LINQ, "Value cannot be null", yet query evaluates

Question

I have the following linq query:

    var test = vendorContact.vendorContactItem
                     .Where(x => x.ItemNumber == vendorContactItem.Item_Number)
                     .FirstOrDefault();

It fails on this piece of code, "Value cannot be null, parameter name: source" ... yet it also displays, in the local variables window, "test" as a variable with all its properties populated.

vendorContact.VendorContactItem is null. Presumably this would be the first element to be added to the list. So how is "test" evaluating correctly while simultaneously throwing up that error?

I'm new to Linq, so excuse me if this is an obvious question.

vendorContact.VendorContactItem cannot be both null and an empty list. Which is it? — Marcelo Cantos
– Marcelo Cantos, Commented Dec 1, 2010 at 21:34
"null" and "a list with no elements" are two different things. — Paul Abbott
– Paul Abbott, Commented Dec 1, 2010 at 21:34
The LINQ extension methods (all extension methods) are invoked as: fn(obj, p1, p2, ...). That is, they are not invoked upon a receiver (they are static methods) and thus can react to null (e.g. when obj == null) how they choose to. Usually throwing an ArgumentNullException is appropriate. Not sure if this answers/explains your question though. — user166390
– user166390, Commented Dec 1, 2010 at 21:35
Good catch guys, you are correct it is not an empty list, it is actually null, I have edited the question to reflect this. — user500038
– user500038, Commented Dec 1, 2010 at 21:37

Albin Sunnanbo · Accepted Answer · 2010-12-01 21:41:16Z

1

If this is in a loop test in the locals window contains the last value of test, from the last iteration of the loop.

Edit: This has really nothing to do with LINQ, but how the debugger works.

edited Dec 1, 2010 at 21:41

answered Dec 1, 2010 at 21:35

Albin Sunnanbo

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1 Comment

user500038 Over a year ago

Ah ha! Well, I did say the answer might be obvious. Thanks a bunch!

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