[英]Different object's type in VB.net and C#
I have 2 codes in vb.net and C# ( I think they are identical ) : 我在vb.net和C#中有2个代码(我认为它们是相同的):
C# C#
class ChildClass
{ }
class MyClass
{
public ICollection<ChildClass> Children { get; set; }
public MyClass()
{
Children = new HashSet<ChildClass>() { new ChildClass() };
}
}
T e1<T>(T entity)
where T : class, new()
{
T copy = new T();
return copy;
}
void Test()
{
MyClass myClass = new MyClass();
dynamic children = typeof(MyClass).GetProperty("Children").GetValue(myClass);
dynamic child = Enumerable.ElementAt(children, 0);
dynamic copy = e1(child);
}
VB.NET VB.NET
Public Class ChildClass
End Class
Public Class MyClass
Public Property Children As ICollection(Of ChildClass) = New HashSet(Of ChildClass)
End Class
Public Function e1(Of T As {Class, New})(entity As T) As T
Dim clone As New T()
Return clone
End Function
Sub Main()
Dim someClass = New MyClass
someClass.Children.Add(New ChildClass)
Dim el = Enumerable.ElementAt(CallByName(someClass, "Children", CallType.Get), 0
Dim el3 = CTypeDynamic(el, GetType(ChildClass))
Dim copy = e1(el3)
End Sub
Now the last line of each code ( where the E1 function is used ) is producing a different object type : 现在,每个代码的最后一行(使用E1函数的位置)正在产生不同的对象类型:
in c# ---- the copy has ChildClass type 在C#中- 副本具有ChildClass类型
in vb.net .... the copy has Object type 在vb.net中.... 副本具有对象类型
What should I do in order that the vb.net code to produce a ChildClass type object ? 为了使vb.net代码生成ChildClass类型的对象,我应该怎么做?
Thank you ! 谢谢 !
The problem is that the dynamic
type in C# tries to mimic the behaviour of the value if the compiler knew its type. 问题在于,如果编译器知道值的类型,则C#中的dynamic
类型会尝试模仿该值的行为。 Therefore, passing it to a method infers the type arguments from its type at run-time. 因此,将其传递给方法会在运行时从其类型推断出类型实参。 VB.NET doesn't do such things, el3
is of type Object
. VB.NET不会做这些事情, el3
是Object
类型的。 One way you can solve it is to remove the generic parameters: 解决问题的一种方法是删除通用参数:
Public Function e1(entity As Object) As Object
Dim clone = Activator.CreateInstance(entity.GetType())
Return clone
End Function
I know that you have already accepted an answer, but, for completeness, I'd still like to answer your question "How can I do this in VB.NET" ? 我知道您已经接受了答案,但是出于完整性考虑,我仍然想回答您的问题“如何在VB.NET中做到这一点” ?
The reason for the behaviour is that generics are a compile-time feature . 出现这种情况的原因是泛型是编译时功能 。 At compile-type, the type of el3
is Object
, so T
is resolved as Object
. 在compile-type处, el3
的类型为Object
,因此T
解析为Object
。 The same would happen if you declared child
as object
in C#. 如果在C#中将child
声明为object
,也会发生同样的情况。
Since VB.NET does not have a dedicated late-binding type ( Object
serves as both a regular type as well as a late-binding type if Option Strict is Off), you will have to resort to reflection for run-time generics resolution. 由于VB.NET没有专用的后期绑定类型(如果Option Strict为Off,则Object
既充当常规类型又充当后期绑定类型),因此对于运行时泛型解析,您将不得不依靠反射。 Basically, you can replace 基本上,您可以更换
Dim copy = e1(el3)
with 同
Dim copy = Me.GetType().GetMethod("e1").MakeGenericMethod(el3.GetType()).Invoke(Me, {el3})
which yields the same result as your C# code. 产生的结果与您的C#代码相同。
Side note: This is actually how dynamic dispatch was commonly done in C# before the introduction of the dynamic
keyword. 旁注:实际上,这是在引入dynamic
关键字之前在C#中通常进行动态分配的方式。
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