为什么在 Contains 期间不调用 GetHashCode？

发布于 2025-01-11 19:07:51 字数 807 浏览 3 评论 0原文

直到今天，我的理解是 HashSet 在 Contains 中使用 GetHashCode。例如这里也这么说。

我写了一些IEqualityComparer：

public class MyComparer : IEqualityComparer<string>
{
    public bool Equals(string? a, string? b)
    {
        return a == b;
    }

    public int GetHashCode(string a)
    {
        throw new NotImplementedException();
    }
}

并像这样使用它：

public void TestMyComparer()
{
    var x = new HashSet<string>(new []{ "hello", "world" });
    bool helloInside = x.Contains("hello", new MyComparer());
}

但是TestMyComparer不会像我预期的那样抛出NotImplementedException。相反，它返回true。

为什么？

原文

Until today my understanding was that a HashSet uses GetHashCode inside Contains. That is also said e.g. here.

I wrote a little IEqualityComparer:

public class MyComparer : IEqualityComparer<string>
{
    public bool Equals(string? a, string? b)
    {
        return a == b;
    }

    public int GetHashCode(string a)
    {
        throw new NotImplementedException();
    }
}

And used it like so:

public void TestMyComparer()
{
    var x = new HashSet<string>(new []{ "hello", "world" });
    bool helloInside = x.Contains("hello", new MyComparer());
}

But TestMyComparer does not throw a NotImplementedException, as I expected. Instead, it returns true.

Why?

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亣腦蒛氧 2025-01-18 19:07:51

如果您想在 HashSet.Contains 中使用自定义比较器，请将其传递给构造函数。

var x = new HashSet<string>(new MyComparer());
x.Add("hello");
x.Add("world");
bool helloInside = x.Contains("hello");

现在使用 GetHashCode 因为您使用基于集合的集合而不是 Enumerable.Contains 它只是枚举所有项目并将它们与 Equals 进行比较。

If you want to use your custom comparer in HashSet.Contains, pass it to the the constructor.

var x = new HashSet<string>(new MyComparer());
x.Add("hello");
x.Add("world");
bool helloInside = x.Contains("hello");

Now GetHashCode is used since you use a set based collection and not Enumerable.Contains which just enumerates all items and compares them with Equals.

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