CODEWITHSUNDEEP
vb.netinheritanceoverriding
Ikke
Ikke

Reputation: 101251

Extending ControlCollection in VB.NET

I want to extend the basic ControlCollection in VB.NET so I can just add images and text to a self-made control, and then automaticly convert them to pictureboxes and lables.

So I made a class that inherits from ControlCollection, overrided the add method, and added the functionality.

But when I run the example, it gives a NullReferenceException.

Here is the code:

        Shadows Sub add(ByVal text As String)
            Dim LB As New Label
            LB.AutoSize = True
            LB.Text = text
            MyBase.Add(LB) 'Here it gives the exception.
        End Sub

I searched on Google, and someone said that the CreateControlsInstance method needs to be overriden. So I did that, but then it gives InvalidOperationException with an innerException message of NullReferenceException.

How do I to implement this?

Upvotes: 0

Views: 1445

Answers (3)

roomaroo
roomaroo

Reputation: 5871

If you're inheriting from Control.ControlCollection then you need to provide a New method in your class. Your New method must call ControlCollection's constructor (MyBase.New) and pass it a valid parent control.

If you haven't done this correctly, the NullReferenceException will be thrown in the Add method.

This could also be causing the InvalidOperationException in your CreateControlsInstance method

The following code calls the constructor incorrectly causing the Add method to throw a NullReferenceException...

Public Class MyControlCollection
    Inherits Control.ControlCollection

    Sub New()
        'Bad - you need to pass a valid control instance
        'to the constructor
        MyBase.New(Nothing)
    End Sub

    Public Shadows Sub Add(ByVal text As String)
        Dim LB As New Label()
        LB.AutoSize = True
        LB.Text = text
        'The next line will throw a NullReferenceException
        MyBase.Add(LB)
    End Sub
End Class

Upvotes: 0

Nescio
Nescio

Reputation: 28443

Why not inherit from UserControl to define a custom control that has properties like Text and Image?

Upvotes: 3

mattlant
mattlant

Reputation: 15451

You are probably better off using just a generic collection anyways. Bieng Control Collection doesnt really do anything special for it.

puclic class MyCollection : Collection<Control>

Upvotes: 0

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