Vb.net how to create an array of controls with different types i.e textbox, combobox, checkbox...based on certain value

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This is the code:

VB

Public Function TestControl(c As Integer())
        Dim a As Control()
        Dim l As Point
        Dim F As New Form
        For i As Integer = 0 To c.Length - 1
            If c(i) Then
                a(i) = New TextBox
            Else
                a(i) = New ComboBox
            End If
            a(i).Width = 400
            a(i).Height = 40
            l.Y = 50 + (3 * i - 2) * a(i).Height / 2
            l.X = 150
            a(i).Location = l
            F.Controls.Add(a(i))
        Next
        F.Show()
        Return Nothing
   End Function

When I am trying to execute the Function the following Error appears:

Variable 'a' is used before it has been assigned a value. A null reference exception could result at runtime.

What I have tried:

I am trying to solve it but i couldn't succeed. any Idea would very helpful for me

Thanks

Posted 31-Jul-17 21:05pm

YahyaSh

Updated 31-Jul-17 21:32pm

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3 solutions

Solution 1

Here is an example of working with groups of objects - Controls in this instance...

VB

Private controls As List(Of Control) = New List(Of Control)

Sub AddControls()

    controls.Add(New PictureBox)
    controls.Add(New TextBox)
    controls.Add(New Label)

End Sub

Posted 31-Jul-17 21:12pm

Graeme_Grant

Solution 3

Well, the error message is clear. You declared the array

Quote:
Dim a As Control()

but never created it.

The statement

VB

a = new Control(c.Length){}

is missing in your code.

In any case, I would follow Graeme_Grant suggestion and use instead a dynamic container, like the List.

Posted 31-Jul-17 21:32pm

CPallini

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Ralf Meier · Accepted Answer · 2017-07-31T21:17:00

You dim a as Array of Control with no Dimension. That ist the Prior mistake.
But for what do you need this Array ? It's enough to instance your control and add it to the Form's Control-collection.

To correct your code (which has the wrong approach in the moment) I would like to know something more about the Goal ...

Additional after clarification :

VB

Public Sub CreateControls (c As Integer())
        Dim l As Point

        For i As Integer = 0 To c.Length - 1
            l.Y = 50 + (3 * i - 2) * 40  ' your Control-Height
            l.X = 150

            If c(i) = ??? Then  ' <- add the right value for comparing here
                dim myTB as New TextBox
                myTB.Width = 400
                myTB.Height = 40
                myTB.Location = l
                myTB.Name = "TextBox"+i.toString  ' <- important for later accessing
                me.Controls.Add(myTB)

            Else  ' if c(i)= ???
                dim myCB as New ComboBox
                myCB.Width = 400
                myCB.Height = 40
                myCB.Location = l
                myCB.Name = "ComboBox"+i.toString  ' <- important for later accessing
                me.Controls.Add(myCB)
            End If
        Next
   End Sub