"(({{}}))" is a valid s
"{[]}" is a valid s
"[{[}]]" is not valid

public class Question_ValidStringWithBrackets
{
    public bool IsValidBracketString(String s, BracketValidator validator = null)
    {
        if (s == null)
            throw new ArgumentNullException("A string is required");

        if (validator == null)
            validator = BracketValidator.CreateDefaultValidator();

        Stack<char> openStack = new Stack<char>();

        foreach (var curBracket in s)
        {
            if (validator.IsOpen(curBracket))
            {
                openStack.Push(curBracket);
            }
            else if (openStack.Count == 0 || validator.IsMatchingPair(openStack.Pop(), curBracket))
            {
                return false;
            }
        }
        return openStack.Count == 0;
    }

    public class BracketValidator
    {
        private HashSet<char> openBrackets = new HashSet<char>();
        private Dictionary<char, char> closedOpenedPair = new Dictionary<char, char>();

        public void AddPair(char open, char close)
        {
            if (char.IsWhiteSpace(open) || char.IsWhiteSpace(close))
                throw new ArgumentException("A bracket must be specified. An empty character is not allowed.");

            if (openBrackets.Contains(open) || openBrackets.Contains(close))
                throw new ArgumentException("Brackets exist already.");

            if (closedOpenedPair.ContainsKey(open) || closedOpenedPair.ContainsKey(close))
                throw new ArgumentException("Brackets exist already.");

            openBrackets.Add(open);
            closedOpenedPair.Add(close, open);
        }

        public Boolean IsOpen(char open)
        {
            return openBrackets.Contains(open);
        }

        public bool IsMatchingPair(char open, char close)
        {
            return closedOpenedPair.TryGetValue(open, out char actualClose) && actualClose == close;
        }

        public static BracketValidator CreateDefaultValidator()
        {
            var validator = new BracketValidator();
            validator.AddPair('{', '}');
            validator.AddPair('[', ']');
            validator.AddPair('(', ')');

            return validator;
        }
    }
}