c# - 不可为空的引用类型 : why is my object considered nullable by the compiler?

Question

我有一个项目，我启用了新的 Nullable reference type feature

 <Nullable>enable</Nullable>

现在让我们考虑这段代码

public class Foo  {    }

var foo = new Foo();

编译器会考虑 foo变量可以为空( Foo? )。
这是为什么？我不明白。

现在使用 Nullable 引用类型功能启用返回的 Foo object 不应该为 null，因为它是不可为 null 的类型。

如果我希望它可以为空，我会指定它有一个 Foo?
那么为什么编译器说它是一个可为空的变量呢？

谢谢

编辑

这是我在这里描述的内容的屏幕截图。当您将鼠标悬停在 foo 上时多变的

Answer 1

在最初的实现中，foo将被推断为 Foo .

然而，人们提示这妨碍了以下事情:

string? GetThing() => ...

var result = "";
if (condition)
{
    result = GetThing();
}

如 result被推断为 string ，然后是 result = GetThing()行导致警告: GetThing()返回 string? ，如果您尝试分配 string?，则会出现警告到 string .

解决方案是推断 result作为 string? ，但编译器知道它当前不为空(它的“流状态”是“NotNull”)。

这意味着:

string? GetThing() => ...

var result = "";

// No warning, as the compiler knows that result isn't null
int l1 = result.Length; 

if (condition)
{
    result = GetThing();
}

// Warning: the compiler knows 'result' might have been re-assigned
int l2 = result.Length; 

有关工作中的流动状态的其他示例，请参阅以下内容:

string? result = GetString();
if (result == null)
    throw new Exception();

// No warning: the compiler knows that result can't be null here: if it was,
// the exception above would have been thrown
int l1 = result.Length;

string? result = GetString();

// Warning: result might be null
int l1 = result.Length; 

// No warning: the compiler knows that result can't be null here: if it was,
// the line above would have thrown
int l2 = result.Length; 

string result = "hello";
if (result == null)
    Console.WriteLine("NULL!");

// Warning: because we checked for null above, the compiler assumes that we
// know something that it doesn't, and so result might be null.
int l1 = result.Length;