Question

What is the best (and fastest) way to retrieve a random row using Linq to SQL when I have a condition, e.g. some field must be true?

Answer 1

You can do this at the database, by using a fake UDF; in a partial class, add a method to the data context:

partial class MyDataContext {      [Function(Name='NEWID', IsComposable=true)]       public Guid Random()       { // to prove not used by our C# code...           throw new NotImplementedException();       } } 

Then just order by ctx.Random(); this will do a random ordering at the SQL-Server courtesy of NEWID(). i.e.

var cust = (from row in ctx.Customers            where row.IsActive // your filter            orderby ctx.Random()            select row).FirstOrDefault(); 

Note that this is only suitable for small-to-mid-size tables; for huge tables, it will have a performance impact at the server, and it will be more efficient to find the number of rows (Count), then pick one at random (Skip/First).

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for count approach:

var qry = from row in ctx.Customers           where row.IsActive           select row;  int count = qry.Count(); // 1st round-trip int index = new Random().Next(count);  Customer cust = qry.Skip(index).FirstOrDefault(); // 2nd round-trip