using System;

using System.Globalization;

public class Program

{

    public static DateTime? SafetyParseExactStringToDate(string dateStr)

    {

        DateTime tempDate;

        string[] formats = new string[] {

            "dd/MM/yyyy",

            "dd-MM-yyyy",

            "dd.MM.yyyy"

        };

        var style = DateTimeStyles.AllowWhiteSpaces;

        dateStr = dateStr.Replace(".", "/").Replace("-", "/");

        var isValid = DateTime.TryParseExact(dateStr, formats, CultureInfo.InvariantCulture, style, out tempDate);

        if (!isValid)

        {

            return null;

        }

        return tempDate;

    }

    public static void Main()

    {

        var result = ParseStringToDate("");

        Console.WriteLine(result);

    }

}