实施 EqualityCompare 与覆盖 GetHashCode 和 Equals

Question

我创建了两个几乎相同的 class。两者都代表一个 Pair (x,y)，但在其中一个中，我覆盖了 GetHashCode 和 Equals 方法。有人告诉我，当 HashCode 不同时，Collections 将它们视为不同的元素，甚至懒得实际将它们与相等的元素进行比较。然而，事实证明，我为 class 实现了一个 EqualityComparer，它没有覆盖 GetHashCode 和 Equals，即使 HashCode 仍然不同，一切也能正常工作。

看看我的控制台项目：

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

namespace matrixExample
{
    class Program
    {

        static void Main(string[] args)
        {
            Console.WriteLine("Same Hash but no insertion: as expected");
            HashSet<MyPair> hash = new HashSet<MyPair>();
            MyPair one = new MyPair { X = 10, Y = 2 };
            MyPair copyOfOne = new MyPair { X = 10, Y = 2 };
            Console.WriteLine(one.GetHashCode() + " " +  hash.Add(one));
            Console.WriteLine(copyOfOne.GetHashCode() + " " + hash.Add(copyOfOne));


            Console.WriteLine("-----------------------------------------");

            Console.WriteLine("Different Hash but no insertion! why?");
            HashSet<MyPairWithoutOverride> hash2 = new HashSet<MyPairWithoutOverride>(new SameHash());
            MyPairWithoutOverride a1 = new MyPairWithoutOverride { X = 10, Y = 2 };
            MyPairWithoutOverride a1copy = new MyPairWithoutOverride { X = 10, Y = 2 };
            Console.WriteLine(a1.GetHashCode() + " " + hash2.Add(a1));
            Console.WriteLine(a1copy.GetHashCode() + " " + hash2.Add(a1copy));

        }

        public class MyPair
        {
            public int X { get; set; }
            public int Y { get; set; }

            public override int GetHashCode()
            {
                return X * 10000 + Y;
            }

            public override bool Equals(object obj)
            {
                MyPair other = obj as MyPair;
                return X == other.X && Y == other.Y;
            }
        }

        public class MyPairWithoutOverride
        {
            public int X { get; set; }
            public int Y { get; set; }
        }

        public class SameHash : EqualityComparer<MyPairWithoutOverride>
        {
            public override bool Equals(MyPairWithoutOverride p1, MyPairWithoutOverride p2)
            {
                return p1.X == p2.X && p1.Y == p2.Y;
            }
            public override int GetHashCode(MyPairWithoutOverride i)
            {
                return base.GetHashCode();
            }
        }

    }
}

Answer 1

你的问题就在这里

public override int GetHashCode(MyPairWithoutOverride i)
{
    return base.GetHashCode();
}

您 returning base.GetHashCode() 实际上是 SameHash class 的哈希码。所以你实际上每次都return输入相同的哈希码。

如果您 return i.GetHashCode() 那么它将按预期运行。

实施 EqualityCompare 与覆盖 GetHashCode 和 Equals

Implementing EqualityCompare vs overriding GetHashCode and Equals

c#

collections

hashcode

iequalitycomparer