Permutate

Program.cs

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;

    public class Program
    {
        public static void Main()
        {
            var randomPassword =
                Permutate(
                    100,
                    new[]
                    {
                        new PermutationRule
                        {
                            AtLeast = 1,
                            Characters = new HashSet<char>("ABCDEFGHIJKLMNOP")
                        },
                        new PermutationRule
                        {
                            AtLeast = 1,
                            Characters = new HashSet<char>("1234567890")
                        },
                        new PermutationRule
                        {
                            AtLeast = 1,
                            Characters = new HashSet<char>("!@#$%^&*()_+")
                        },
                    }
                );

            Console.WriteLine(randomPassword);
        }

        public static string Permutate(int minLength, params PermutationRule[] rules)
        {
            if (rules.Length == 0)
            {
                throw new ArgumentException("rules");
            }

            var random = new Random();

            var sb = new StringBuilder();

            foreach (var rule in rules)
            {
                for (var i = 0; i < rule.AtLeast; i++)
                {
                    sb.Insert(
                        random.Next(0, sb.Length),
                        rule.Characters.ElementAt(random.Next(0, rule.Characters.Count - 1))
                    );
                }
            }

            for (var i = sb.Length; i < minLength; i++)
            {
                var rule = rules[random.Next(0, rules.Length)];

                sb.Append(rule.Characters.ElementAt(random.Next(0, rule.Characters.Count)));
            }

            return sb.ToString();
        }

        public class PermutationRule
        {
            public int AtLeast { get; set; }
            public HashSet<char> Characters { get; set; }
        }
    }