try catch 塊的使用原則

舉一個NHibernate的例子

ISession session;

ITransaction tx;

try

{

session = factory.OpenSession();

tx = session.BeginTransaction();

// do database work

tx.Commit();

session.Close();

}

catch (Exception ex)

{

tx.Rollback();

session.Close();

// further exception handling

}

I can’t claim to know the inner workings of OpenSession but if there’s a chance it throws and surfaces an exception, the catch is going to access a null reference, tx. General advice:

Acquire Resource;

Try

Do Something with Resource

Catch

Do something

Finally

Release Resource

Actually there’s a semantic implication of OpenSession. To me it should either return a valid ISession reference to an open reference or it should throw an exception. The same goes for BeginTransaction. So, IMHO this is how to write it (my 2c):

ISession session = factory.OpenSession();

try

{

  ITransaction tx = session.BeginTransaction();

  try

  {

    // do db work

    tx.Commit();

  }

  catch

  {

    tx.Rollback();

  }

}

finally

{

  session.Close();

}

Both session in finally and tx in catch will be valid references provided that OpenSession and BeginTransaction returns what they atomically should return, i.e. an open session in the former case and a started transaction in the latter case.