Weird behaviour with conditional operator in .Net

Question

This has me pretty stumped. Maybe I'm too tired right now.

    Rectangle rectangle = new Rectangle(0, 0, image.Width, image.Height);
    Rectangle cropArea = inputArea == null ? rectangle : inputArea.Value;

    if (inputArea == null)
        cropArea = rectangle;

inputArea is a nullable Rectangle, which in my particular case is null.

The first two statements yields a cropArea initialized to 0. The second, however, yields the correct cropArea based on the image width and height. Have I misunderstood anything with the conditional operator? It seems it does not return rectangle when inputArea = null? Is there any quirks when working with value types?

EDIT: Alright, I should have tried this first: restarted VS. It seems the debugger lied to me, or something. Anyway, works now. Thanks.

Answer 1

Rectangle cropArea = (!inputArea.HasValue) ? rectangle : inputArea.Value;

Answer 2

Your code appears correct. The conditional expression (or conditional operator, or originally called the ternary operator... everyone happy now? :)) should be interchangeable with if/else statements.

Rectangle cropArea = inputArea == null ? rectangle : inputArea.Value;

should be exactly the same as:

Rectangle cropArea;
if (inputArea == null)
{
    cropArea = rectangle;
}
else
{
    cropArea = inputArea.Value;
}

(in fact they should generate the same IL code).

Trace through with the debugger and see if anything jumps out at you.

Answer 3

So are you saying that when inputArea is null, without the if statement you get a rectangle initialized to something else than the image size? I just tried running that and it works fine. Make sure that image has a size and that inputArea is actually null.

Answer 4

What the hell?

Rectangle rectangle = ...;
Rectangle cropArea;
if (inputArea == null)
    cropArea = rectangle;
else
    cropArea = inputArea.Value;

if (inputArea == null)
    cropArea = rectangle;

Why have the second if? It's totally and entirely redundant. The scenario in which cropArea might still be null or zero is if inputArea.Value is null/zero, since you didn't check for that (only if inputArea is null).

Answer 5

That seems like a nasty bug in Visual Studio debug mode which is fooling you:

Now F10 to step over this line and you get:

On the console correct values are printed.

WTF.