Managing data conversion for user-defined data types within a DBMS. A custom data type definition and one or more converters are stored within a database management system. Each converter can convert instances of the custom data type between a binary format and a programming language format. The database management system receives a request for a custom data type instance in a specified programming language format. A converter among the one or more converters converts the custom data type instance expressed in the binary format into a custom data type instance expressed in the specified programming language format, using a custom data type definition from the database management system. The request is responded to with the converted custom data type instance in the specified programming language format.