界面和转换，表单和面板 [英] Interfaces and Casting, Forms and Panels

问题描述

我有一个只能在特定情况下实现的控件(例如:只能添加到特定面板或窗体的ControlCollection中).为此，我重写了控件的OnParentChanged方法，以查看父级是否已实现给定的接口:

I have a control that can only be implemented in specific situations (eg: it can only be added to the ControlCollection of a specific panel or form). To achieve this, I have overridden the OnParentChanged method of the control to see if the parent has implemented a given interface:

Public Class MyControl

Private MyParentControl As IMyControlInterface

Protected Overrides Sub OnParentChanged(ByVal e As System.EventArgs)
    If MyBase.Parent IsNot Nothing Then
        Me.MyParentControl = TryCast(MyBase.Parent, IMyControlInterface)
        If Me.MyParentControl Is Nothing Then
            Throw New Exception("Control can only be added to an object that implements IMyControlInterface.")
        End If
    End If
    MyBase.OnParentChanged(e)
End Sub

End Class

我试图启用此代码，以便可以将控件从Visual Basic设计器的控件面板拖动到设计图面上.我将面板定义为:

I am trying to enable this code so that the control can be dragged from the controls panel of the Visual Basic designer to the design surface. I have defined the panel as:

Public Class MyControlPanel
    Inherits Panel
    Implements IMyControlInterface

    ...

End Class

表单是由设计人员生成的(即:表单的PartialClass扩展名)，即:

and the form is designer-generated (ie: a PartialClass extension of the form), to wit:

Public Class Form1
    Implements IMyControlInterface

    ...

End Class

由于某些原因，当我将MyControl从控件面板拖到窗体时，它会调用该异常.但是，当我将MyControlPanel拖到窗体上，然后将MyControl拖到MyControlPanel1(设计者给定的名称)上时，它将接受被拖动的控件.

我不明白为什么一个调用接受添加，而另一个则不接受.有谁对此有任何想法吗?

在此先感谢您的帮助...

For some reason, when I drag MyControl to the form from the controls panel, it invokes the exception. However, when I drag MyControlPanel to the form, and then drag MyControl onto MyControlPanel1 (the name given by the designer), it accepts the dragged control.

I do not understand why one invocation accepts the add, while the other does not. Does anyone have any idea about this?

I thank you in advance for your help...

推荐答案

请参阅本页上的答案1"，因为它仅说明了您的痛苦:http://stackoverflow.com/questions/4546311/c-windows-forms-design -time-form-constructor-not-调用 [ ^ ]

Please see "Answer 1" on this page as it just about explains your pains: http://stackoverflow.com/questions/4546311/c-windows-forms-design-time-form-constructor-not-called[^]

这种行为是设计使然.VisualStudio中的表单设计器无法实例化所设计的类，而是将其直接父级实例化.无法更改此行为.如果需要在设计期间执行某些逻辑，则可以必须创建一个单独的祖先，封装该逻辑并从中继承."

Ondrej Tucny 在stackoverflow上

"This behavior is by design. The form designer in Visual Studio cannot instantiate the class being designed — instead, it instatiates its immediate parent. There is no way to change this behavior. If you need some logic to be executed during design time, you have to create a separate ancestor encapsulating that logic and inherit from it."

Ondrej Tucny on stackoverflow

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