具有泛型返回类型的可空引用类型 [英] Nullable reference types with generic return type

问题描述

我正在尝试使用新的 C# 8 可空引用类型功能，在重构我的代码时，我发现了这个(简化的)方法:

I'm playing around a bit with the new C# 8 nullable reference types feature, and while refactoring my code I came upon this (simplified) method:

public T Get<T>(string key)
{
    var wrapper = cacheService.Get(key);
    return wrapper.HasValue ? Deserialize<T>(wrapper) : default;
}

现在，这是一个警告

可能的空引用返回

这是合乎逻辑的，因为 default(T) 将为所有引用类型提供 null.一开始我以为我会改成下面这样:

which is logical, since default(T) will give null for all reference types. At first I thought I would change it to the following:

公共T?获取(字符串键)

但这是做不到的.它说我要么必须添加一个通用约束 where T : class 或 where T : struct.但这不是一个选项，因为它可以同时存在(我可以存储 int 或 int? 或 FooBar 的实例或任何缓存).我还读到了一个假设的新通用约束 where class? 但这似乎不起作用.

But this cannot be done. It says I either have to add a generic constraint where T : class or where T : struct. But that is not an option, as it can be both (I can store an int or int? or an instance of FooBar or whatever in the cache). I also read about a supposed new generic constraint where class? but that did not seem to work.

我能想到的唯一简单的解决方案是使用空原谅运算符更改返回语句:

The only simple solution I can think of is changing the return statement using a null forgiving operator:

return wrapper.HasValue ? Deserialize<T>(wrapper) : default!;

但这感觉不对，因为它肯定可以为空，所以我在这里基本上是在对编译器撒谎..

But that feels wrong, since it can definitely be null, so I'm basically lying to the compiler here..

我该如何解决这个问题?我在这里遗漏了一些非常明显的东西吗?

How can I fix this? Am I missing something utterly obvious here?

推荐答案

你们很亲近.只需像这样编写您的方法:

You were very close. Just write your method like this:

[return: MaybeNull]
public T Get<T>(string key)
{
    var wrapper = cacheService.Get(key);
    return wrapper.HasValue ? Deserialize<T>(wrapper) : default!;
}

您必须使用 default! 来消除警告.但是你可以用 [return: MaybeNull] 告诉编译器它应该检查 null，即使它是一个不可为 null 的类型.

You have to use the default! to get rid of the warning. But you can tell the compiler with [return: MaybeNull] that it should check for null even if it's a non-nullable type.

在这种情况下，如果开发人员使用您的方法并且不检查 null，则他可能收到警告(取决于流分析).

In that case, the dev may get a warning (depends on flow analytics) if he uses your method and does not check for null.

有关详细信息，请参阅 Microsoft 文档:指定后置条件:MaybeNull 和 NotNull

For further info, see Microsoft documentation: Specify post-conditions: MaybeNull and NotNull

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