How to change return of function from string to generic list ?

create proc getcompannies
as
select compnyid from companes where compnyid > 10

public static string ExecuteProcedureReturnString(string connString, string procName,
           params SqlParameter[] paramters)
       {
           string result = "";
           using (var sqlConnection = new SqlConnection(connString))
           {
               using (var command = sqlConnection.CreateCommand())
               {
                   command.CommandType = System.Data.CommandType.StoredProcedure;
                   command.CommandText = procName;
                   if (paramters != null)
                   {
                       command.Parameters.AddRange(paramters);
                   }
                   sqlConnection.Open();
                   var ret = command.ExecuteScalar();
                   if (ret != null)
                       result = Convert.ToString(ret);
               }
           }
           return result;
       }