当 T 是接口并且派生类型之间的哈希码不同时覆盖 IEquatable<T>

Question

我有A 和B 类都实现了接口I。

public interface I
{
    int SomeInt { get; }
    bool SomeBool { get; }
    float SomeFloat { get; }
}


public class A : I
{
    public int SomeInt { get; }
    public bool SomeBool { get; }
    public float SomeFloat { get; }

    private readonly string _someARelatedStuff;
    // Rest of class...
}

public class B : I
{
    public int SomeInt { get; }
    public bool SomeBool { get; }
    public float SomeFloat { get; }

    private string readonly _someBRelatedStuff;
    private double readonly _someOtherBRelatedStuff;
    // Rest of class...
}

有时我想根据I 属性的相等性（SomeInt、@987654333 @, SomeFloat)，所以我在两者上都实现了IEquatable<I>，并根据它们共享的I 属性值比较它们。

问题是我已经在 A 和 B 上实现了 GetHashCode() 的实现，它会产生不同的哈希值，因为我考虑了其他成员。

B 不依赖于A，所以我使用接口I 来比较它们，它有一个带有getter 的属性列表。

我读到in a StackOverflow answer：

如果您正在实现一个类，则应始终确保两个相等的对象具有相同的哈希码。

这是否意味着每当一个类A 想要实现接口I，并且我希望能够比较实现I 的实例时，我必须确保哈希码的计算方式相同I 的所有实例的方式，并且只使用 I 属性？

当 T 是一个接口时，我确实觉得我不打算实现 IEquatable<T>，但我的替代方案是：

1. 对基类使用常规继承 - 我宁愿尽可能避免继承，如果 B 由于单继承而需要从某个框架 C 类派生，则此解决方案将不起作用
2. 使用A 或B 上的方法在A 和B 之间实现相等检查 - 将创建代码重复
3. 在I 中定义的I 实例之间有一个相等检查方法 - 听起来是最好的选择

有没有我遗漏的选项？

Answer 1

考虑创建一个IEqualityComparer<> 类来比较常用值。

_{为了便于阅读，我已将界面重命名为ICommon}

public interface ICommon
{
    int SomeInt { get; }
    bool SomeBool { get; }
    float SomeFloat { get; }
}

public class CommonComparer : IEqualityComparer<ICommon>
{
    public bool Equals(ICommon x, ICommon y)
    {
        return x.SomeInt.Equals(y.SomeInt)
            && x.SomeBool.Equals(y.SomeBool)
            && x.SomeFloat.Equals(y.SomeFloat);
    }

    public int GetHashCode(ICommon obj)
    {
        unchecked
        {
            int hc = -1817952719;
            hc = (-1521134295)*hc + obj.SomeInt.GetHashCode();
            hc = (-1521134295)*hc + obj.SomeBool.GetHashCode();
            hc = (-1521134295)*hc + obj.SomeFloat.GetHashCode();
            return hc;
        }
    }
}

并且程序可以区分两个列表中的相等项。

class Program
{
    static void Main(string[] args)
    {
        var listA = new List<A>
        {
            new A(1001, true, 1.001f, "A1"),
            new A(1002, true, 1.002f, "A2"),
            new A(1003, false, 1.003f, "A1"),
            new A(1004, false, 1.004f, "A4")
        };

        var listB = new List<B>
        {
            new B(1001, true, 1.001f, "B1", 2.5),
            new B(1002, false, 1.002f, "B2", 2.8),
            new B(1003, true, 1.003f, "B3", 2.9),
            new B(1004, false, 1.004f, "B4", 2.9)
        };

        var common = Enumerable.Intersect(listA, listB, new CommonComparer()).OfType<ICommon>();


        Console.WriteLine($"{"SomeInt",-8} {"Bool",-6} {"SomeFloat",-10}");
        foreach (var item in common)
        {
            Console.WriteLine($"{item.SomeInt,-8} {item.SomeBool,-6} {item.SomeFloat,-10}");
        }
        //SomeInt  Bool   SomeFloat
        //1001     True   1.001
        //1004     False  1.004

    }
}

以及其余的代码定义

public class A : ICommon, IEquatable<A>
{
    static readonly CommonComparer comparer = new CommonComparer();

    public int SomeInt { get; }
    public bool SomeBool { get; }
    public float SomeFloat { get; }

    private readonly string _someARelatedStuff;
    // Rest of class...
    public A(ICommon other, string someARelatedStuff)
        : this(other.SomeInt, other.SomeBool, other.SomeFloat, someARelatedStuff)
    { }
    public A(int someInt, bool someBool, float someFloat, string someARelatedStuff)
    {
        this.SomeInt = someInt;
        this.SomeBool = someBool;
        this.SomeFloat = someFloat;
        this._someARelatedStuff = someARelatedStuff;
    }

    public override string ToString() => _someARelatedStuff;

    #region IEquatable Members
    public override bool Equals(object obj)
    {
        if (obj is A other)
        {
            return Equals(other);
        }
        return false;
    }


    public virtual bool Equals(A other)
    {
        return comparer.Equals(this, other)
            && _someARelatedStuff.Equals(other._someARelatedStuff);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            int hc = comparer.GetHashCode(this);
            hc = (-1521134295)*hc + _someARelatedStuff.GetHashCode();
            return hc;
        }
    }

    #endregion

}

public class B : ICommon, IEquatable<B>
{
    static readonly CommonComparer comparer = new CommonComparer();

    public int SomeInt { get; }
    public bool SomeBool { get; }
    public float SomeFloat { get; }

    readonly string _someBRelatedStuff;
    readonly double _someOtherBRelatedStuff;
    // Rest of class...

    public B(ICommon other, string someBRelatedStuff, double someOtherBRelatedStuff)
        : this(other.SomeInt, other.SomeBool, other.SomeFloat, someBRelatedStuff, someOtherBRelatedStuff)
    { }
    public B(int someInt, bool someBool, float someFloat, string someBRelatedStuff, double someOtherBRelatedStuff)
    {
        this.SomeInt = someInt;
        this.SomeBool = someBool;
        this.SomeFloat = someFloat;
        this._someBRelatedStuff = someBRelatedStuff;
        this._someOtherBRelatedStuff = someOtherBRelatedStuff;
    }

    public override string ToString() => $"{_someBRelatedStuff}, {_someOtherBRelatedStuff.ToString("g4")}";

    #region IEquatable Members

    public override bool Equals(object obj)
    {
        if (obj is B other)
        {
            return Equals(other);
        }
        return false;
    }

    public virtual bool Equals(B other)
    {
        return comparer.Equals(this, other)
            && _someBRelatedStuff.Equals(other._someBRelatedStuff)
            && _someOtherBRelatedStuff.Equals(other._someOtherBRelatedStuff);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            int hc = comparer.GetHashCode(this);
            hc = (-1521134295)*hc + _someBRelatedStuff.GetHashCode();
            hc = (-1521134295)*hc + _someOtherBRelatedStuff.GetHashCode();
            return hc;
        }
    }

    #endregion
}