using System;

using System.Security.Cryptography;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(GeneratePassword(15, 3));

        Console.WriteLine(GeneratePassword(15, 3));

        Console.WriteLine(GeneratePassword(15, 3));

        Console.WriteLine(GeneratePassword(15, 3));

    }

    public static string GeneratePassword(int length, int numberOfNonAlphanumericCharacters)

    {

        const string allowedChars = "abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ0123456789";

        const string nonAlphanumericChars = "!@#$%^&*()_-+=[{]};:<>|./?";

        var randNum = new byte[4];

        using (var rng = RandomNumberGenerator.Create())

        {

            rng.GetBytes(randNum);

            var randomSeed = BitConverter.ToInt32(randNum, 0);

            var random = new Random(randomSeed);

            var chars = new char[length];

            var allowedCharCount = allowedChars.Length;

            var nonAlphanumericCharCount = nonAlphanumericChars.Length;

            var numNonAlphanumericCharsAdded = 0;

            for (var i = 0; i < length; i++)

            {

                if (numNonAlphanumericCharsAdded < numberOfNonAlphanumericCharacters && i < length - 1)

                {

                    chars[i] = nonAlphanumericChars[random.Next(nonAlphanumericCharCount)];

                    numNonAlphanumericCharsAdded++;

                }

                else

                {

                    chars[i] = allowedChars[random.Next(allowedCharCount)];

                }

            }

            return new string(chars);

        }

    }

​

}