`using` statement with uninitialized variable not possible, nor is finally, so how to properly dispose it?

Question

Suppose I have a class DisposableObject which implements IDisposable. There is a risk it can throw an exception from any constructor and from the DoStuff function. I want to do something like this, so that no matter what happens, the object is disposed properly:

edit: I would like to avoid duplicate code, so a using inside each if/else block is not desirable (the code is a lot more complex than here)

edit2: Yes the try/finally error was wrong, changed it to use of unassigned local variable.

using (DisposableObject worker)
{
    if(/*condition*/)
        worker = new DisposableObject(/*args*/)
    else
        worker = new DisposableObject(/*other args*/)
    worker.DoStuff();
}

But I can't, because the compiler says I must provide an initializer in the using statement. I can't use try/finally either:

DisposableObject worker;
try
{
    if(/*condition*/)
        worker = new DisposableObject(/*args*/)
    else
        worker = new DisposableObject(/*other args*/)
    worker.DoStuff();
}
finally
{
    worker.Dispose();
}

The compiler says use of unassigned local variable 'worker'.

So what can I do? Obviously I can't simply go like this...

DisposableObject worker;
if (/*condition*/)
    worker = new DisposableObject(/*args*/)
else
    worker = new DisposableObject(/*other args*/)
worker.DoStuff();
worker.Dispose();

Because I can't be sure the Dispose() function is ever reached.

I'm not sure how the compiler could throw such an error for your try/finally example, since the variable is declared (albeit not initialized until in the try block). What's your actual code? — BoltClock
– BoltClock, Commented Feb 21, 2012 at 15:30
What BoltClock pointed out, your try/finally code should be working. Try adding a catch { } in there and see if the problem persists. — Brian
– Brian, Commented Feb 21, 2012 at 15:32
constructors that throw exceptions are very evil. Is there any way to refactor the code so that this doesn't happen. Also if a instance is so broken that it failed to initialize, why do you trust it enough to want to try to dispose it? — Conrad Frix
– Conrad Frix, Commented Feb 21, 2012 at 15:33
@AakashM you are correct, I changed it to use of unassigned local variable. — David S.
– David S., Commented Feb 21, 2012 at 15:34
@BoltClock exact error would be "use of unassigned local variable" he's just missing an DisposableObject worker = null; instantiation outside the try/finally — Eoin Campbell
– Eoin Campbell, Commented Feb 21, 2012 at 15:36

Dark Falcon · Accepted Answer · 2012-02-21 15:31:14Z

7

Actually, the compiler says that you are using a potentially uninitialized variable, so just initialize it:

DisposableObject worker = null;
try
{
    if(/*condition*/)
        worker = new DisposableObject(/*args*/)
    else
        worker = new DisposableObject(/*other args*/)
    worker.DoStuff();
}
finally
{
    if(worker != null)
        worker.Dispose();
}

answered Feb 21, 2012 at 15:31

Dark Falcon

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2 Comments

David S. Over a year ago

Thanks, I changed the message.

David S. Over a year ago

I had no idea assigning null was considered initalizing. How about that... :) Thanks.

Rich O'Kelly · Accepted Answer · 2012-02-21 16:00:14Z

I find it useful to move the creation and usage to separate methods (often using a factory, but private method shown here for sake of brevity):

private DisposableObject CreateWorker() {
  if (condition) return new DisposableObject(/*args*/);
  return new DisposableObject(/*other args*/);
}

This means that a using block can be used (rather than having to intialise to null and use a try... finally):

using (DisposableObject worker = CreateWorker())
{
  worker.DoStuff();
}

NB If the factory method needs to do any work with the created entity after it is instantiated the object should still be disposed and the exception re-thrown:

public DisposableObject Create() 
{
  DisposableObject entity = new DisposableEntity();
  try 
  {
    //Some operations involving entity
  }
  catch
  {
    entity.Dispose();
    throw;
  }
}

score 4 · Accepted Answer · 2012-02-21 17:23:06Z

4

There are two fairly straightforward options with using:

{
    DisposableObject worker;
    if(/*condition*/)
        worker = new DisposableObject(/*args*/);
    else
        worker = new DisposableObject(/*other args*/);
    using (worker)
    {
        worker.DoStuff();
    }
}

or

using (DisposableObject worker = /* condition */
    ? new DisposableObject(/*args*/)
    : new DisposableObject(/*other args*/))
{
    worker.DoStuff();
}

Others have given other equally valid possibilities that either avoid using or refactor code, but I believe this is closest to what you have already.

edited Feb 21, 2012 at 17:23

answered Feb 21, 2012 at 15:35

user743382

1 Comment

David S. Over a year ago

I really like the latter option; very consise. Not sure if it's fitting a much more complicated version of the problem though.

ken2k · Accepted Answer · 2012-02-21 15:32:34Z

1

How about:

DisposableObject worker = null;
try
{
    if(/*condition*/)
        worker = new DisposableObject(/*args*/)
    else
        worker = new DisposableObject(/*other args*/)

    worker.DoStuff();
}
finally
{
    if (worker != null)
    {
        worker.Dispose();
    }
}

answered Feb 21, 2012 at 15:32

ken2k

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Comments

Chris · Accepted Answer · 2012-02-21 15:32:45Z

1

If the arguments are too complex, you can create a wrapper class that contains the arguments. Then create the arguments in another method that contains your complex If/else. From there you can properly use the using block from your first example & you'll be sure that Dispose() is called.

answered Feb 21, 2012 at 15:32

Chris

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1 Comment

Dark Falcon Over a year ago

I like the concept. Could be uglier sometimes (or even cleaner), but still another interesting way to address it.

Mike Corcoran · Accepted Answer · 2012-02-21 15:32:47Z

0

you could write a wrapper class around DisposableObject that takes the arguments, and does the try/catch internally somehow and notifies you of the error?

answered Feb 21, 2012 at 15:32

Mike Corcoran

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