我应该为 .NET 字典使用什么比较器来使用 EqualityComparer<t> 钥匙的方法？</t>

Question

My Position class is derived from the EqualityComparer<T> class. I would like the dictionary to use the overridden methods but it is using the methods from Object.Equals and Object.GetHashCode . What comparer should I use for the Dictionary to use the EqualityComparer<T> methods of the Position class?

using System.Collections.Generic;
using System.Diagnostics;

namespace ConsoleApp1
{
    class Position : EqualityComparer<Position>
    {
        public int X { get; set; }
        public int Y { get; set; }

        public override bool Equals(Position left, Position right)
        {
            if (left == null || right == null)
                return false;

            return left.X == right.X && left.Y == right.Y;
        }

        public override int GetHashCode(Position cell)
        {
            if (cell == null)
                return 0;

            return cell.X * 31 + cell.Y;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var dictionary = new Dictionary<Position, int>();
            var position = new Position();
            position.X = 1;
            position.Y = 1;
            dictionary[position] = 1;
            position = new Position();
            position.X = 1;
            position.Y = 1;
            var found = dictionary.TryGetValue(position, out var result);
            Debug.Assert(found);
            Debug.Assert(result == 1);
        }
    }
}

Answer 1

While it is very awkward unusual to implement comparer on object itself you simply need to pass an instance of comparer to the Dictionary constructor as shown in EqualityComparer example.

var dictionary = new Dictionary<Position, int>(new Position());

More common approach is to

• use separate class for comparer (if you need multiple variations of comparison) - implement IEquatable<T> with typed Equals and GetHashCode on the type itself (if there is a natural equality like in your case). This is preferable over object.Equals approach in case of value types as Equals(object) forces boxing of value types.
• just implement object.Equals and object.GetHashCode as shown in John's answer - this is most straightforward approach for if your type has natural equality and it is a reference type ( class ).

Answer 2

Alternative answer: You could just override Position 's GetHashCode and Equals method:

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

    public override bool Equals(object other)
    {
        if (other == null || !(other is Position))
        {
            return false;
        }
        var otherPosition = (Position)other;
        return otherPosition.X == this.X && otherPosition.Y == this.Y;
    }

    public override int GetHashCode()
    {
        return this.X * 31 + this.Y;
    }
}

That way you can simply declare your dictionary like so:

var dict = new Dictionary<Position, string>();

And it will work as you expect.

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