using System.Globalization;
public static void Main()
DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
Calendar cal = dfi.Calendar;
int startWeek = cal.GetWeekOfYear(new DateTime(2015, 3, 1), dfi.CalendarWeekRule, DayOfWeek.Monday);
int endWeek = cal.GetWeekOfYear(new DateTime(2015, 3, 31), dfi.CalendarWeekRule, DayOfWeek.Monday);
int numberOfWeeks = 1 + endWeek - startWeek;
Console.WriteLine(numberOfWeeks);