LINQ with Skip and Take

Question

I used the below code to take some items from IEnumerable, but it is always returning the source as null and count as 0 and actually there are items exists in IEnumerable

private void GetItemsPrice(IEnumerable<Item> items, int customerNumber)
{
    var a = items.Skip(2).Take(5);
}

When i try to access a it has count 0. Anything goes wrong here?

Solution

Remember, that variable a in your code is a query itself. It is not result of query execution. When you are using Immediate Window to watch query (actually that relates to queries which have deferred execution otherwise you will have results instead of query), it will always show

{System.Linq.Enumerable.TakeIterator<int>}
    count: 0
    source: null

You can verify that with this code, which obviously has enough items:

int[] items = { 1, 2, 3, 4, 5, 6, 7 };
var a = items.Skip(2).Take(3);

So, you should execute your query to see results of query execution. Write in Immediate Window:

a.ToList()

And you will see results of query execution:

Count = 3
    [0]: 3
    [1]: 4
    [2]: 5

Comments

[Grant Thomas]

How many items are in the collection initially?

[Admin]

No i have something more, but for my question this alone enough

[Darren]

What does items contain?

[Admin]

@GrantThomas, it contain 102 items

[Jon Skeet]

What do you mean by "returning the source as null"? And if a.Count() returns 0, then basically there were at most 2 elements in items. If you think there were 102 items, I suspect your diagnostics are incorrect.

[Admin]

@DarrenDavies, it is a object collections which contains itemid and itemname

[Admin]

Downvoters care to comment, whats wrong with my question ?

[Jonas Høgh]

And what does the output of dumping items to the immediate window look like?

[Admin]

@JonSkeet, pls check my attachment and it taken from immediate window

[Thomas Lindvall]

You still don't seem to be Enumerating your collection. IEnumrables are lazy loaded and won't do what you want until you tell them.

[Jon Skeet]

@SSS: There's no indication from the screenshot that items.Count() is more than 2.

[Admin]

@JonSkeet, pls check my updated screenshot and i have added count in it

[Jon Skeet]

@SSS: Ah... lazyberezovsky has discovered the issue. This is why you shouldn't usually try to interpret the values of private members you don't understand. If you'd called a.Count() you'd have seen something very different.

[Xaruth]

that's it. msdn.microsoft.com/en-us/library/bb503062.aspx "The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach "

[Sergey Berezovskiy]

@Xaruth yes that is what simply called as query: an object that stores all the information that is required to perform the action

[Admin]

Thanks for clarification I need to pass this as parameter items.Skip(2).Take(5)to another method. Do i need to use .ToList() to pass this as parameter?

[Jon Skeet]

As I noted in another comment: this is why you shouldn't use the debugger to show you fields you don't know the meaning of, and then assume you do understand the meaning...

[Jon Skeet]

@SSS: Well you might want to. It depends. But you need to understand that the only reason you're seeing 0 is because you're looking at a field which is an implementation detail you wouldn't normally be able to observe. It's not the length of the sequence.

[Sergey Berezovskiy]

@SSS as Jon stated, it depends. If you will return a, then query will be executed by caller later (it is not always possible). If you will return result of query execution by adding ToList() then caller will work with objects in memory (that could be problem if your query gets items from database).