理解可能的空引用 return

Question

我开始在 VS19 预览版中收到这些警告，

<TargetFramework>net5.0</TargetFramework>
<LangVersion>preview</LangVersion>`
<Nullable>enable</Nullable>

这是一个来自 class 的示例，它将它的许多方法代理到 ImmutableList<T>:

class C<T> {
  readonly ImmutableList<T> composed;
  public C() => composed = ImmutableList<T>.Empty;
  ...
  public T Find(Predicate<T> match) => composed.Find(match);
  ...
}

warning CS8603: Possible null reference return. for the Find method

我不明白为什么它与 ImmutableList<T>.Find 具有相同的签名？解决此问题的最佳方法是什么？

Answer 1

例如Find method will return default(T) when no match, which is null when T is a reference type. If you want to prevent that, you could constrain your class只允许值类型。

Answer 2

在the docs

中有描述

Returns

The first element that matches the conditions defined by the specified predicate, if found; otherwise, the default value for type T.

所以 return T? 或者可能使用 ?? 来 return 一个非空值。

关于相同的签名，在代码中您可以看到：

[return: MaybeNull]
public T Find(Predicate<T> match)

见the code on github

还有 docs for [MaybeNull]

Specifies that an output may be null even if the corresponding type disallows it.

理解可能的空引用 return

understanding Possible null reference return

c#

c#-8.0

nullable-reference-types

c#-9.0