开发中从后台请求数据，返回如下：

2014-12-05 16:44:52.535 掌麦[6984:613] getDefaultAddress: reuslt == {
    item =     {
        data =http://www.mamicode.com/ {
            address = "<null>";
            area = "<null>";
            city = "<null>";
            name = "";
            phone = 18503051318;
            province = "<null>";
        };
        mark = 1;
        msg = "\U6210\U529f";
    };
    status = 1;
}

我直接解析出字典dic，赋值：

<span style="font-size:18px;">    NSString *address = [dic objectForKey:@"address"];
    //NSString *area = [dic objectForKey:@"area"];
    NSString *name = [dic objectForKey:@"name"];
    //NSString *city = [dic objectForKey:@"city"];
    NSString *phone = [dic objectForKey:@"phone"];
    //NSString *province = [dic objectForKey:@"province"];    self.addressLabel.text = address;    self.nameLabel.text    = name;    self.phoneLabel.text   = phone;</span>

原因及时后台返回有些值为null，在object－c中就是nsull的空值

所以解析后，应加一判断，防止出错

<span style="font-size:18px;">    if (![address isKindOfClass:[NSNull class]]) {
        self.addressLabel.text = address;
    }
    if (![name isKindOfClass:[NSNull class]]) {
        self.nameLabel.text    = name;
    }
    if (![phone isKindOfClass:[NSNull class]]) {
        self.phoneLabel.text   = phone;
    }
</span>

CRASH: -[NSNull length]: unrecognized selector sent to instance错误的处理办法