An alternative way is to use DateTime.DaysInMonth to get the number of days in the current month as suggested by @Jade
Since we know the first day of the month will always 1 we can use it as default for the first day with the current Month & year as current.year,current.Month,1.
var now = DateTime.Now; // get the current DateTime
//Get the number of days in the current month
int daysInMonth = DateTime.DaysInMonth (now.Year, now.Month);
//First day of the month is always 1
var firstDay = new DateTime(now.Year,now.Month,1);
//Last day will be similar to the number of days calculated above
var lastDay = new DateTime(now.Year,now.Month,daysInMonth);
//So
rdpStartDate.SelectedDate = firstDay;
rdpEndDate.SelectedDate = lastDay;