Why Re-throw Exceptions?

Question

I\'ve seen the following code many times:

try
{
    ... // some code
}
catch (Exception ex)
{
    ... // Do something
    throw new CustomException(ex);

            


        

Answer 1

Actually there is a difference between

throw new CustomException(ex);

and

throw;

The second will preserve the stack information.

But sometimes you want to make the Exception more "friendly" to your application domain, instead of letting the DatabaseException reach your GUI, you'll raise your custom exception which contains the original exception.

For instance:

try
{

}
catch (SqlException ex)
{
    switch  (ex.Number) {
        case 17:
        case 4060:
        case 18456:
           throw new InvalidDatabaseConnectionException("The database does not exists or cannot be reached using the supplied connection settings.", ex);
        case 547:
            throw new CouldNotDeleteException("There is a another object still using this object, therefore it cannot be deleted.", ex);
        default:
            throw new UnexpectedDatabaseErrorException("There was an unexpected error from the database.", ex);
    } 
}