为什么该代码给出“可能的空引用返回"?编译器警告? [英] Why does this code give a "Possible null reference return" compiler warning?

问题描述

考虑以下代码:

using System;

#nullable enable

namespace Demo
{
    public sealed class TestClass
    {
        public string Test()
        {
            bool isNull = _test == null;

            if (isNull)
                return "";
            else
                return _test; // !!!
        }

        readonly string _test = "";
    }
}

构建此文件时，标有!!!的行会发出编译器警告:warning CS8603: Possible null reference return..

When I build this, the line marked with !!! gives a compiler warning: warning CS8603: Possible null reference return..

鉴于_test是只读的并且已初始化为非null，因此我对此感到有些困惑.

I find this a little confusing, given that _test is readonly and initialised to non-null.

如果我将代码更改为以下内容，则警告消失:

If I change the code to the following, the warning goes away:

        public string Test()
        {
            // bool isNull = _test == null;

            if (_test == null)
                return "";
            else
                return _test;
        }

任何人都可以解释这种行为吗?

Can anyone explain this behaviour?

推荐答案

可为空的流分析会跟踪变量的空状态，但不会跟踪其他状态，例如bool变量(如上面的isNull一样)，它不会跟踪单独变量(例如isNull和_test)的状态之间的关系.

The nullable flow analysis tracks the null state of variables, but it does not track other state, such as the value of a bool variable (as isNull above), and it does not track the relationship between the state of separate variables (e.g. isNull and _test).

一个实际的静态分析引擎可能会做这些事情，但是在某种程度上也可能是启发式"或任意性"的:您不一定要告诉它遵循的规则，而且这些规则甚至可能随着时间而改变.

An actual static analysis engine would probably do those things, but would also be "heuristic" or "arbitrary" to some degree: you couldn't necessarily tell the rules it was following, and those rules might even change over time.

这不是我们可以直接在C#编译器中执行的操作.可空警告的规则非常复杂(正如乔恩的分析所示！)，但是它们是规则，并且可以进行推理.

That's not something we can do directly in the C# compiler. The rules for nullable warnings are quite sophisticated (as Jon's analysis shows!), but they are rules, and can be reasoned about.

当我们推出该功能时，感觉好像我们大多数时候已经达到了适当的平衡，但是确实有些尴尬的地方出现了，我们将在C#9.0中进行重新讨论.

As we roll out the feature it feels like we mostly struck the right balance, but there are a few places that do come up as awkward, and we'll be revisiting those for C# 9.0.

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