iOS NSDictionary与NSMutableDictionary差异导致的报错

报错：

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<__NSDictionary0 0x600000000de0> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key doctorCode.'

解决：

NSDictionary *params = [[NSDictionary alloc] init];

[params setValue:doctorCode forKey:@"doctorCode"];

改为

NSMutableDictionary *params = [[NSMutableDictionary alloc] init];

[params setValue:doctorCode forKey:@"doctorCode"];

bug消失，访问数据成功！！！

应该是NSDictionary不支持为未知的key赋值，只能是它原有的key才可以。

这就是基础知识的不足导致的失误！！！