处理泛型时VB.Net中的多态

问题描述

我有一个名为Modify的函数.就像这样:

I have a function called Modify. It is delcared like so:

Public Function Modify(Of SIMType As {New, DAOBase})(ByVal obj As DAOBase) As Boolean

您可以看到此功能是通用的.它以对象为参数，该对象是DAOBase或DAOBase的子类.

You can see that this function is generic. It takes as a paramer a object that is a DAOBase or subclasses of DAOBase.

在Modify函数内部有一个调用，如下所示:

Inside the modify function there is a call like so:

DAOToGP(obj)

这就是多态性发挥作用的地方.我创建了四个左右的DAOBase子类.我为每种类型写了DAOToGP().因此，在Modify()函数中，当它调用DAOToGP(obj)时，应该引入多态性，并且应该根据我传递给Modify()的类型来调用DAOToGP()的正确实现.

This is where the polymorphism comes into play. There are four or so subclasses I have created of DAOBase. I have written a DAOToGP() for each of these types. So in the Modify() function, when it calls the DAOToGP(obj), polymorphism should kick in and it should call the correct implementation of DAOToGP() depending on the type that I pass into Modify().

但是，出现以下错误:

Error   20  Overload resolution failed because no accessible 'DAOToGP' can be called without a narrowing conversion:
'Public Shared Function DAOToGP(distributor As Distributors) As Microsoft.Dynamics.GP.Vendor': Argument matching parameter 'distributor' narrows from 'SierraLib.DAOBase' to 'IMS.Distributors'.
'Public Shared Function DAOToGP(product As Products) As Microsoft.Dynamics.GP.SalesItem': Argument matching parameter 'product' narrows from 'SierraLib.DAOBase' to 'IMS.Products'. C:\Users\dvargo.SIERRAWOWIRES\Documents\Visual Studio 2010\Projects\SIM\Dev_2\SIM\IMS\DVSIMLib\GP\GPSIMRunner\Runners\RunnerBase.vb 66  39  IMS

我有点不知所措.我不知道为什么它不能弄清楚要调用哪个函数.有什么想法吗?

I am kind of at a loss here. I am not sure why it cant figure out which function to call. Any ideas?

推荐答案

您必须将obj指定为SIMType而不是DAOBase:

You have to specify obj as SIMType instead of DAOBase:

Public Function Modify(Of SIMType As {New, DAOBase})(ByVal obj As SIMType) As Boolean

否则，您的通用类型参数将毫无用处.

Otherwise your generic type parameter would be useless.

您的DAOToGP函数具有不同的签名，并且显然不是从基类派生的.试试这个:

Your DAOToGP function have different signatures and are apparently not derived from a base class. Try this:

Public Class DAOBase(Of Tin, Tout)
    Public Function DAOToGP(ByVal obj As Tin) As Tout
    End Function
End Class

Public Module Test_DAOBase
    Public Function Modify(Of Tin, Tout)(ByVal obj As DAOBase(Of Tin, Tout)) As Boolean
    End Function
End Module

---

您还可以将DAOBAse声明为抽象类(MustInherit)，将DAOToGP声明为抽象函数(MustOverride):

---

You could also declare DAOBAse as an abstract class (MustInherit) and DAOToGP as an abstract function (MustOverride):

Public MustInherit Class DAOBase(Of Tin As {New}, Tout)
    Public MustOverride Function DAOToGP(ByVal obj As Tin) As Tout
End Class

Public Class DAOProduct
    Inherits DAOBase(Of Products, SalesItem)

    Public Overrides Function DAOToGP(ByVal obj As Products) As SalesItem
        Return Nothing
    End Function
End Class

Public Module Test_DAOBase
    Public Function Modify(Of Tin As {New}, Tout)(ByVal obj As DAOBase(Of Tin, Tout)) As Boolean
        Dim out As Tout = obj.DAOToGP(New Tin())     'This is OK
    End Function

    Public Sub TestModify()
        Dim daoProd = New DAOProduct()
        Modify(daoProd)  'This is OK
    End Sub
End Module

然后为参数类型的不同组合(在我的示例中为DAOProduct)声明从DAOBase继承的不同类.

Then declare different classes inheriting from DAOBase for the different combinations of parameter types (DAOProduct in my example).

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