首页 \ 问答 \ 为什么使用块不能安全地初始化var?(Why can a use block not safely initialize a var?)

为什么使用块不能安全地初始化var?(Why can a use block not safely initialize a var?)

为什么会出现编译错误?

val autoClosable = MyAutoClosable()
var myVar: MyType
autoClosable.use {
    myVar= it.foo()
}
println(myVar) // Error: Variable 'myVar' must be initialized

也许编译器只看到{ myVar= it.foo() }作为一个传递给另一个函数的函数,并且不知道何时甚至是否会被执行?

但是,由于use不仅仅是一个功能,而是Kotlin替代Java的资源尝试,一些关于它的特殊知识将是合适的,不是吗? 现在,我被迫用一些虚拟值初始化myVar ,这根本不符合Kotlin的精神。


Why does this give a compile error?

val autoClosable = MyAutoClosable()
var myVar: MyType
autoClosable.use {
    myVar= it.foo()
}
println(myVar) // Error: Variable 'myVar' must be initialized

Maybe the compiler just sees { myVar= it.foo() } as a function that is passed to another function and has no knowledge about when or even if it will be executed?

But since use is not just a function but Kotlin's replacement for Java's try-with-resource, some special knowledge about it would be appropriate, wouldn't it? Right now, I am forced to initialize myVar with some dummy value, which is not in the spirit of Kotlin at all.


原文:https://stackoverflow.com/questions/45238746
更新时间:2023-05-07 10:05

最满意答案

当你说:-

var _db = new PortOfTroyCustomers.Models.PortOfTroyContext();

编译器推断赋值右侧的表达式类型,这在C#中称为隐式类型。

现在,您正在尝试分配,这样的查询,我们右边有System.Data.Entity.DbSet ,左边有System.Linq.IQueryable ,它们是不同的类型: -

IQueryable<Accommodation> query = _db.Accommodations;

因此,您需要一个明确的类型转换: -

IQueryable<Accommodation> query = (IQueryable<Accommodation>)_db.Accommodations;

When you say:-

var _db = new PortOfTroyCustomers.Models.PortOfTroyContext();

The compiler infers the type of the expression to the right of the assignment, this is known as implicit type in C#.

Now, you are trying to assign, your query like this, we have System.Data.Entity.DbSet on right & System.Linq.IQueryable on left which are different types:-

IQueryable<Accommodation> query = _db.Accommodations;

Thus, you need an explicit typecast like this:-

IQueryable<Accommodation> query = (IQueryable<Accommodation>)_db.Accommodations;

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