using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        string sDate = "2018-12-31 15:31:56.000";

        string ssDate = "2018-12-31 15:31:56.000";

        DateTime dDate = DateTime.Parse(sDate);

        Console.WriteLine(dDate);

        DateTime eDate =  DateTime.ParseExact(ssDate, "yyyy-MM-dd HH:mm:ss.fff", CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None);

        Console.WriteLine(eDate);

    }

}