C# 枚举：可空或“未知”价值？
如果我有一个带有 enum 成员的类，并且我希望能够表示未定义该成员的情况，那么哪个更好？
a) 使用可为空类型在类中将成员声明为可为空。例如：
public SomeEnum? myEnum;
b) 向枚举添加默认的“未知”值。例如：
public enum SomeEnum {
Unknown,
SomeValueA,
SomeValueB,
SomeValueC,
}
我真的看不出任何主要的优点/缺点；但也许其中一个比另一个更好？
If I have a class with an enum member and I want to be able to represent situations where this member is not defined, which is it better?
a) Declare the member as nullable in the class using nullable types. E.g.:
public SomeEnum? myEnum;
b) Add a default, 'unknown' value to the enumeration. E.g.:
public enum SomeEnum {
Unknown,
SomeValueA,
SomeValueB,
SomeValueC,
}
I can't really see any major pros/cons either way; but perhaps one is preferable over the other?
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评论（6）
绝对使用可为空的值类型 - 这就是它们的用途。它明确地表明了您的意图。这也意味着您可以使用
Enum.IsDefined（如果您想要泛型类型安全，则可以使用 Unconstrained Melody 中的等效项）轻松确定特定值是否为真实值，而不必担心“假”值。Definitely use a nullable value type - that's what they're for. It explicitly states your intention. It also means you can use
Enum.IsDefined(or the equivalent from Unconstrained Melody if you want generic type safety) to easily determine whether a particular value is a real value without worrying about the "fake" one too.我同意 archimed7592 的观点，即缺席值和“未知”值之间存在真正的区别。
例如：
“你的血型是什么？”
Null = 缺少值 -->问题尚未得到解答 -->提出问题
“未知”-->病人表示他不知道 -->订购血型实验室测试
I agree with archimed7592 that there is a real difference between absent value and "Unknown" value.
Example:
"what is your blood type?"
Null = absent value --> question hasn't been answered --> ask the question
"Unknown" --> patient indicates he doesn't know --> order lab test for the blood type
您只需决定是否需要一个值来表示未知值，或者确实需要一种方法来表示任何值的缺失。
在需要表示未知值的情况下，额外的枚举成员听起来是一个不错的解决方案。
如果需要表示不存在任何值，请将其设为可为空。
请记住，“未知”枚举成员和枚举本身同时可为空并没有什么问题。
HTH。
You just have to decide whether you need a value to represent unknown values, or you do need a way to represent absence of any value.
In the case of a need to represent unknown values, an extra enum-member sounds good as a solution.
In case of a need to represent absence of any value, make it nullable.
Have in mind, that there is nothing wrong with having both "unknown" enum-member and the enum itself being nullable at the same time.
HTH.
视情况而定！
Bill Wagner 列出了一些在有意义时添加未定义
枚举的充分理由。我建议您找到完整的项目，但这里是预览：现在，我遇到一种情况，用户需要从组合框中选择某种类型，或者他们可以不选择任何类型。我使用带有
[Description("a description")]属性的枚举作为要选择的对象。如果“none/Unknown”是枚举的自然选择，那么我将用它来表示没有选择任何内容。否则，我将使用Nullable。IT DEPENDS!
Bill Wagner has listed some good reasons to add an Undefined
enumwhen it makes sense. I suggest that you find the full item, but here's a preview:Now, I have a situation where the users need to select some type from a ComboBox, or they can have no type selected. I am using an enum with a
[Description("a description")]attribute as the object to be selected. If none/Unknown is a natural choice for an enum, then I would use that to represent that nothing is selected. Otherwise, I will use aNullable<MyEnum>.如果它可以为空，您将把它作为默认值，如果您尝试在空时使用它，您会得到异常（这是一件好事！）
If it's nullable, you'd get it as a default value, you'd get exceptions if you tried to use it when null (which is a good thing!)
如果枚举具有
[Flags]属性，则一切都会改变。Everything changes if the enum is attributed with
[Flags]Attribute.