Troubleshooting Exceptions: System.Data.ConstraintException
A ConstraintException exception is thrown when an action is attempted that violates a constraint.
Make sure you are not trying to assign a value to a primary key field where the primary key already exists in the data table.
If the primary key exists, this exception is thrown.
Clear datasets before loading them from view state.
If there is data in the dataset when you load it, this exception may be thrown.