Why use Finally in Try … Catch

Question

I see that the Finally in Try .. Catch will always execute after any parts of the execution of the try catch block.

Is it any different to

Answer 1

The difference is when the code in the try block throws an exception that isn't caught by the catch block.

Normally a catch block would catch a specific type of exception, and let anything else through. In that case, the finally block will still run.

The finally block will also run if the code in the try block returns.

Answer 2

Doing clean up in a finally block is to ensure that it is run. If the catch block doesn't deal with the exception (ie. it just logs it), or even causes another exception, the code in the finally block will still run.

Answer 3

Code with four radio buttons:

• Return in TRY
• Return in CATCH
• Throw in CATCH

• Finish CATCH

  private void checkFinally()
  {
      try
      {
          doFinally();
      }
      catch
      {
          Console.WriteLine(" Breaking news: a crash occured. ");
      }
  }
  
  private void doFinally()
  {
      Console.WriteLine(" ");
      Console.Write("Here goes: " 
          + (radioReturnInTry.Checked ? "2. Return in try: " 
                  : (radioReturnInCatch.Checked? "3. Retrun in catch: "
                      : (radioThrowInCatch.Checked? "4. Throw in catch: "
                          : "1. Continue in catch: "))) );
      try
      {
          if (radioReturnInTry.Checked)
          {
              Console.Write(" Returning in try. ");
              return;
          }
          Console.Write(" Throwing up in try.  ");
          throw new Exception("check your checkbox.");
      }
      catch (Exception ex)
      {
          Console.Write(" ...caughtcha! ");
          if (radioReturnInCatch.Checked)
          {
              Console.Write("Returning in catch. ");
              return;
          }
          if (radioThrowInCatch.Checked)
          {
              Console.Write(" Throwing up in catch. ");
              throw new Exception("after caught");
          }
      }
      finally { Console.Write(" Finally!!"); }
      Console.WriteLine(" Done!!!"); // before adding checkboxThrowInCatch, 
      // this would never happen (and was marked grey by ReSharper)
  
  }
  

---

Output:

• Here goes: 1. Continue in catch: Throwing up in try. ...caughtcha! Finally!! Done!!!
• Here goes: 2. Return in try: Returning in try. Finally!!
• Here goes: 3. Retrun in catch: Throwing up in try. ...caughtcha! Returning in catch. Finally!!
• Here goes: 4. Throw in catch: Throwing up in try. ...caughtcha! Throwing up in catch. Finally!! Breaking news: a crash occured.

---

To summarize: Finally takes care of two things:

1. Of code that returned in the try or in the catch.
2. Or If you had an exception in the try, AND THROW an exception in the catch,
3. or, if you had an exception in the try, AND DID NOT CATCH that exception,

Finally to summarize "FINALLY": Finally does nothing special if you tried,and

1. DID NOT RETURN,
2. and caught any exceptions during the trial, and then
3. DID NOT RETURN in the catch either, and
4. DID NOT THROW or have code that throws up.

And last but not least (finally): If you have an exception in your code that YOU DID NOT CATCH, your code will fly, WITHOUT REACHING THE FINALLY.

Hope this is clear. (Now it is to me...)

Moshe

Answer 4

This is a good idea when dealing with database connections or anytime objects need to be disposed of. Just in case something goes wrong while running queries, you can still close the connection safely. It also helps to clean up code that the block outside the try/catch/finally block is not able to access.

Answer 5

Finally should be used to everything that needs to be done in order to keep a system consistent. This usually means release resources

Finally is always executed, no matter what exception was thrown. It should be used to release resources, in the following cases:

• Finalize a connection
• Close a file handler
• Free memory
• Close a database connection

Let me give a complete example. Imagine that that you are sending messages through the network. In pseudo-code:

// With finally                  |  //Without finally
try{                             |  try{  
  send_message()                 |    send_message() 
} catch(NetworkError){           |  } catch(NetworkError){ 
  deal_with_exception()          |    deal_with_exception()
} finally {                      |  }
  finalizes_connection()         |  finalizes_connection() 
}                                |

The only difference of both codes is when what is hold in the try block raises an exception that is not NetworkError, for example, MethodNotFound. In the first case, the method finalizes_connection() will be called, and in the second one, it will not.

A connection is naturally done through more than one program. So what happens in the case of a MethodNotFound exception to the other program? In the first case, your program will finish the connection and the other program and it will be happy. In the second case, the other program can be waiting for your response forever. What if the other program can only receive one connection per time? You just bugged the other program as well.

This would also apply for a file, for example, that you opened and other programs wouldn't be able to open for reading (in Windows). And for memory, it is never released and now you have a memory leak.

Answer 6

The Finally block will execute regardless of if the function exits because of an exception. (there are some exceptions to this rule, see this stackoverflow question for more info).

For example:

Try
    'Do something
Catch ex As Exception
    if 'Some Condition
       throw ex
    else
       'Handle exception
Finally
    'Do cleanup
End Try

In this case the Finally block will still be executed even though you may throw an exception out of the function.

This is a good practice to get into because it ensures that your cleanup code always executes. Of course using the Resoource Acquisition Is Initialization idiom is a much cleaner way of ensuring that resources get cleaned up, but I'm not versed enough in VB.net to know if this is possible to do.