using System;   

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Random random = new Random();

        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

        var str = new string(System.Linq.Enumerable.Repeat(chars, 250)

                          .Select(s => s[random.Next(s.Length)]).ToArray());

        Console.WriteLine(str);

    }

}