In effect, such a null value is a type of data column overloading, and in this context, a null value might be understood to represent some aspect of the data.
A general, good practice is not to allow null (that is, no value) columns in a relational database, because when you attempt to read such a column, you get a null value exception.
What if someone inadvertently passes in a null value for the Class parameter?
如果有人不经意地为Class参数传入一个null值会怎么样呢?
If the corresponding class, field, or method is not generic, a null value is passed when calling the method.
若相应的类、字段或方法非泛型,则在调用方法时将返回null值。
A filter function takes as input the unmodified data, and returns modified data (or in some cases, a null value to indicate the data should be deleted or disregarded).
The default model binder also cannot assign a null value to a property that does not accept nulls. In particular, the model binder cannot assign a null value to the UnitsInStock property.