Will a LINQ expression returning multiple items followed by the First method iterate through all items in the preceding method in .NET?

Question

In the following statement will the where method iterate through all elements before First is executed on the returned elements, or will the First method take effect already in the Where method? In VB.NET:

Items.Where(Function(Item) Item.IsMatch).First

Items implements IEnumerable(Of T)

Read my own question asked some time ago: http://stackoverflow.com/questions/10110013/order-of-linq-extension-methods-does-not-affect-performance — Tim Schmelter, Jul 31 '14 at 08:42
It depends on the filter. In a worst case it will loop through the whole set and raise an exception on First. — Wiktor Zychla, Jul 31 '14 at 08:43
@TimSchmelter read it, and I'm glad to hear that it works as I was hoping! — SuppaiKamo, Jul 31 '14 at 08:57
@WiktorZychla If used on a collection where not finding an element is a possibility, I will use FirstOrDefault instead — SuppaiKamo, Jul 31 '14 at 08:58

score 1 · Accepted Answer · answered Jul 31 '14 at 08:56

From MSDN document for Where:

This method is implemented by using deferred execution. 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.

That is when First() is invoked on Wheres output, which in turn invokes GetEnumerator.

Will a LINQ expression returning multiple items followed by the First method iterate through all items in the preceding method in .NET?

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