(转)在VB.NET中处理构造函数时值得注意的两个陈述

在VB.NET中处理构造函数时值得注意的两个陈述

　　在VB.NET中处理构造函数时，有两个陈述值得注意：

　　(1)如果类型不包含任何实例构造函数声明，则VB.NET提供默认的构造函数；
　　默认参数调用直接基类的无参数构造函数。如果基类没有可访问的无参数构造函数，则发生编译期错误。

　　(2)构造函数不能继承；

　　陈述（1）包含两个方面的信息：①是如果类型不包含任何实例构造函数声明，则VB.NET提供默认的构造函数，即下面的声明是允许的，在类Shape中我们没有显式声明任何构造函数，但是我们仍然可以通过VB.NET自动提供的无参数构造函数New()来实例化类；

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ModuleTest

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PublicSubMain()

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Dimshape1AsShape=NewShape

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EndSub

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EndModule

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Public ClassShape

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PublicSubDraw()

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'somecodeshere.

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EndSub

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EndClass

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　　②是如果类型中包含有实例构造函数的声明，那么VB.NET不再提供默认的构造函数，例如，我们在类Shape中添加了一个带参数的构造函数，

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Public ClassShape

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Private_ptAsPoint

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PublicSubNew(pAsPoint)

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_pt=p

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EndSub

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PublicSubDraw()

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'somecodeshere.

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EndSub

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EndClass

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　　那么，如果直接实例化类时，则会产生错误；

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Dim shape1 As Shape = New Shape

　　只能通过调用Shape类中显式声明的带参数的构造函数来实例化类，如下，

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Dim shape1 As Shape = New Shape( New Point( 0 , 0 ))

　　因此，如果要在类中同时使用无参数和带参数的构造函数，必须同时显式地声明两种构造函数。

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Public ClassShape

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Private_ptAsPoint

PublicSubNew()

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'somecodeshere.

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EndSub

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PublicSubNew(pAsPoint)

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_pt=p

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EndSub

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PublicSubDraw()

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'somecodeshere.

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EndSub

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EndClass

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　　陈述（２），构造函数不能继承，即，如果要使子类与基类有相同的构造函数，那么必须在每个子类中都显式的声明与基类构造相同的构造函数，例如：

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Public MustInherit ClassShape

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Private_ptAsPoint

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PublicSubNew()

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'somecodeshere.

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EndSub

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PublicSubNew(pAsPoint)

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_pt=p

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EndSub

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PublicMustOverrideSubDraw()

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EndClass

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Public ClassLine

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InheritsShape

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PublicSubNew()

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EndSub

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PublicSubNew(pAsPoint)

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MyBase.New(p)

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PublicOverridesSubDraw()

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'somecodeshere.

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EndClass

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　　这一点非常重要而且也很有意义。因为我们往往倾向于认为子类继承了基类所有的属性和方法，这往往导致我们的程序出现编译错误。

　　综合以上两个陈述，在实际编程中应遵循一个原则：在类中，无论是基类还是子类，显式地声明所有需要的无参数或带参数构造函数。遵循这个原则，既有利于提高编译正确代码的速度，也有利于自己和其他程序员理解代码。

转自 http://www.cnblogs.com/zsi/archive/2005/02/02/100971.html

(转)在VB.NET中处理构造函数时值得注意的两个陈述

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