Пример склонения личных имен и фамилий с использованием класса Cyriller.CyrName.

Cyriller.CyrName.Program.cs

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

/* https://github.com/miyconst/Cyriller */
namespace CyrillerTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.OutputEncoding = System.Text.Encoding.UTF8;

            Cyriller.CyrName cyrName = new Cyriller.CyrName();
            Cyriller.CyrDeclineCase[] cases = Cyriller.CyrDeclineCase.List;

            {
                string name = "Петров Сергей Витальевич";
                int gender = 0;
                bool shorten = false;

                foreach (Cyriller.CyrDeclineCase @case in cases)
                {
                    Console.WriteLine($"{@case.NameRu}: " + cyrName.Decline(name, @case.Index, gender, shorten));
                }
            }

            Console.WriteLine("--");

            {
                string name = "Петров Сергей Витальевич";
                int gender = 0;
                bool shorten = true;

                foreach (Cyriller.CyrDeclineCase @case in cases)
                {
                    Console.WriteLine($"{@case.NameRu}: " + cyrName.Decline(name, @case.Index, gender, shorten));
                }
            }

            Console.WriteLine("--");

            {
                string name = "Семенова Дарья Николаевна";
                int gender = 1;
                bool shorten = false;

                foreach (Cyriller.CyrDeclineCase @case in cases)
                {
                    Console.WriteLine($"{@case.NameRu}: " + cyrName.Decline(name, @case.Index, gender, shorten));
                }
            }

            Console.WriteLine("--");

            {
                string name = "Семенова Дарья Николаевна";
                int gender = 1;
                bool shorten = true;

                foreach (Cyriller.CyrDeclineCase @case in cases)
                {
                    Console.WriteLine($"{@case.NameRu}: " + cyrName.Decline(name, @case.Index, gender, shorten));
                }
            }
        }
    }
}