c# - 为什么不 List.GetHashCode 和 ObservableCollection.GetHashCode 评估他们的项目？

Question

我认为这些集合的 GetHashCode 函数不将它们的哈希码基于列表中的项目是很奇怪的。

我需要它来工作以提供脏检查(您有未保存的数据)。我已经编写了一个覆盖 GetHashCode 方法的包装类，但我发现这不是默认实现很奇怪。

我猜这是性能优化？

class Program
{
    static void Main(string[] args)
    {
        var x = new ObservableCollection<test>();
        int hash = x.GetHashCode();
        x.Add(new test("name"));
        int hash2 = x.GetHashCode();

        var z = new List<test>();
        int hash3 = z.GetHashCode();
        z.Add(new test("tets"));
        int hash4 = z.GetHashCode();

        var my = new CustomObservableCollection<test>();
        int hash5 = my.GetHashCode();
        var test = new test("name");
        my.Add(test);
        int hash6 = my.GetHashCode();
        test.Name = "name2";
        int hash7 = my.GetHashCode();
    }
}

public class test
{
    public test(string name)
    {
        Name = name;
    }

    public string Name { get; set; }

    public override bool Equals(object obj)
    {
        if (obj is test)
        {
            var o = (test) obj;
            return o.Name == this.Name;
        }
        return base.Equals(obj);
    }

    public override int GetHashCode()
    {
        return Name.GetHashCode();
    }
}

public class CustomObservableCollection<T> : ObservableCollection<T>
{
    public override int GetHashCode()
    {
        int collectionHash = base.GetHashCode();

        foreach (var item in Items)
        {
            var itemHash = item.GetHashCode();
            if (int.MaxValue - itemHash > collectionHash)
            {
                collectionHash = collectionHash * -1;
            }
            collectionHash += itemHash;
        }
        return collectionHash;
    }
}

Answer 1

如果这样做，它会破坏一些 guidelines for implementing GetHashCode .即:

the integer returned by GetHashCode should never change

既然列表的内容可以改变，那么它的哈希码也会改变。

the implementation of GetHashCode must be extremely fast

根据列表的大小，您可能会减慢其哈希码的计算速度。

此外，我认为您不应该使用对象的哈希码来检查数据是否脏。 The probability of collision is higher than you think .