无法找出此NSDate组件错误

Question

我有以下语句:

NSDate *Today = [NSDate dateWithTimeIntervalSinceNow:0];
NSDate  *cDate =  [Lines objectAtIndex:(NSUInteger) 0];
NSLog(@"cDate: %@  Today: %@", cDate, Today); 
NSCalendar *calendar = [NSCalendar currentCalendar];
NSInteger desiredComponents = (NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit);
NSDateComponents *secondComponents = [calendar components:desiredComponents fromDate:Today];
NSDateComponents *firstComponents  = [calendar components:desiredComponents fromDate:cDate]; 

NSLog语句打印:cDate: 2011-08-16 18:35:29 +0000 Today: 2011-08-17 15:53:19 +0000

最后一个语句(firstComponents)与前一个语句(secondComponents)非常相似，它崩溃并出现以下错误:无法识别的选择器发送到实例

谢谢你的帮助

Answer 1

检查cDate是否真的是NSDate的实例。例如，它也可以是NSString实例，其值为"2011-08-16 18:35:29 +0000"。