具有通用返回类型的可空引用类型 [英] Nullable reference types with generic return type

问题描述

我正在使用新的C#8可为空的引用类型功能，并且在重构代码时遇到了这种(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，这会给出警告Possible null reference return，这是合乎逻辑的，因为default(T)将为所有引用类型提供null.一开始我以为可以将其更改为以下内容:

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

public T? Get<T>(string key)

但这无法完成.它说我必须添加通用约束where T : class或where T : struct.但这不是一个选择，因为两者都可以(我可以在缓存中存储int或int?或FooBar的实例或其他内容). 我还读了关于所谓的新通用约束where class?的信息，但这似乎行不通.

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 null宽容运算符来更改return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!;

但是这感觉很不对，因为它肯定可以为null，所以我基本上在这里对编译器撒谎:-)

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?我在这里遗漏了一些显而易见的东西吗?

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

推荐答案

您非常亲密.只需这样编写您的方法:

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!摆脱警告.但是您可以使用[return: MaybeNull]告诉编译器，即使它是不可为null的类型，也应检查null.

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是否为空，则可能会得到警告(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.

有关更多信息，请参阅Microsoft文档:

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

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