Question

For example, i don’t know if it is a good practice to use Linq whenever possible.

     Class Aclass : Dictionary<string,int>
     {
        public Aclass(Aclass myAclass, HashSet<string> blacklist)
        {
            foreach (var item in myAclass)
            {
                if ((item.Value > 0) && (!blacklist.Contains(item.Key)))
                {
                    Add(item.Key, item.Value);
                }
            }
         }
      }

Answer 1

It looks to me like you’d be fine with:

Dictionary<string, int> other = ...;
HashSet<string> blacklist = ...;

var dictionary = other.Where(item => item.Value > 0 && 
                                     !blackList.Contains(item.Key)
                      .ToDictionary(item => item.Key, item => item.Value);

No need for a separate type at all as far as I can see – deriving from Dictionary<,> or List<> is almost always a bad idea IMO.