为什么New（）在我继承的控件上触发两次？ （的WinForms）

Question

Step 1 : Create inhered control class

Public Class Test_Control
    Inherits ListBox

    Public Sub New()
        Items.Add("test")
    End Sub
End Class

Step 2 : Drag class to form in the designer

Step 3 : Run the project

Result :

Why is this happening?! I am completely stumped here.. I have googled and googled and I cannot find any solution or answer to this.

This is causing some major issues for me. I am simply trying to add an initial "Select one..." option to every newly created Combobox. Same thing happens with every inherited control class, regardless of control type (textbox/combobox/listbox/etc).

Same thing happens if I use a message box within New(). Two message boxes appear as soon as I run my application.

Answer 1

You need to tell the designer to not serialize the items collection:

Public Class Test_Control
  Inherits ListBox

  Public Sub New()
    Items.Add("test")
  End Sub

  <DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)> _
  Public Shadows ReadOnly Property Items As ListBox.ObjectCollection
    Get
      Return MyBase.Items
    End Get
  End Property
End Class

As far as the two message boxes go, MessageBoxes are just not a good debugging tool. You are probably getting the WinForms designer calling new while the runtime calling new, too (or something like that).

Answer 2

The first test is from the designer and you are adding a second one in the constructor.

Either remove the test from the designer or clear the items in the constructor before adding, like this:

Items.Clear()