private void GetDicKeyByValue()

         {

             Dictionary<string, string> dic = new Dictionary<string, string>();

             dic.Add("", "");

             dic.Add("", "");

             dic.Add("", "");

             //foreach KeyValuePair traversing

             foreach (KeyValuePair<string, string> kvp in dic)

             {

                 if (kvp.Value.Equals(""))

                 {

                     //...... kvp.Key;

                 }

             }

             //foreach dic.Keys

             foreach (string key in dic.Keys)

             {

                 if (dic[key].Equals(""))

                 {

                     //...... key

                 }

             }

             //Linq

             var keys = dic.Where(q => q.Value == "").Select(q => q.Key);  //get all keys

             List<string> keyList = (from q in dic

                                     where q.Value == ""

                                     select q.Key).ToList<string>(); //get all keys

             var firstKey = dic.FirstOrDefault(q => q.Value == "").Key;  //get first key

         }