using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var sessionId = "ggySK4Q8406KWOF6BHIOdQ";

        Console.WriteLine(IsValidSessionId(sessionId));

        Console.WriteLine(ToGuid(sessionId));

    }

        public static Guid ToGuid(string base64UrlSafeId)

        {

            Guid guid;

            if (!TryParse(base64UrlSafeId, out guid))

            {

                throw new ArgumentException(base64UrlSafeId);

            }

​

            return guid;

        }

            public static bool TryParse(string base64Id, out Guid id)

        {

            id = Guid.Empty;

            if (string.IsNullOrWhiteSpace(base64Id))

            {

                return false;

            }

​

            base64Id = base64Id.Trim().Replace('-', '+').Replace('_', '/');

            if (!IsValidSessionId(base64Id))

            {

                return false;

            }

​

            // restoring chars '==' in the end for proper Base64 convertion

            base64Id += "==";

            id = new Guid(Convert.FromBase64String(base64Id));

            return true;

        }

        public static bool IsValidSessionId(string sessionId)

        {

            if (string.IsNullOrWhiteSpace(sessionId))

            {

                return false;

            }

​

            return (sessionId.Length == 22) && Regex.IsMatch(sessionId, @"^[a-zA-Z0-9\+/]*={0,3}$", RegexOptions.None);

        }

}