Question

I have someone on GitHub asking for the ability to compare HashSets for my project on GitHub: https://github.com/GregFinzer/Compare-Net-Objects/. I need to be able to determine if a type is a hash set and then get the enumerator. I am not sure what to cast it to. Here is what I have for an IList:

private bool IsIList(Type type)
{
    return (typeof(IList).IsAssignableFrom(type));
}


private void CompareIList(object object1, object object2, string breadCrumb)
{
    IList ilist1 = object1 as IList;
    IList ilist2 = object2 as IList;

    if (ilist1 == null) //This should never happen, null check happens one level up
        throw new ArgumentNullException("object1");

    if (ilist2 == null) //This should never happen, null check happens one level up
        throw new ArgumentNullException("object2");

    try
    {
        _parents.Add(object1);
        _parents.Add(object2);

        //Objects must be the same length
        if (ilist1.Count != ilist2.Count)
        {
            Differences.Add(string.Format("object1{0}.Count != object2{0}.Count ({1},{2})", breadCrumb,
                                              ilist1.Count, ilist2.Count));

            if (Differences.Count >= MaxDifferences)
                return;
        }

        IEnumerator enumerator1 = ilist1.GetEnumerator();
        IEnumerator enumerator2 = ilist2.GetEnumerator();
        int count = 0;

        while (enumerator1.MoveNext() && enumerator2.MoveNext())
        {
            string currentBreadCrumb = AddBreadCrumb(breadCrumb, string.Empty, string.Empty, count);

            Compare(enumerator1.Current, enumerator2.Current, currentBreadCrumb);

            if (Differences.Count >= MaxDifferences)
                return;

            count++;
        }
    }
    finally
    {
        _parents.Remove(object1);
        _parents.Remove(object2);
    }
}

Answer 1

I believe it is enough works directly with the ISet<T>, the ICollection<T> or the IEnumerable<T> generic interfaces instead of the HashSet<T>. You can detect these types using the following approach:

// ...
    Type t = typeof(HashSet<int>);
    bool test1 = GenericClassifier.IsICollection(t); // true
    bool test2 = GenericClassifier.IsIEnumerable(t); // true
    bool test3 = GenericClassifier.IsISet(t); // true
}
//
public static class GenericClassifier {
    public static bool IsICollection(Type type) {
        return Array.Exists(type.GetInterfaces(), IsGenericCollectionType);
    }
    public static bool IsIEnumerable(Type type) {
        return Array.Exists(type.GetInterfaces(), IsGenericEnumerableType);
    }
    public static bool IsISet(Type type) {
        return Array.Exists(type.GetInterfaces(), IsGenericSetType);
    }
    static bool IsGenericCollectionType(Type type) {
        return type.IsGenericType && (typeof(ICollection<>) == type.GetGenericTypeDefinition());
    }
    static bool IsGenericEnumerableType(Type type) {
        return type.IsGenericType && (typeof(IEnumerable<>) == type.GetGenericTypeDefinition());
    }
    static bool IsGenericSetType(Type type) {
        return type.IsGenericType && (typeof(ISet<>) == type.GetGenericTypeDefinition());
    }
}