写时？ [英] Why is it important to override GetHashCode when Equals method is overridden?

问题描述

考虑下面的类

public class Foo
{
    public int FooId { get; set; }
    public string FooName { get; set; }

    public override bool Equals(object obj)
    {
        Foo fooItem = obj as Foo;

        return fooItem.FooId == this.FooId;
    }

    public override int GetHashCode()
    {
        // Which is preferred?

        return base.GetHashCode();

        //return this.FooId.GetHashCode();
    }
}

我已经覆盖了等于方法，因为富重新present为<$ C $一行C>富桌上。这是重写preferred方法 GetHash code ？

I have overridden the Equals method because Foo represent a row for the Foos table. Which is the preferred method for overriding the GetHashCode?

为什么很重要重写 GetHash code ？

推荐答案

是的，这是很重要的，如果你的项目将被用作在字典中的关键，或的HashSet＆LT; T＆GT; 等等 - 因为这是使用（在没有自定义的的IEqualityComparer＆LT的; T＆GT; ）组项目入桶。如果散列code两个项目不匹配，他们可能的从不的被认为是等于（等于根本不会被调用）。

Yes, it is important if your item will be used as a key in a dictionary, or HashSet<T>, etc - since this is used (in the absence of a custom IEqualityComparer<T>) to group items into buckets. If the hash-code for two items does not match, they may never be considered equal (Equals will simply never be called).

的 GetHash code（）方法应当反映等于逻辑;规则是：

The GetHashCode() method should reflect the Equals logic; the rules are:

• 如果两件事情是相等的（等于（...）==真），那么他们的必须的返回相同的值 GetHash code（）


• 如果在 GetHash code（）是平等的，它的不的需要为他们是相同的;这是一个碰撞，和等于将被调用，看它是否是一个真正的平等与否。

• if two things are equal (Equals(...) == true) then they must return the same value for GetHashCode()
• if the GetHashCode() is equal, it is not necessary for them to be the same; this is a collision, and Equals will be called to see if it is a real equality or not.

在这种情况下，它看起来像返回FooId; 是一个合适的 GetHash code（）的实施。如果您正在测试多个属性，通常使用code像下面，以减少碰撞对角（即让新的Foo（3,5）有将它们组合不同的散列code到新的Foo（5,3））

In this case, it looks like "return FooId;" is a suitable GetHashCode() implementation. If you are testing multiple properties, it is common to combine them using code like below, to reduce diagonal collisions (i.e. so that new Foo(3,5) has a different hash-code to new Foo(5,3)):

int hash = 13;
hash = (hash * 7) + field1.GetHashCode();
hash = (hash * 7) + field2.GetHashCode();
...
return hash;

哦 - 为了方便起见，你也可以考虑提供覆盖时== 和 = 运营商<$ C！ $ C>等于和 GetHash code 。

当你犯了这个错误会发生什么情况的演示是<一个href=\"http://stackoverflow.com/questions/638761/c-gethash$c$c-override-of-object-containing-generic-array/639098#639098\">here.

A demonstration of what happens when you get this wrong is here.

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