Change properties of an unknown object in VB.NET

Question

I have a sub that handles when 14 ComboBoxes have their Index changed. I am able to cast the sender of the event, and obtain properties from there. However, after that, I want to be able to change the properties of the actual sender, rather than the cast one. How would I do this?

Current code:

Private Sub ComboBoxIndexChange(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged, ComboBox2.SelectedIndexChanged [etc]
        Dim myComboBox As ComboBox = sender

        Select Case myComboBox.Text
            Case "Will"
                Me.Controls(myComboBox.Name).Text = "555-555-555"
            Case "Bob"
                Me.Controls(myComboBox.Name).Text = "555-124-1234"
            [etc]
        End Select
    End Sub
End Class

As you can see, I am currently trying to use

Me.Controls(myComboBox.Name).Text

But I get the error: Object reference not set to an instance of an object.

What can I do?

Answer 1

ComboBox is a reference type, so if you assign directly to the Text property of the myComboBox variable, it will update the control.

Answer 2

The sender parameter in an event handler will (typically) contain a reference to the object that raised the event. In the case of the SelectedIndexChanged event of a ComboBox control, it will be the ComboBox that had its SelectedIndex property changed. So in your code sample above myComboBox is referring the ComboBox that raised the event.

To clarify: if you select an item in the drop-down list of a ComboBox control, so that it raises the SelectedIndexChanged event, the sender parameter of the event handler will be that same ComboBox, not a copy of it. This is true for all reference types.

Had it been a value type raising the event it would have been a completely different story, but that is a very rare case (it it is never the case when it comes to controls on a form).

Answer 3

This should work:

 Dim ctl As Control
 Dim cmb As ComboBox

 For Each ctl In Me.Controls
   If sender Is ctl Then
     cmb = ctl
     cmb.Text = "string"
     End If
   Next ctl

You can add the specific string assignment code in place of cmd.Text = "string". Assigning cmb allows you to use the specific combobox properties rather than the generic control properties.

Answer 4

To answer the original question of why you were getting a null object reference exception, is the Control you are searching for a direct child of the Form? Does it sit in a panel or some other container on the form? If so then I don't think you will find that control in the Form's Controls collection. So this line:

Me.Controls(myComboBox.Name).Text

Only searches through the forms direct children and not recursively through the children of those controls.

Answer 5

If I using the following code, I will get the same err: Object reference not set to an instance of an object.

Me.Controls(TBName).DataBindings.Add("Text", dt, dt.Columns(colindex).ColumnName)

Sugestion from Christian Pena fit my case, so the right code should be:

Me.TCAll.TabPages(0).Controls(TBName).DataBindings.Add("Text", dt, dt.Columns(colindex).ColumnName)