nullable value type 可空值类型
以上来源于: WordNet
In C# you can flag a value type as a nullable type by adding ? after the type (like int? x;).
在C#中，你可以在一个允许为空的值类型后面加上一个问号（比如，int? x;）。
If a column in the database is marked as being nullable, then the corresponding property in the data model class created by the LINQ to SQL designer will be a nullable type.
如果数据库中的一列被标志为可空，那么数据模型类相应的属性将是一个可空的类型。
Specifically, any type defined as Nullable (Of t) wasn't allowed to be optional.
特别地，定义为Nullable (Of T)的任何类型都不允许是可选的。
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