using System;

using Microsoft.Extensions.DependencyInjection;

using System.Globalization;

​

public class Program

{

​

    public static void Main()

    {       

        decimal? d = null;

        Console.WriteLine(d.ToDecimalString());

        d = 23.30m;

        Console.WriteLine(d.ToDecimalString());

        decimal d2 = 3.14m;

        Console.WriteLine(d2.ToDecimalString());

​

        DateTime dt = DateTime.Now;

        Console.WriteLine(dt.ToString(CultureInfo.InvariantCulture));

        Console.WriteLine(dt.ToString("yyyy-MM-ddTHH\\:mm\\:ss.fff"));

        Console.WriteLine(dt.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture));

        Console.WriteLine(dt.ToString("s", CultureInfo.InvariantCulture));  

        Console.WriteLine(DateTime.UtcNow.ToString("o"));

        Console.WriteLine(dt.ToString("MMM dd, yyyy", CultureInfo.InvariantCulture));

    }   

}

​

public static class Utils

{        

    const string DATE_FORMAT = "yyyy'/'MM'/'dd";

    const string TIME_FORMAT = "HH:mm:ss:fff";

    public static string String(this object value)

    {

        if(value?.GetType() == typeof(string))

        {

            return (string)value;

        }

        else if (value?.GetType() == typeof(decimal))

        {

            return ((decimal)value).ToDecimalString();

        }

        else if (value?.GetType() == typeof(decimal?))

        {

            return ((decimal?)value).ToDecimalString();

        }

        else if(value?.GetType() == typeof(DateTime))

        {

            return ((DateTime)value).ToDateString();

        }

        return value?.ToString();

    }

            public static string ToDateTimeString(this DateTime date)

        {

            return date.ToString(string.Format("{0} {1}", DATE_FORMAT, TIME_FORMAT), CultureInfo.InvariantCulture);

        }

​

        public static string ToDateString(this DateTime date)

        {

            return date.ToString(string.Format("{0}", DATE_FORMAT), CultureInfo.InvariantCulture);

        }

​

        public static string ToDecimalString(this decimal? value)

        {

            if (value == null) {

                Console.WriteLine("null");

                return null;

            }

            Console.WriteLine("Not null");

            return ((decimal)value).ToDecimalString();

        }

​

        public static string ToDecimalString(this decimal value)

        {

            return value.ToString("0.00####", CultureInfo.InvariantCulture);

        }

}