StringHelpers is a class, that contains GenerateRandomUTF8String function.

StringHelpers

using System;
using System.Globalization;
using System.Linq;

namespace Konard.Helpers
{
    public static class StringHelpers
    {
        // TODO: Исправить
        //string randomString = StringHelpers.GenerateRandomUTF8String(100, UnicodeCategory.CurrencySymbol, UnicodeCategory.OtherSymbol);

        public static readonly Random rnd = new Random();

        public static readonly string EnglishAlphabet = "abcdefghijklmnopqrstuvwxyz";
        public static readonly string RussianAlphabet = "абвгдеёжзийклмнопрстуфхцчшщъыьэюя";

        public static unsafe string GenerateRandomUTF8String(int length, string alphabet)
        {
            if (length <= 0)
                return String.Empty;
            if (string.IsNullOrWhiteSpace(alphabet))
                throw new ArgumentNullException("alphabet");

            byte[] randomBytes = rnd.NextBytes(length);

            string s = new string(alphabet[0], length);

            fixed (char* p = s)
            {
                for (int i = 0; i < s.Length; i++)
                {
                    *(p + i) = alphabet[randomBytes[i] % alphabet.Length];
                }
            }
            return s;
        }

        public static unsafe string GenerateRandomUTF8String(int length, params UnicodeCategory[] unicodeCategories)
        {
            if (length <= 0)
                return String.Empty;
            if (unicodeCategories == null)
                throw new ArgumentNullException("unicodeCategories");
            if (unicodeCategories.Length == 0)
                return rnd.NextString(length);

            byte[] randomBytes = rnd.NextBytes(length);

            string s = randomBytes.ConvertToString();
            fixed (char* p = s)
            {
                for (int i = 0; i < s.Length; i++)
                {
                    while (!unicodeCategories.Contains(char.GetUnicodeCategory(*(p + i))))
                        *(p + i) += (char)*(p + i);
                }
            }
            return s;
        }
    }
}