why can't I assign object variables within the try block?

Question

Why can't I assign object variables within the try block?

If I attempt to do this and clean up the variable in the finally block I get a compiler error: "use of unassigned local variable". This makes no sense because the variable is declared before the try block, and in the finally block I am first checking whether the variable is null.

Why can't the following code compile? I am checking whether dbc is null so there's no chance of it trying to do something with an unassigned variable.

eg:

DbConnection dbc;
try {
    dbc = <some method call returning an open DbConnection>
    // do stuff
} catch (Exception e) { // do stuff }
finally { 
    if (dbc != null) {
        dbc.Close();
    }
}

I bet Jon Skeet will show up from nowhere and get the best answer award. Yawn. — Hamish Grubijan
– Hamish Grubijan, Commented Aug 11, 2010 at 3:21

Anthony Pegram · Accepted Answer · 2010-08-11 03:08:52Z

12

Change your declaration to DbConnection dbc = null; so the compiler can know for certain that the variable is assigned. (Merely declaring dbc is not the same as assigning it a value of null, you must be explicit with a local.)

The reason your existing code fails is that it is entirely possible for an exception to occur before dbc is set. As such, the compiler cannot assume that dbc is assigned by the time the finally block executes.

For more info, see section 5.3 of the language specification on definite assignment.

http://msdn.microsoft.com/en-us/library/aa691172(VS.71).aspx

answered Aug 11, 2010 at 3:08

Anthony Pegram

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9 Comments

CJ7 Over a year ago

Surely the variable is null if it is not assigned?

Anthony Pegram Over a year ago

@Craig, no, the variable is merely unassigned. null does not constitute the lack of assignment.

Hamish Grubijan Over a year ago

Global and member variables get auto-nulled, I believe. Local variables do not ... for efficiency purposes. Don't quote me on this one, look it up.

josh3736 Over a year ago

@Craig: Javascript behaves the same way: null and undefined mean two slightly different things.

Hans Passant Over a year ago

Just like VB.NET, C# also automatically nulls local variables. However, unlike VB.NET, the C# compiler thinks you probably made a mistake by having a path through your code that never initializes it. That catches a lot of bugs.

|

jwsample · Accepted Answer · 2010-08-11 03:10:25Z

1

Change this

DbConnection dbc;

to this

DbConnection dbc = null;

answered Aug 11, 2010 at 3:10

jwsample

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