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 (OfT)的任何类型都不允许是可选的。
Note the values for name, data type, default value and Nullable.
请注意字段名、数据类型、默认值和Nullable。
If the return type of the method is nullable, the distinguished null value for the type is returned.
如果该方法的返回类型可以为空，则返回类型的突出默认空值。
If the return type is non-nullable, the default CLR value for the type is returned.
如果返回类型可以为非空型，则返回类型的默认clr值。
If the return type is non-nullable, the default CLR value for the type is returned.
如果返回类型可以为非空型，则返回类型的默认clr值。
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