Nullables的隐式界面演员 [英] Implicit Interface casts of Nullables

问题描述

使用VB的 Option Strict On ，为什么 Nullable（Of T）不需要显式转换为T的界面，当它需要一个 T ？

With VB's Option Strict On, why does a Nullable(Of T) not require an explicit cast to an interface of T when it does require one to T?

即

Dim x As Integer? = 5
Dim y As Integer
Dim z As IComparable

y = x ' Fails to compile with error
      ' "Option Strict On disallows implicit conversions from 'Integer?' to 'Integer'."
z = x ' Succeeds

编辑：由@SSS显示为（某种），部分答案是 Nullable 值是否可以为空，并且可以 Nothing ，这对参考来说没问题像一个界面。因此转换将始终成功，不像转换为 T 的情况（当 Nullable 没有值时失败）所以它可以看作是一种隐含的转换。

As (sort of) shown by @SSS, part of the answer is that Nullable values are, well, nullable, and can be Nothing, which is fine for a reference like an interface. So this conversion will always succeed, unlike the conversion to T case (which fails when the Nullable has no value), and so it can be seen as an implicit conversion.

我的问题现在变成了怎么样？。如何从 Nullable（Of T）（它没有自己的接口）到 T 理论上协商了吗？

My question now becomes "how?". How is the conversion from a Nullable(Of T) (which has no interfaces of its own) to an interface of T theoretically negotiated?

我知道实现是框Nullable< T> ，它有效地剥离了 Nullable 包装器，但我在这里确认了这个概念......

I know the implementation is box Nullable<T>, which effectively strips the Nullable wrapper, but I'm confirming the concept here...

（所以我会查看文档并查看是否他们解释了这一点。）

(So I'll review the documentation and see if they explain this.)

推荐答案

如果没有实际阅读文档，我将尝试回答：

Without actually reading documentation yet, I'm going to attempt an answer:

首先，更高级别的答案是向界面投射 Nullable 是安全的并且不会抛出，所以它是逻辑上是加宽运算符，不应该是显式的（与 T 的转换相比，当 .HasValue 是 False 它会抛出，所以它不应该隐含 Option Strict On ）。

Firstly, the higher-level answer is that casting a Nullable to an interface is "safe" and will not throw, so it is logically a Widening operator and should not need to be explicit (compared to casting to T, when .HasValue is False it throws, so it should not be implicit with Option Strict On).

然而，从技术上讲，如何是bi t obscure：尽管 Nullable 的某些行为是通过反射提供的元数据中编码的，但它的大部分魔力都隐藏在：

However, technically the "how" is a bit obscure: Even though some of the behaviour of Nullable is encoded in the metadata available via reflection, much of its "magic" is hidden in:

• 框的运行时行为 Nullable （因此编译器知道何时离开提升，并且


• Eric Lippert在他的回答C＃及其在VB.NET中的等价物。

• the runtime behaviour of box on a Nullable (and thus the compiler knows when to leave "lifting" to that), and
• the other points made by Eric Lippert in his answer for C# and their equivalent in VB.NET.

它看起来像是 S. Somasegar的博客文章宣布在VS2k5的后期测试版中对 Nullable 支持的更改也与此相关。

It looks like S. Somasegar's blog post announcing changes to Nullable support in a late beta release for VS2k5 is also relevant here.

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