每个循环的VB.Net是怎样看待推断类型的 [英] What does VB.Net For Each Loop look at to Infer the Type

问题描述

在以下代码中，

 对于MyCollection中的每个项目
 ... 
下一个
  

编译器用于确定项目

<$ p

$ p $ 公共类BaseDataObjectGenericCollection（Of T）
继承BaseDataObjectCollection
结束类


每个循环的A依然推断Item类型为 Object 。编辑：每Beatles1692的答案，实现IEnumerator（Of T）有点作品。如何修改上面的类，以使类型推断工作？

基类已经有一个 GetEnumerator 函数，继承自 CollectionBase ，所以我的实现看起来像这样，

  Public Function GetEnumerator1（）As System.Collections.Generic.IEnumerator（Of T）Implements System.Collections.Generic.IEnumerable（Of T） .GetEnumerator 
返回MyBase.Cast（Of T）（）。GetEnumerator 
 End Function 
  

然而，for循环仍然会将该类型推断为对象。但是，如果我将接口实现更改为此，

  Public Shadows Function GetEnumerator（）As System.Collections.Generic.IEnumerator （T）实现System.Collections.Generic.IEnumerable（Of T）.GetEnumerator 
返回MyBase.Cast（Of T）（）。GetEnumerator 
 End Function 
  

这可以起作用，for循环获得正确的类型推断。所以我想现在的问题是， For Each 只需寻找一个名为 GetEnumerator ？

的函数

解决方案

好吧，只有一个地方可以解决这个问题。规范！

第10.9.3节讨论For Each语句。根据它：

[if]使用局部变量类型推断，则标识符定义一个新的局部变量，其范围是整个For循环，其类型是集合的元素类型（如果枚举器表达式类型为Object，则为Object）。

collection这里看起来很模糊，但是它在下一页精确定义。实质上，该类型必须具有GetEnumerator（）调用，并且该枚举器必须（a）具有返回布尔类型的 MoveNext（）方法，并且（b）具有当前属性。 Current属性的类型是由编译器推断的类型。请注意，它实际上与IEnumerator或IEnumerable无关......你只需要符合规定的模式。考虑下面的代码：

 选项推断
 $ b公共模块M 
 Sub Main（） 
 For Each x In New SomeClass（）
 
 Next 
 End Sub 
 End Module 
 
 Public Class SomeClass 
 Public Function GetEnumerator（）作为MyEnumerator 
返回新的MyEnumerator（）
结束函数
结束类
 
 
公共类MyEnumerator 
公共ReadOnly属性当前As整数
获取
返回42 
结束获取
结束属性
 
公共函数MoveNext（）作为布尔
返回True 
 End函数
结束类
  

Sub Main（）中的x类型是Integer ，因为Current属性返回Integer。

In the following code,

For Each item in MyCollection
...
Next

What does the compiler use to determine the type of item?

For example let say I have this class, which is inheriting a non generic collection,

Public Class BaseDataObjectGenericCollection(Of T)
    Inherits BaseDataObjectCollection
End Class

A for each loop still infers the Item type as Object. How would I have to modify the above class to make the type inference work?

Edit: Per Beatles1692's answer, Implementing IEnumerator(Of T) kinda works. The base class already has a GetEnumerator function, inherited from CollectionBase, so I my implementation looked like this,

    Public Function GetEnumerator1() As System.Collections.Generic.IEnumerator(Of T) Implements System.Collections.Generic.IEnumerable(Of T).GetEnumerator
        Return MyBase.Cast(Of T)().GetEnumerator
    End Function

However, the for loop still infers the type as object. But, if I change the interface implementation to this,

    Public Shadows Function GetEnumerator() As System.Collections.Generic.IEnumerator(Of T) Implements System.Collections.Generic.IEnumerable(Of T).GetEnumerator
        Return MyBase.Cast(Of T)().GetEnumerator
    End Function

That works, the for loop gets the type inference correct. So I guess the question is now, does For Each just look for a function called GetEnumerator ?

解决方案

Well, there's only one place to go for a question like this. The spec!

Section 10.9.3 discusses For Each statements. According to it:

[if] local variable type inference is being used, then the identifier defines a new local variable whose scope is the entire For loop and whose type is the element type of the collection (Object if the enumerator expression is typed as Object).

"collection" here seems vague, but it's precisely defined on the next page. Essentially, the type must have a GetEnumerator() call, and this enumerator must (a) have a MoveNext() method that returns a boolean type, and (b) have a Current property. The type of the Current property is the type that will be inferred by the compiler. Note it actually has nothing to do with IEnumerator or IEnumerable...you just have to fit the prescribed pattern. Consider this code:

Option Infer On

Public Module M
    Sub Main()
        For Each x In New SomeClass()

        Next
    End Sub
End Module

Public Class SomeClass
    Public Function GetEnumerator() As MyEnumerator
        Return New MyEnumerator()
    End Function
End Class


Public Class MyEnumerator
    Public ReadOnly Property Current As Integer
        Get
            Return 42
        End Get
    End Property

    Public Function MoveNext() As Boolean
        Return True
    End Function
End Class

The type of "x" in the Sub Main() is Integer, since the Current property returns Integer.

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