Question

Today I ran into an issue with LINQ to objects (not SQL) that popped up due to a typo. I had a .Select one place and a .Where in another place. I was expecting same result but they are showing different numbers. Assume somelist has 10 elements with all elements having qty = 0

//returns 10 - basically count of all rows. I am expecting 0
 somelist.Select(p => p.qty > 0).Count() 

//returns 0 - the correct count
 somelist.Where(p => p.qty > 0).Count() 

if both select and where return IEnumerable<T> then why the ambiguity? Thank you.

Answer 1

Select is a projection, so what you get is the expression p.qty > 0 evaluated for each element in somelist. i.e. lots of true/false values (the same number as your original list). So when you do Count on it, you get the same number. If you look the select will return IEnumerable<bool> (because the type of p.qty > 0 is a bool).

Where filters the results so count runs on the filtered list, and give you the expected results. The type of this is an IEnumerable<TypeOfElementInOriginalList>.

Note you can also do: somelist.Count(p => p.qty > 0) because Count has an overload that accepts a predicate to filter by.