How to convert PearsiandCalender to GregorianCalendar?

I found, Thanks my friend. 

All the Calendars have a ToDateTime() method which we used above to convert directly from the Persian to the Gregorian calendars because the DateTime structure is based on the latter.

Even the GregorianCalendar class itself has a ToDateTime() method (inherited from Calendar) but it doesn't have a ToPersianCalendar() method as it would be unwieldy to have to provide direct conversion methods for all other calendars.

You therefore have to create a PersianCalendar object and use the methods in that to convert back from Gregorian:

     PersianCalendar pCalendar = new PersianCalendar();
     DateTime dt = DateTime.Today; // uses Gregorian
     int year = pCalendar.GetYear(dt);
     int month = pCalendar.GetMonth(dt);
     int day = pCalendar.GetDayOfMonth(dt); 
     Console.WriteLine("{0}/{1}/{2}", month, day, year); // 8/2/1392

any time i want to convert Ger... to Per... i have to use PersianCalendar, but now I want to convert Per... to Gre.... !!!!!!!!!!!!!!!!!!!!!!!!
I couldn't found, Why???????????
Could you tell me a little more? 

It's because the DateTime structure is based on the Gregorian calendar and you always need to go through this to convert to some other calendar (Persian, Japanese, Korean etc).

You therefore need to create an object of the other calendar's type to expose the methods needed to do the conversion.

Thanks a million dear friend,
when i want to convert Pea... to Gre... I use PersianCalender, Why when i want to revers it, I have to use PersianCalendder again?

Try: