DateTime是否可以为null ？
Any variables that are not mandatory can be null.
任何非必需的变量都可以为空。
The nillable=false means the parameter cannot be null.
nillable=false意味着参数不得为 null。
The annotated element must not be null. Accepts any type.
带注释的元素不能为空。接受任何类型。
We added the NOT NULL flag to indicate that the fields must not be null.
我们添加了NOTNULL标志来指示字段必须不能为空。
If there was a problem creating the BIO object, the pointer will be NULL.
如果创建BIO对象时遇到问题，指针将会是NULL。
Article 58 Civil ACTS in the following categories shall be null and void.
第五十八条下列民事行为无效。
Exclude columns: the name of the columns that you want to be excluded can be null.
排除列：您希望被排除的列名称可以为空。
Because top would be NULL, accessing &top is a sure-fire way to crash—clearly an avoidable bug.
因为top是NULL，访问&top 肯定会导致崩溃 —这显然是可以避免的bug。
Line 7 gets the session key from the session if it's already present, else it would be null.
如果会话已存在，第7行代码将从会话中获取会话键，否则它将为null。
For example, if an order status field is' Open ', then the shipment date should be null or blank.
例如，如果一个订单的状态字段是‘Open’，那么发货日期应该为null或空白。
So, for any columns that can potentially be null, you can just define an additional Boolean column.
因此，对于任何有可能存在null值的列，只需要定义一个额外的布尔列。
If you have strings and columns that should not be null, make sure you code your migration appropriately.
如果字符串和列都不应该为空，那么就请确保正确编写了迁移。
The second parameter (state) can either be null or an object containing application-specific information.
public void TimerCallback(objectstate);第二个参数（state）可以为空或者是包含程序规范信息的对象。
Oops! You're greeted with the following error: Property [author] of class [class Entry] cannot be null.
您将看到以下错误：Property [author]ofclass [classEntry] can notbe null。
If you want to create a column that cannot be null, you must explicitly add the parameter :null => false.
如果希望创建一个不能为空的列，您必须显式地添加参数 :null =>false。
Primary keys are the unique identifiers for each row. They must contain unique values and cannot be null.
主键是每行的唯一标识符，必须包含不重复的值，而且不能为空。
Neither a key nor a value can be null (if a null value in a dictionary is required, use "NSNull" instead).
密钥或值可能是null(如果需要字典中的null值，请使用NSNull替代)。
If the value turns out to be null, I then throw a RuntimeException to alert everyone of the potential problem.
如果其值最终为null，我就会抛出一个RuntimeException来警告他人注意这个潜在问题。
The next parameter should be null unless you want to use a custom software rasterizer , which is very unlikely.
下一个参数应该是空，除非你想使用自定义的软件光栅，这是非常不可能的。
This line is supposed to ensure that instance will only ever be null or refer to a fully constructed Singleton object.
此行应该确保instance只为null或引用一个构造完整的Singleton对象。
Most of the time this can be null, but it's possible to supply both OSGi-specific and general properties about the URLs.
大多情况下属性可以为null，但是也可以提供OSGi特定或关于URL的通用属性。
In todo_foreign, that would require two columns; the deletion date might be null to indicate that the item is still active.
在todo_foreign中，这将需要两个列；删除日期可能为null，表示该事项仍然在活动中。
If the applicant has no insurable interest to the subject matter of insurance, the insurance contract will be null and void.
保险利益决定保险合同的效力问题，投保人对保险标的不具有保险利益的，保险合同无效。
A clause assigning the benefit of insurance of the goods in favour of the carrier or any similar clause shall be null and void.
将货物的保险利益转让给承运人的条款或者类似条款，无效。
The default item class on a RepoItem is null, so the item class on all Folder objects in the example connector will also be null.
RepoItem 上的默认条目类是null，所以示例连接器中所有Folder对象上的条目类也是 null。
In such a case, the policy framework determines the compatible policy to be null and thus, invocation of the service doesn't work.
在这种情况下，策略框架判定兼容策略为null，服务的调用将不会成功。
If no comment is associated with a given purchase order object, you might expect this column to be null after persisting the entity.
如果没有任何注释与给定订单对象相关，那么您可能希望在持久化实体后将该列设为null。
If no comment is associated with a given purchase order object, you might expect this column to be null after persisting the entity.
如果没有任何注释与给定订单对象相关，那么您可能希望在持久化实体后将该列设为null。
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