Is there a clever way to handle this null object reference?

Question

So I have a Retrieve() function, which either gets me an object or a null (if that object is not found). I'm using an if statement with a boolean attribute of that object. It's set up like this.

if(Retrieve(index).IsForm == true) {}

The issue with this is that if it doesn't find an object, it'll throw a null reference exception. There are some ways around this, of course, but none that I find concise. There's a try...catch, but that seems pointless when I expect the error. I can check if the object is null first, if(Retrieve(index) != null), but that seems like adding needless nesting. Is there a clever way to handle this? I thought of using the null coalescing operator but it doesn't work in this situation.

Conciseness is not always the best way. Readability is generally far more important. — Steve, Aug 24 '12 at 17:00
Depending on your situation you could use a Null Object pattern. See this question http://stackoverflow.com/q/11972281/1066291. — Mark Rucker, Aug 24 '12 at 17:02

score 2 · Accepted Answer · answered Aug 24 '12 at 17:00

2

You can either call the method twice:

if(Retrieve(index) != null && Retrieve(index).IsForm == true) { }

Or you can break the lines apart and store the result before the if:

var result = Retrieve(index);
if(result != null && result.IsForm == true) { }

answered Aug 24 '12 at 17:00

Justin Niessner

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score 2 · Answer 2 · answered Aug 24 '12 at 17:03

2

You could write an IsForm function to do both operations for you:

bool IsForm(int index)
{
    var result = Retrieve(index);
    return result != null && result.IsForm;
}

if (IsForm(index))
    ...

answered Aug 24 '12 at 17:03

Gabe

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score 1 · Answer 3 · answered Aug 24 '12 at 17:02

1

The Null Object pattern would be helpful here. It keeps your calling code clean but does add an additional class.

class NullWhatever : Whatever
{
    public NullWhatever() { IsForm = false; } 
}


Whatever Retrieve(...) 
{
     ...
     return new NullWhatever();  // instead of null
}

answered Aug 24 '12 at 17:02

Austin Salonen

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Jinx. I just said the same thing in the comments :) – Mark Rucker Aug 24 '12 at 17:04

Mark Rucker · Answer 4 · 2012-08-24T20:23:32.533

You could make a Nullable_IsForm extension method. Then you could check for the null condition.

public static class RetrieveExtension
{
    public static bool? Nullable_IsForm(this Retrieve retrieved)
    {
        if(retrieved == null)
        {
            return null;
        }
        else
        {
            return retrieved.IsForm;
        }
    }
}

Then in your code you'd check it against bool values

if(Retrieve(index).Nullable_IsForm == true) 
{}
else if (Retrieve(index).Nullable_IsForm == false) 
{}
else if (Retrieve(index).Nullable_IsForm == null ) 
{}

score 0 · Answer 5 · answered Aug 24 '12 at 17:02

I don't think there is any more concise way to do it, no.

Shortest I can think of is:

if(Retrieve(index)!=null && Retrieve(index).IsForm == true) {}

but I don't like this because it calls Retrieve(index) multiple times. I'd go for

var foo = Retrieve(index);
if(foo!=null && foo.IsForm == true) {}

but that is obviously not doing anything clever or more concise. It is probably more efficeint than some of the alternatives.

score 0 · Answer 6 · answered Aug 24 '12 at 17:04

0

You could put both conditions in the same if:

if(Retrieve(index)!= null && Retrieve(index).IsForm == true) {}

Thanks to short-circuit, if the null-check fails, rest of the expression is not evaluated.

answered Aug 24 '12 at 17:04

Timoteo Brasil

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calling Retrieve twice? What if it's slow? – Austin Salonen Aug 24 '12 at 17:05
just split in two statements: `var whatever = Retrieve(index); if(whatever!=null && whatever.IsForm) {}` – Timoteo Brasil Aug 24 '12 at 17:08

Is there a clever way to handle this null object reference?

6 Answers6