WPF: DatePicker

获取DatePicker：date_Start上显示的时间date_Start.SelectedDate

比较两个dateTime类型的时间：

DateTime StartDate =Convert.ToDateTime(date_Start.SelectedDate);
DateTime EndDate = date_End.DisplayDate;

//if (DateTime.Compare(StartDate, EndDate)>0)

//或
if (StartDate>EndDate)
{
block_Instruction.Text = "选择错误：起始时间在结束时间之后。";
}