public TypeMap this [SQL_C_TYPE sqlCType]
 {
         get {
                 foreach (TypeMap map in List){
                         if (map.SqlCType == sqlCType
                             && (map.BitMask & TypeMap.DefaultForSQLCType) > 0 )
                                 return map;
                 }
                 throw new ArgumentException (String.Format ("Type mapping for odbc type {0} is not found", 
                                                             sqlCType.ToString ()
                                                             )
                                              );
         }
         set {
                 int i = IndexOf (sqlCType);
                 if (i == -1)
                         Add (value);
                 List [i] = value;
         }
         
 }