Dart类型测试在局部变量和类成员变量之间的工作方式不同 [英] Dart type tests work differently between local variable and class member variables

问题描述

我在使用dart时不允许出现隐式的动态变化和强制转换，我注意到以下几点:

I am working with dart without allowing implicit dynamics and casts and I noticed the following:

在使用局部变量时，我可以对该变量进行类型检查，如果测试通过，编译器将假定我可以将该变量用作该类型:

When working with a local variable, I can use a type check on that variable and if the test passes, the compiler will just assume that I can use that variable as that type:

var emp; // set to something
if (emp is Person) {
  // The compiler infers that emp is a person within this scope
  // so it allows me to use Person's member functions and variables
  // without the need for explicit typecast
  // https://dart.dev/guides/language/language-tour#type-test-operators
  emp.firstName = 'Bob';
}

但是，如果变量是对象的成员变量，则此操作不起作用:

However, this does not work if the variable is the member variable of an object:

class SuperPerson {
  Object _emp;

  /* Various things that could be doing things with _emp here */

  void memberFun() {
    if (_emp is Person) {
      _emp.firstName = 'Bob'; // ERROR: The setter firstName is not defined for type Object.
      (_emp as Person).firstName = 'Bob'; // workaround but would like to avoid casts that could fail.
    }
  }
}

为什么会这样，我该如何克服呢?可能是因为其他线程可能在测试和使用之间更改_emp的值吗?

Why is that and how can I overcome it? Could it be because of potentially other threads changing the value of _emp in between the test and the use?

推荐答案

我忘记了我已经回答了这个问题.改为看到那个.

I had forgotten that I had already answered this question. See that one instead.

(由于此答案在编辑时已被接受，因此我无法将其删除.)

(Since this answer had already been accepted at the time of this edit, I cannot delete it.)

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