Order list "randomly" per day

Question

I have an IQueryable<Post> and to sort it randomly I do the following:

IQueryable<Post> posts = getPosts();

posts = posts.OrderBy(x => Guid.NewGuid());

Is it possible to sort it randomly but for each day?

If I order it on 18 of April I would always get the same order.

But on 19 of April I would get another random order.

Answer 1

Guids are not guaranteed to be random, only unique. Do not use Guids for random data. There are specific classes in the .NET framework designed to give (pseudo) random numbers - Random is the easiest one, but if you want crypographical quality try System.Security.Cryptography.RandomNumberGenerator.

In your case Random is your friend. You can seed it with a value created from today's date.

Try this:

var seed = (int)DateTime.Now.Date.Ticks;
var random = new Random(seed);

IQueryable<Post> posts = getPosts();

Post[] sorted =
    posts
        .ToArray()
        .Select(post => new { post, random = random.Next() })
        .OrderBy(x => x.random)
        .Select(x => x.post)
        .ToArray();

When I run that with the harness code:

IQueryable<Post> getPosts()
{
    return Enumerable.Range(0, 5).Select(x => new Post() { Id = x }).AsQueryable();
}

public class Post
{
    public int Id;  
}

I get 4, 3, 1, 0, 2 today. Tomorrow it will be 4, 2, 3, 0, 1. And Tuesday it will be 3, 4, 2, 0, 1.