public class WordDictionary {

    private readonly Dictionary<int, HashSet<string>> _data = new Dictionary<int, HashSet<string>>();
    private readonly Dictionary<string, bool> _searchResults = new Dictionary<string, bool>();

    /** Initialize your data structure here. */
    public WordDictionary() {

    }

    /** Adds a word into the data structure. */
    public void AddWord(string word) {
        if(string.IsNullOrEmpty(word))
            return;

        if(!_data.ContainsKey(word.Length))
            _data[word.Length] = new HashSet<string>();

        if(_data[word.Length].Add(word))
        {
            foreach(var kvp in _searchResults.ToArray())
            {
                if(!kvp.Value && kvp.Key.Length == word.Length)
                    _searchResults.Remove(kvp.Key);
            }
        }
    }

    /** Returns if the word is in the data structure. A word could contain the dot character '.' to represent any one letter. */
    public bool Search(string searchWord) {

        if(string.IsNullOrEmpty(searchWord))
            return false;

        if(searchWord.Contains("."))
        {
            if(_searchResults.ContainsKey(searchWord))
                return _searchResults[searchWord];

            if(!_data.ContainsKey(searchWord.Length))
                return false;

            var regex = new System.Text.RegularExpressions.Regex($"^{searchWord}$");
            foreach(var word in _data[searchWord.Length])
            {
                if(regex.IsMatch(word))
                    return _searchResults[searchWord] = true;
            }

            return _searchResults[searchWord] = false;
        }

        return _data.ContainsKey(searchWord.Length)
            && _data[searchWord.Length].Contains(searchWord);
    }
}

/**
 * Your WordDictionary object will be instantiated and called as such:
 * WordDictionary obj = new WordDictionary();
 * obj.AddWord(word);
 * bool param_2 = obj.Search(word);
 */