One of the things I've lerned the hard way is that not allwais the error ADO.NET raises, is of any help to solve issues like this...

One of these days, I had a problem with a Field... the name od the field was USER... turned out to give me an error completly strange... after a few hours, changed the USER to USERID, and the problem was solved....

In this case I would sugest you to check if you SQL String includes in both table, the primary key, and the related key. Also, make sure that on the database, the key fields, are indexed.

Is both these are correct, can you post your SQL query for both tables and the code you are using to stablish the relation?

Best regards

Joao Cardoso