Random element of List from LINQ SQL

Question

I'm using C# 3.5 and am currently using Linq to get all users from a user table and put them in a list.

Now I would like to return a random user from that list. What's the best way to go about doing that?

Edit: Found it here: How to get a Random Object using Linq

score 80 · Accepted Answer · answered Jan 06 '11 at 13:35

80

Like this:

var rand = new Random();
var user = users[rand.Next(users.Count)];

answered Jan 06 '11 at 13:35

SLaks

868,454
176
1,908
1,964

19

Note: In real code `var rand = new Random();` should be initialized somewhere else so that it will only be seeded once. If this code is called multiple times in quick succession, the new instances of `Random` may end up using the same seed and thus returning the same value when calling `Next`. – Brian Jan 06 '11 at 19:17
4

Indeed, this rand could be `static` – Junior Mayhé May 04 '17 at 20:27
I belive users collection is zero base so it should be like: var user = users[rand.Next(users.Count-1)]; – Roman Feb 22 '19 at 08:36
1

@Roman Count -1 isn't needed, `rand.Next` "Returns a non-negative random integer that is **less** than the specified maximum" – 4imble Nov 12 '19 at 14:10

score 37 · Answer 2 · answered Jan 06 '11 at 14:17

37

Use ElementAt:

var rand = new Random();
var user = users.ElementAt( rand.Next( users.Count() ) );

answered Jan 06 '11 at 14:17

Danko Durbić

7,077
5
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score 16 · Answer 3 · answered Apr 11 '13 at 23:31

Why not create a generic helper and/or extension?!

namespace My.Core.Extensions
{
    public static class EnumerableHelper<E>
    {
        private static Random r;

        static EnumerableHelper()
        {
            r = new Random();
        }

        public static T Random<T>(IEnumerable<T> input)
        {
            return input.ElementAt(r.Next(input.Count()));
        }

    }

    public static class EnumerableExtensions
    {
        public static T Random<T>(this IEnumerable<T> input)
        {
            return EnumerableHelper<T>.Random(input);
        }
    }
}

Usage would be:

        var list = new List<int>() { 1, 2, 3, 4, 5 };

        var output = list.Random();

score 10 · Answer 4 · answered Aug 24 '12 at 13:27

for Entity Framework or Linq 2 Sql, can use this extension method

public static T RandomElement<T>(this IQueryable<T> q, Expression<Func<T,bool>> e)
{
   var r = new Random();
   q  = q.Where(e);
   return q.Skip(r.Next(q.Count())).FirstOrDefault();
}
// persons.RandomElement(p=>p.Age > 18) return a random person who +18 years old
// persons.RandomElement(p=>true) return random person, you can write an overloaded version with no expression parameter

score 3 · Answer 5 · answered Jan 06 '11 at 13:37

3

How about something like this?

var users = GetUsers();
var count = user.Count();
var rand = new System.Random();
var randomUser = users.Skip(rand.Next(count)).FirstOrDefault();

answered Jan 06 '11 at 13:37

ilivewithian

19,476
19
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If it's an `IList`, you don't need LINQ. – SLaks Jan 06 '11 at 13:38
Quite right, but the way the question was phrased there is a lot of room for ambiguity. – ilivewithian Jan 06 '11 at 13:41

score 2 · Answer 6 · answered Jun 17 '14 at 13:55

The Random class can be used to generate pseudo-random numbers. Use it to generate a random number within the range of valid indices into your array or list.

Random rand = new Random();
var user = Users[rand.Next(Users.Count)];

If you want to see more examples, I created several random-oriented LINQ extensions and published it in the article Extending LINQ with Random Operations.

score 2 · Answer 7 · answered Jun 07 '22 at 06:32

2

list.OrderBy(x=> Guid.NewGuid()).Take(10)

answered Jun 07 '22 at 06:32

TheKido

99
3

Not really what was asked - also this is highly inefficient. – Frederik Jun 07 '22 at 11:18
OpenAI says this is the best way to do it. – Ricardo Araújo May 22 '23 at 15:58

score 0 · Answer 8 · answered Apr 15 '21 at 18:19

Not exactly applicable in all cases but below is an alternative solution that I found handy since I was already using Bogus in my project.

List<User> myUserList = _context.Users.ToList();

var _faker = new Faker("en");
User randomUser = _faker.Random.ListItem<User>(myUserList);

Random element of List from LINQ SQL

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