hi,
i want to take backup of database(which include large object) created in postgres8.4 and restore it
i m using backup and restore method of pgsqldumb class
but to restore large file (some what 46 mb) i got error
"Exception of type 'System.OutOfMemoryException' was thrown."

how do i restore large object stored in my database
please help

I have no idea what "pgsqldumb" class is, it sure isn't a tool provided by Postgres.
'System.OutOfMemoryException' seems to be some exception from your programming langauge (whatever that is), so it's a problem with your class and your program.

I recommend you use the standard backup and restore procedures described in the manual:
PostgreSQL: Documentation: Manuals: PostgreSQL 8.4: Backup and Restore

You can use a GUI Tool to backup the database, i'm using DbSchema, you can backup your database, work on it also OFFLINE and after that synchronize it with your database.
See the screenshots for details, hope it helps.