HashSet.CreateSetComparer() cannot specify IEqualityComparer, is there an alternative?

Question

In the internal source there is such a constructor public HashSetEqualityComparer(IEqualityComparer comparer) but it\'s internal so I can\'t use it.<

Answer 1

I agree @InBetween, using SetEquals is the best way. Even if add the constructor still can not achieve what you want.

please see this code: http://referencesource.microsoft.com/#System.Core/System/Collections/Generic/HashSet.cs,1360

Here is I try to do:

class HashSetEqualityComparerWrapper : IEqualityComparer>
{
    static private Type HashSetEqualityComparerType = HashSet.CreateSetComparer().GetType();
    private IEqualityComparer> _comparer;

    public HashSetEqualityComparerWrapper()
    {
        _comparer = HashSet.CreateSetComparer();
    }
    public HashSetEqualityComparerWrapper(IEqualityComparer comparer)
    {
        _comparer = HashSet.CreateSetComparer();
        if (comparer != null)
        {
            FieldInfo m_comparer_field = HashSetEqualityComparerType.GetField("m_comparer", BindingFlags.NonPublic | BindingFlags.Instance);
            m_comparer_field.SetValue(_comparer, comparer);
        }
    }

    public bool Equals(HashSet x, HashSet y)
    {
        return _comparer.Equals(x, y);
    }
    public int GetHashCode(HashSet obj)
    {
        return _comparer.GetHashCode(obj);
    }
}

UPDATE

I took 5 mins to implement another version form HashSetEqualityComparer source code. And rewrite the bool Equals(HashSet x, HashSet y) method. It is not complex. All code just copy and paste from source, I just revise a bit.

class CustomHashSetEqualityComparer : IEqualityComparer>
{
    private IEqualityComparer m_comparer;

    public CustomHashSetEqualityComparer()
    {
        m_comparer = EqualityComparer.Default;
    }

    public CustomHashSetEqualityComparer(IEqualityComparer comparer)
    {
        if (comparer == null)
        {
            m_comparer = EqualityComparer.Default;
        }
        else
        {
            m_comparer = comparer;
        }
    }

    // using m_comparer to keep equals properties in tact; don't want to choose one of the comparers
    public bool Equals(HashSet x, HashSet y)
    {
        // http://referencesource.microsoft.com/#System.Core/System/Collections/Generic/HashSet.cs,1360
        // handle null cases first
        if (x == null)
        {
            return (y == null);
        }
        else if (y == null)
        {
            // set1 != null
            return false;
        }

        // all comparers are the same; this is faster
        if (AreEqualityComparersEqual(x, y))
        {
            if (x.Count != y.Count)
            {
                return false;
            }
        }
        // n^2 search because items are hashed according to their respective ECs
        foreach (T set2Item in y)
        {
            bool found = false;
            foreach (T set1Item in x)
            {
                if (m_comparer.Equals(set2Item, set1Item))
                {
                    found = true;
                    break;
                }
            }
            if (!found)
            {
                return false;
            }
        }
        return true;
    }

    public int GetHashCode(HashSet obj)
    {
        int hashCode = 0;
        if (obj != null)
        {
            foreach (T t in obj)
            {
                hashCode = hashCode ^ (m_comparer.GetHashCode(t) & 0x7FFFFFFF);
            }
        } // else returns hashcode of 0 for null hashsets
        return hashCode;
    }

    // Equals method for the comparer itself. 
    public override bool Equals(Object obj)
    {
        CustomHashSetEqualityComparer comparer = obj as CustomHashSetEqualityComparer;
        if (comparer == null)
        {
            return false;
        }
        return (this.m_comparer == comparer.m_comparer);
    }

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

    static private bool AreEqualityComparersEqual(HashSet set1, HashSet set2)
    {
        return set1.Comparer.Equals(set2.Comparer);
    }
}