我如何使用HashSet <T> 作为字典键？

Question

I wish to use HashSet<T> as the key to a Dictionary:

Dictionary<HashSet<T>, TValue> myDictionary = new Dictionary<HashSet<T>, TValue>();

I want to look up values from the dictionary such that two different instances of HashSet<T> that contain the same items will return the same value.

HashSet<T> 's implementations of Equals() and GetHashCode() don't seem to do this (I think they're just the defaults). I can override Equals() to use SetEquals() but what about GetHashCode() ? I feel like I am missing something here...

Answer 1

您可以使用HashSet<T>提供的set comparer：

var myDictionary = new Dictionary<HashSet<T>, TValue>(HashSet<T>.CreateSetComparer());

Answer 2

digEmAll's answer is clearly the better choice in practice, since it uses built in code instead of reinventing the wheel. But I'll leave this as a sample implementation.

---

You can use implement an IEqualityComparer<HashSet<T>> that uses SetEquals . Then pass it to the constructor of the Dictionary. Something like the following(Didn't test it):

class HashSetEqualityComparer<T>: IEqualityComparer<HashSet<T>>
{
    public int GetHashCode(HashSet<T> hashSet)
    {
        if(hashSet == null)
           return 0;
        int h = 0x14345843; //some arbitrary number
        foreach(T elem in hashSet)
        {
            h = unchecked(h + hashSet.Comparer.GetHashCode(elem));
        }
        return h;
    }

    public bool Equals(HashSet<T> set1, HashSet<T> set2)
    {
        if(set1 == set2)
            return true;
        if(set1 == null || set2 == null)
            return false;
        return set1.SetEquals(set2);
    }
}

Note that the hash function here is commutative, that's important because the enumeration order of the elements in the set is undefined.

One other interesting point is that you can't just use elem.GetHashCode since that will give wrong results when a custom equality comparer was supplied to the set.

Answer 3

您可以向Dictionary构造函数提供IEqualityComparer<HashSet<T>> ，并在该比较器中进行所需的实现。