C#7编译器错误-模式匹配 [英] C# 7 Compiler Error - Pattern Matching

问题描述

由于某些原因， M1()会导致编译器错误，而执行相同操作的 M2()则不会导致错误.知道为什么吗?

For some reason, M1() causes a compiler error, while M2(), which does the same thing, causes no error. Any idea why?

使用 false == 应该与使用not运算符！相同.

Using false == should be the same as using the not operator, !.

使用未分配的局部变量"i"

Use of unassigned local variable 'i'

class Program {
    static void Main(string[] args) {
        int x = 8;

        M1(x);
        M2(x);
    }  // Main()

    public static void M1(Object obj) {
        if (false == (obj is int i))  // Causes ERROR on WriteLine
            return;

        System.Console.WriteLine(i); // Use of unassigned local variable 'i'
    }

    public static void M2(Object obj) {
        if (!(obj is int i))  // OKAY
            return;

        System.Console.WriteLine(i);
    }
} // class Program

推荐答案

此处的问题在于编译器处理"true时绝对分配"的方式.！将其反转； == false 不会.因此，对于代码:

The issue here is with the way the compiler handles "definitely assigned when true". ! inverts that; == false doesn't. So for the code:

if (!(obj is int i))
        return;

System.Console.WriteLine(i);

编译器可以推断出，如果 obj is int i 为false，则！会将其求反，因此如果 return 不是 int .因此，可以允许 i 安全地泄漏"到后续代码中.

The compiler can infer that if obj is int i is false, the ! inverts that, thus return will occur if it's not an int. Therefore i can be allowed to "leak" into subsequent code safely.

但是，相同的规则不适用于 == false .尽管在语义上与人类代码阅读者相同，但编译器将！和 == false 视为完全不同的事物.因此:

However, the same rules do not apply to == false. Whilst semantically identical to a human reader of the code, the compiler treats ! and == false as very different things. So for:

if (false == (obj is int i))

编译器冒充并认为它不知道 i 的赋值状态，从而导致错误.

the compiler baulks and takes the view it cannot know the assignment state of i, thus the error.

有关此问题的讨论，请参见对于不正确的使用未分配的局部变量"(x是T y)==假 .

For a discussion on this, please see Incorrect "Use of unassigned local variable" for (x is T y) == false.

故事的寓意:使用C#模式时，避免与 false 进行比较，并使用！.

The moral of the story: avoid comparing to false and use ! when using C# patterns.

编辑

应注意， == false 在这里不是特殊情况.任何使用 == 的方法都会使编译器无法确定在true时绝对分配".例如，以下代码编译:

It should be noted that == false is not a special case here. Any use of == removes the ability of the compiler to determine "definitely assigned when true". For example, the following code compiles:

object x = 1;
if (!(x is bool y))
    return 0;

var z = y;

但是添加一个 == true ，它将不再起作用:

But add a == true and it no longer does:

object x = 1;
if (!(x is bool y == true))
    return 0;

var z = y; // error: use of an unassigned variable

EDIT2

顺便说一句，对于使用 if(expression == false)的人，因为他们发现难以理解的 if(！expression)，您可能会想知道<一种href ="https://github.com/dotnet/csharplang/issues/882" rel ="noreferrer">语法，对于C#8，正在考虑 if！(expression) >.

Incidently, for anyone who uses if (expression == false) because they find if (!expression) difficult to read, you may be interested to know that the syntax, if !(expression) is being considered for C# 8.

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