Why does my LINQ query always return 0?

Question

I'm facing a weird problem, I haven't programmed much with c# and only started recently, so I apologise in advance if the question is in fact just a beginner mistake.

int i = 0;
var index = from x in (
                from v in Category.Items 
                select new { Key = i++, Value = v }) 
            where ((MenuCategory) x.Value).id == menuItems[items.SelectedIndex].category 
            select x.Key;

I'm trying to get the index of a specific object in Category.Items[] (where the field id is a specific value, menuItems[items.SelectedIndex].category)

is it because the index of the column is 0? remember everything starts at 0 — WhatsThePoint, Feb 07 '17 at 13:49
Right, but unless something else is wrong in the code the index should be 1 for the value I'm testing it with — user1494162, Feb 07 '17 at 13:50
If you need the index you can use method syntax and there is an overload of `Select` that will include the index in the lambda. https://msdn.microsoft.com/en-us/library/bb534869(v=vs.110).aspx — juharr, Feb 07 '17 at 13:51
`i++` stores the current value and THEN increments the value. So if `i` starts at 0, the first call to ++ will return 0, are you expecting 1? If so, use `++i` instead, it increments before storing. — Bradley Uffner, Feb 07 '17 at 13:51
check this answer: http://stackoverflow.com/a/17995757/521446 — Alexandru Pupsa, Feb 07 '17 at 13:53
you can get a list of (Value, Index) pairs like this: list.Select((Value, Index) => new { Value, Index }); — Alexandru Pupsa, Feb 07 '17 at 13:58

score 5 · Accepted Answer · answered Feb 07 '17 at 13:53

5

LINQ queries should not cause side effects like this. You can get what you want with method syntax and the overload of Select:

var selectedCatId = menuItems[items.SelectedIndex].category;
var indexes = Category.Items
    .Select((c, index) => new { Key = index, Value = c })
    .Where(x => ((MenuCategory)x.Value).id == selectedCatId)
    .Select(x => x.Key);

answered Feb 07 '17 at 13:53

Tim Schmelter

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this worked, thank you. can you please elaborate on why it's not a good idea to have LINQ queries with side effects? – user1494162 Feb 07 '17 at 14:01
1

@user1494162: well, you have provided a good example. Queries are supposed to collect informations provided in one or multiple sources. If this query also modifies something it often depends on how the query is executed(f.e. if you have an `OrderBy` or `Where` and in which order they appear). If someone changes this logic ´he might also change the effect but unknowingly. – Tim Schmelter Feb 07 '17 at 14:04

Why does my LINQ query always return 0?

1 Answers1