using System;

using System.Linq;

using System.Collections.Generic;

    using System.Globalization;

    using System.IO;

    using System.Text;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

using System.Text.Json;

​

public class Program

{

    static void Main(string[] args)     

    {

        var inputs = new []{20201010, 20203030, 20200101 };

        foreach(var input in inputs){

            var result = ConvertDate(input);

            Console.WriteLine($"{input} => {(result==null? "cannot convert": ((DateTime)result).ToShortDateString())}.");

        }

    }

    static DateTime? ConvertDate(decimal? date, string format = "yyyyddMM")

    {

​

        if (date == null) return null;

​

        if (DateTime.TryParseExact(

                date.ToString(), 

                format, 

                System.Globalization.CultureInfo.InvariantCulture,

                System.Globalization.DateTimeStyles.None, 

                out DateTime result))

            return result;

​

        return null;

    }

}