How to use OrderBy in Linq - linq

Question

I am using OrderBy, and I have figured out that I have to use OrderBy as a last method, or it will not work. Distinct operator does not grant that it will maintain the original order of values, or if I use Include, it cannot sort the children collection.
Is there any reason why I shouldn't do Orderby always last and don't worry if order is preserved?
Edit:
In general, is there any reason, like performance impact, why I should not use OrderBy last. Doesnt metter if I use EnityFramework to query a database or just querying some collection.
dbContext.EntityFramework.Distinct().OrderBy(o=> o.Something); // this will give me ordered result
dbContext.EntityFramework.OrderBy(o=> o.Something).Distinct().; // this will not, because Distinct doesnt preserve order.
Lets say that I want to Select only one property.
dbContext.EntityFramework.Select(o=> o.Selected).OrderBy(o=> o.Something);
Will order be faster if I order collection after one property selection? So in that case I should use Order last. And I am just asking is there any situation where ordering shoudnt be done as last command?

Answer 1

Is there any reason why I shouldn't do OrderBy always last
There may be reasons to use OrderBy not as the last statement. For example, the sort property may not be in the result:
var result = context.Entities
.OrderBy(e => e.Date)
.Select(e => e.Name);
Or you want a sorted collection as part of the result:
var result = context.Customers
.Select(c => new
{
Customer = c,
Orders = c.Orders.OrderBy(o => o.Date)
Address = c.Address
});
Will order be faster if I order collection after one property selection?
Your examples show that you're working with LINQ to Entities, so the statements will be translated into SQL. You will notice that...
context.Entities
.OrderBy(e => e.Name)
.Select(e => e.Name)
... and ...
context.Entities
.Select(e => e.Name)
.OrderBy(s => s)
... will produce exactly the same SQL. So there is no essential difference between both OrderBy positions.
Doesn't matter if I use Entity Framework to query a database or just querying some collection.
Well, that does matter. For example, if you do...
context.Entities
.OrderBy(e => e.Date)
.Select(e => e.Name)
.Distinct()
... you'll notice that the OrderBy is completely ignored by EF and the order of names is unpredictable.
However, if you do ...
context.Entities
.AsEnumerable() // Continue as LINQ to objects
.OrderBy(e => e.Date)
.Select(e => e.Name)
.Distinct()
... you'll see that the sort order is preserved in the distinct result. LINQ to objects clearly has a different strategy than LINQ to Entities. OrderBy at the end of the statement would have made both results equal.
To sum it up, I'd say that as a rule of the thumb, try to order as late as possible in a LINQ query. This will produce the most predictable results.

Answer 2

I don't know if you misundertood the meaning of Distinct. According to definition it does:
Returns distinct elements from a sequence by using the default equality comparer to compare values.
So if you have a list of int and you want to remove repeated values, you use Distinct. Distinct uses the default equality comparer and it does the comparison by comparing the current element to the next one. So, you have to sort first to get the expected result.
And about OrderBy method, in fact, it does the sort. So if you want to sort something and distinct after you use:
List<int> myNumbers = new List<int>{ 102, 2817, 82, 2, 1, 2, 1, 9, 4 };
Sorting and removing duplicated numbers
// returns 1, 2, 4, 9, 82, 102, 2817
var sortedUniques = myNumbers.OrderBy(n => n).Distinct();
Removing duplicated numbers and sorting
// returns 1, 1, 2, 2, 4, 9, 82, 102, 2817
// It occurs because the Distinct compares current number to the next one
var sortedUniques = myNumbers.Distinct().OrderBy(n => n);
Just removing duplicated numbers
// returns 102, 2817, 82, 2, 1, 9, 4
var sortedUniques = myNumbers.Distinct().OrderBy(n => n);
Just sorting
// returns 1, 1, 2, 2, 4, 9, 82, 102, 2817
var sortedUniques = myNumbers.Distinct().OrderBy(n => n);
I hope it helps you \o/