How does Enumerable.OrderBy use keySelector

Question

Using the proceeding code, I successfully managed to produce a collection of numbers and shuffle the elements' position in the array:

var randomNumbers = Enumerable.Range(0, 100)
                    .OrderBy(x => Guid.NewGuid());

Everything functions fine but it's kind of thrown a spanner in my works when trying to understand Enumerable.OrderBy. Take the following code for example:

var pupils = new[] 
{ 
    new Person() { Name = "Alex", Age = 17 },
    new Person() { Name = "Jack", Age = 21 } 
};

var query = pupils.OrderBy(x => x.Age);

It's my understanding that I am passing the property I wish to sort by and I presume that LINQ will use Comparer<T>.Default to determine how to order the collection if no explicit IComparer is specified for the second overload. I really fail to see how any of this reasonable logic can be applied to shuffle the array in such a way. So how does LINQ let me shuffle an array like this?

Answer 1

How does Enumerable.OrderBy use keySelector?

Enumerable.OrderBy<T> lazily returns - keySelector is not called directly. The result is an IOrderedEnumerable<T> that will perform the ordering when enumerated.

When enumerated, keySelector is called once for each element. The order of the keys defines the new order of the elements.

Here's a nifty sample implementation.

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So how does LINQ let me shuffle an array like this?

var randomNumbers = Enumerable
  .Range(0, 100)
  .OrderBy(x => Guid.NewGuid());

Guid.NewGuid is called for each element. The call for the second element may generate a value higher or lower than the call for first element.

randomNumbers is an IOrderedEnumerable<int> that produces a different order each time it is enumerated. KeySelector is called once per element each time randomNumbers is enumerated.