linq＆distinct，实现equals和gethashcode

Question

所以我正在努力使这项工作，我似乎无法知道为什么它不起作用

演示代码;

namespace ConsoleApplication1
{
class Program
{
    static void Main(string[] args)
    {
        var myVar = new List<parent >();
        myVar.Add(new parent() { id = "id1", blah1 = "blah1", c1 = new child() { blah2 = "blah2", blah3 = "blah3" } });
        myVar.Add(new parent() { id = "id1", blah1 = "blah1", c1 = new child() { blah2 = "blah2", blah3 = "blah3" } });

        var test = myVar.Distinct();

        Console.ReadKey();

    }
}


public class parent : IEquatable<parent>
{
    public String id { get;set;}
    public String blah1 { get; set; }
    public child c1 { get; set; }

    public override int GetHashCode()
    {
        unchecked // Overflow is fine, just wrap
        {
            int hash = 17;
            // Suitable nullity checks etc, of course :)
            hash = hash * 23 + id.GetHashCode();
            hash = hash * 23 + blah1.GetHashCode();
            hash = hash * 23 + (c1 == null ? 0 : c1.GetHashCode());
            return hash;
        }
    }

    public bool Equals(parent other)
    {
        return object.Equals(id, other.id) &&
            object.Equals(blah1, other.blah1) &&
            object.Equals(c1, other.c1);
    }

}

public class child : IEquatable<child>
{
    public String blah2 { get; set; }
    public String blah3 { get; set; }

    public override int GetHashCode()
    {
        unchecked // Overflow is fine, just wrap
        {
            int hash = 17;
            // Suitable nullity checks etc, of course :)
            hash = hash * 23 + blah2.GetHashCode();
            hash = hash * 23 + blah3.GetHashCode();
            return hash;
        }
    }

    public bool Equals(child other)
    {
        return object.Equals(blah2, other.blah2) &&
            object.Equals(blah3, other.blah3);
    }

}
}

任何人都可以发现我的错误？

Answer 1

您需要覆盖Equals(object)方法：

public override bool Equals(object obj) {
    return Equals(obj as parent);
}

object.Equals方法（与EqualityComparer<T>.Default不同）不使用IEquatable接口。因此，当你写object.Equals(c1, other.c1)时，它不会调用你的Child.Equals(Child)方法。

你也不一定非常需要为parent这样做，但你真的应该这样做。

Answer 2

要么你做SLaks建议的，要么你在EqualityComparer<child>.Default类中使用parent来使用你的IEquatable<child>实现：

  public bool Equals(parent other)
  {
   return object.Equals(id, other.id) &&
    object.Equals(blah1, other.blah1) &&
    EqualityComparer<child>.Default.Equals(c1, other.c1);
 }

Answer 3

在添加计算哈希时，您可能想要尝试类似的东西

hash ^= id.GetHashCode();

不确定这是否是导致您的问题的原因。

Answer 4

这里有几件事要做。如果我要在诸如GetHashCode，overriding ==或IEquatable这样的类中实现相等的任何方面，我总是使用以下模式。

1. 覆盖等于
2. 覆盖GetHashCode
3. 实施IEquatable<T>意味着实施Equals(T)
4. 实施！=
5. 实施==

所以，如果我有一个名为ExpiryMonth的类具有属性Year和Month，那么这就是该实现的外观。现在适应其他类型的课程是一项相当盲目的任务。

我已将此模式基于其他几个stackoverflow答案，这些答案都值得赞扬，但我一直没有跟踪。

通过始终将所有这些元素一起实现，它确保在各种上下文中进行适当的相等操作，包括字典和Linq操作。

    public static bool operator !=(ExpiryMonth em1, ExpiryMonth em2)
    {
        if (((object)em1) == null || ((object)em2) == null)
        {
            return !Object.Equals(em1, em2);
        }
        else
        {
            return !(em1.Equals(em2));
        }
    }
    public static bool operator ==(ExpiryMonth em1, ExpiryMonth em2)
    {
        if (((object)em1) == null || ((object)em2) == null)
        {
            return Object.Equals(em1, em2);
        }
        else
        {
            return em1.Equals(em2);
        }
    }
    public bool Equals(ExpiryMonth other)
    {
        if (other == null) { return false; }
        return Year == other.Year && Month == other.Month;
    }
    public override bool Equals(object obj)
    {
        if (obj == null) { return false; }
        ExpiryMonth em = obj as ExpiryMonth;
        if (em == null) { return false; }
        else { return Equals(em); }
    }
    public override int GetHashCode()
    {
        unchecked // Overflow is not a problem
        {
            var result = 17;
            result = (result * 397) + Year.GetHashCode();
            result = (result * 397) + Month.GetHashCode();
            return result;
        }
    }