using System;

using System.Globalization;

​

public class Program

{

    public static void Main()

    {

        var input = "2021-12-29 08:06:59.953000000";

        Console.WriteLine(input);

        var d = DateTime.ParseExact(input, "yyyy-MM-dd HH:mm:ss.fffffffff", CultureInfo.InvariantCulture, DateTimeStyles.None);

        Console.WriteLine(d);

    }

}