Check an object is not null before iterating over it

Question

I'm getting an error System.ArgumentNullException: Value cannot be null. on the following:

 foreach (var pmt in payment.NewInvoiceViewModels
   .Where(x => x.PaymentReceived != 0) ?? 
    Enumerable.Empty<NewInvoiceViewModel>())

All I'm trying to do is check of payment.NewInvoiceViewModels is null before I iterate over it (if it is null, it causes an error).

Is there a better way of achieving this?

Your approach is correct. You can however write a generic extension method to improve it. http://stackoverflow.com/questions/11734380/check-for-null-in-foreach-loop — emre nevayeshirazi, Sep 26 '13 at 13:36
Hi Emrie - thanks - I thought it was correct, but if the object is empty, I get the Value cannot be null error - that's what I'm trying to get around. The link to the other question, is what I'm doing (I believe). Thanks, Mark — Mark, Sep 26 '13 at 13:38
@emrenevayeshirazi: No, the approach is most definitely *not* correct at the moment. Calling `Enumerable.Where(null, predicate)` will throw an exception - just as the OP is reporting. — Jon Skeet, Sep 26 '13 at 13:40

score 9 · Accepted Answer · answered Sep 26 '13 at 13:38

You've got the null-coalescing operator in the wrong place - currently you're calling Where on payment.NewInvoiceModels unconditionally and then checking whether the result will be null... it never will be (Where simply doesn't return null). You potentially want:

foreach (var pmt in (payment.NewInvoiceViewModels ?? 
                     Enumerable.Empty<NewInvoiceViewModel>())
                   .Where(x => x.PaymentReceived != 0))

Personally I'd extract this out though:

var allModels = payment.NewInvoiceViewModels ?? 
                     Enumerable.Empty<NewInvoiceViewModel>();

foreach (var pmt in allModels.Where(x => x.PaymentReceived != 0))

Or perhaps introduce an extension method:

public static IEnumerable<T> NullToEmpty(this IEnumerable<T> source)
{
    return source ?? Enumerable.Empty<T>();
}

Then:

foreach (var pmt in payment.NewInvoiceViewModels
                           .NullToEmpty()
                           .Where(x => x.PaymentReceived != 0))

Perfect - thank you - I'll mark as the answer as soon as SO allows (6 mins) - cheers, Mark — Mark, Sep 26 '13 at 13:41

score 3 · Answer 2 · answered Sep 26 '13 at 13:33

3

Do you want something like this

if(payment.NewInvoiceViewModels!=null)
{
  foreach (var pmt in payment.NewInvoiceViewModels
   .Where(x => x.PaymentReceived != 0) ?? 
    Enumerable.Empty<NewInvoiceViewModel>())
}

answered Sep 26 '13 at 13:33

Sachin

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he is allready dealing with problem by using Enumerable.Empty(). Your answer is obviously worse. – emre nevayeshirazi Sep 26 '13 at 13:35
Hi - thank you - so am I as well removing the: ?? Enumerable.Empty() - as I thought that is what that would do? Thanks, Mark – Mark Sep 26 '13 at 13:35
4

@emrenevayeshirazi: Not at all. If `payment.NewInvoiceViewModels` is null, then `Where` throws ArgumentNullException. – Jon Skeet Sep 26 '13 at 13:37
@JonSkeet yes my bad. I missed the .Where() – emre nevayeshirazi Sep 26 '13 at 13:51

Check an object is not null before iterating over it

2 Answers2