具有泛型的VB类型运算符 [英] VB typeof operator with generics

问题描述

在VB中比较类型时，以下方式可以正常工作，并且可以将当前实例与特定的继承类进行比较，在这种情况下返回 False （从 LINQPad ）

  Sub Main 
 Dim a As New MyOtherChildClass 
 
 a.IsType（）。Dump（）
 End Sub 
 
'在这里定义其他方法和类
 MustInherit类MyBaseClass 
 Public Function IsType（）As Boolean 
返回TypeOf Me MyChildClass 
 End Function 
 End Class 
 
类MyChildClass 
继承MyBaseClass 
结束类
 
类MyOtherChildClass 
继承MyBaseClass 
结束类
  

但是，当引入泛型时，VB编译器会失败并返回错误类型为'UserQuery.MyBaseClass（Of T）'的表达式永远不能是'UserQuery.MyChildClass '。

<$
MustInherit类MyBaseClass（Of T）
公共函数IsType（）作为布尔
返回TypeOf Me MyChildClass
结束函数
结束类

类MyChildClass
继承MyBaseClass（Of String）
结束类

类MyOtherChildClass
继承MyBaseClass（Of String）
End Class

C＃ / strong>编译并允许比较，返回正确的结果

$ b

  void Main（）
 {
 var a = new MyOtherChildClass（）; 
 
 a.IsType（）。Dump（）; 
} 
 
 //在这里定义其他方法和类
 
抽象类MyBaseClass< T> 
 {
 public bool IsType（）
 {
 return this MyChildClass; 
} 
} 
 
 class MyChildClass：MyBaseClass< string> 
 {
} 
 
 class MyOtherChildClass：MyBaseClass< string> 
 {
} 
  

为什么VB编译器不允许这种比较？

解决方案

您提出了一个有关VB / C＃编译的有趣观点，我无法真正了解。如果您正在寻找解决方案，可以通过如何识别泛型类？这个问题来实现。

定义这些函数：


$ b

  Public Function IsSubclassOf（ByVal childType As Type，ByVal parentType As Type）As Boolean 
 Dim isParentGeneric As Boolean = parentType.IsGenericType 
 
返回IsSubclassOf（childType，parentType ，isParentGeneric）
 End Function 
 
 Private Function IsSubclassOf（ByVal childType As Type，ByVal parentType As Type，ByVal isParentGeneric As Boolean）As Boolean 
如果childType是Nothing那么
返回False 
 End If 
 
如果isParentGeneric AndAlso childType.IsGenericType Then 
 childType = childType.GetGenericTypeDefinition（）
 End If 
 
 If childType是parentType Then 
返回True 
 End If 
 
返回IsSubclassOf（childType.BaseType，parentType，isParentGeneric）
 End Function 
  

像这样调用：
$ b

Dim baseType As Type = GetType（MyBaseClass（Of））
Dim typeType As Type = GetType（MyOtherChildClass）

Console.WriteLine（IsSubclassOf（childType，baseType））
'写入：True


以下是 Microsoft Connect Ticket 可能会解决这个问题并给出一些解释，一个功能或一般的键入错误。

尽管这种情况似乎并不支持 类型 文档说明对于类，typeof将在以下情况下返回true：

$ b

objectexpression 类型 typename 或从 typename继承

When comparing types in VB the following works as expected and enables the current instance to be compared against a specific inherited class, in this case returning False (snippet from LINQPad)

Sub Main
    Dim a As New MyOtherChildClass

    a.IsType().Dump()
End Sub

' Define other methods and classes here
MustInherit class MyBaseClass
    Public Function IsType() As Boolean
        Return TypeOf Me Is MyChildClass
    End Function
End Class

Class MyChildClass 
    Inherits MyBaseClass
End Class

Class MyOtherChildClass
    Inherits MyBaseClass
End Class

However when generics are introduced the VB compiler fails with the error Expression of type 'UserQuery.MyBaseClass(Of T)' can never be of type 'UserQuery.MyChildClass'.

' Define other methods and classes here
MustInherit class MyBaseClass(Of T)
    Public Function IsType() As Boolean
        Return TypeOf Me Is MyChildClass
    End Function
End Class

Class MyChildClass 
    Inherits MyBaseClass(Of String)
End Class

Class MyOtherChildClass
    Inherits MyBaseClass(Of String)
End Class

The equivalent code in C# compiles and allows the comparison, returning the correct result

void Main()
{
    var a = new MyOtherChildClass();

    a.IsType().Dump();
}

// Define other methods and classes here

abstract class MyBaseClass<T>
{
    public bool IsType()
    {
        return this is MyChildClass;
    }
}

class MyChildClass : MyBaseClass<string>
{
}

class MyOtherChildClass : MyBaseClass<string>
{
}

Why does the VB compiler not allow this comparison?

解决方案

You raise an interesting point about VB/C# compilation that I can't really speak to. If you're looking for a solution, here's a way to do it from the question How can I recognize a generic class?

Define these functions:

Public Function IsSubclassOf(ByVal childType As Type, ByVal parentType As Type) As Boolean
    Dim isParentGeneric As Boolean = parentType.IsGenericType

    Return IsSubclassOf(childType, parentType, isParentGeneric)
End Function

Private Function IsSubclassOf(ByVal childType As Type, ByVal parentType As Type, ByVal isParentGeneric As Boolean) As Boolean
    If childType Is Nothing Then
        Return False
    End If

    If isParentGeneric AndAlso childType.IsGenericType Then
        childType = childType.GetGenericTypeDefinition()
    End If

    If childType Is parentType Then
        Return True
    End If

    Return IsSubclassOf(childType.BaseType, parentType, isParentGeneric)
End Function

Call like this:

Dim baseType As Type = GetType(MyBaseClass(Of ))
Dim childType As Type = GetType(MyOtherChildClass)

Console.WriteLine(IsSubclassOf(childType, baseType))
'Writes: True

Here's a Microsoft Connect Ticket that might deal with this issue and give some explanation as to whether this was a feature or a bug of generic typing.

Although this case doesn't seem supported by the Type Of documentation which states that for classes, typeof will return true if:

objectexpression is of type typename or inherits from typename

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