c# - Randomly ordered IEnumerable changes order each time it is used, instead of being reordered once

Question

I am shuffling an IEnumerable using OrderBy and Random() and then selecting some of them using ElementAt.

Simplified version:

var r = new Random(DateTime.Now.Millisecond);
IEnumerable<IPublishedContent> testimonialsShuffled = testimonials.OrderBy(x => r.Next());

var testimonialA = testimonialsShuffled.ElementAt(0);
var testimonialB = testimonialsShuffled.ElementAt(1);
var testimonialC = testimonialsShuffled.ElementAt(2);

It turns out that this does not always return three distinct elements as I expected, because every time testimonialsShuffled is used, it seems to get reordered.

var testimonialA = testimonialsShuffled.ElementAt(0);
var testimonialB = testimonialsShuffled.ElementAt(0);
var testimonialC = testimonialsShuffled.ElementAt(0);

Similarly, I would expect this code to result in three equal variables, but instead they usually differ (as the first element in testimonialsShuffled changes each time).

My goal is just order my IEnumerable randomly once (so that its order changes on different page loads, but not multiple times in a single view). What am I doing wrong?

Answer 1

OrderBy results in an IEnumerable, which is re-evaluated every time you access it (simply put).
To get one fixed result, you have to "force" it into a list.

var r = new Random(DateTime.Now.Millisecond);
var testimonialsShuffled = testimonials.OrderBy(x => r.Next()).ToList();

var testimonialA = testimonialsShuffled.ElementAt(0);
var testimonialB = testimonialsShuffled.ElementAt(1);
var testimonialC = testimonialsShuffled.ElementAt(2);

should do the trick.