怎么样:

MyEnum? val = value == null ? (MyEnum?) null : (MyEnum) value;

从装箱的 int 到 MyEnum 的转换(如果 value 为非空)，然后使用从 MyEnum 的隐式转换 到 Nullable.

The cast from boxed int to MyEnum (if value is non-null) and then use the implicit conversion from MyEnum to Nullable<MyEnum>.

没关系，因为您可以从枚举的装箱形式拆箱到其底层类型，反之亦然.

That's okay, because you're allowed to unbox from the boxed form of an enum to its underlying type, or vice versa.

我相信这实际上是一种转换，不保证 C# 规范可以正常工作，但 保证 CLI 规范可以正常工作.所以只要你在 CLI 实现上运行你的 C# 代码(你将是 :)，你会没事的.

I believe this is actually a conversion which isn't guaranteed to work by the C# spec, but is guaranteed to work by the CLI spec. So as long as you're running your C# code on a CLI implementation (which you will be :) you'll be fine.