Object reference not set to an instance of an object [VB.NET]

Question

 Public Class Form1
    Private Function AllEnabled(ByVal b As Boolean) As Boolean
        For i As Integer = 0 To 2
            Dim c As CheckBox = CType(Me.Controls("CheckBox" & i.ToString), CheckBox)
            c.Enabled = b
        Next
    End Function

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Call AllEnabled(False)
     End Sub
    End Class

getting error with highlight in b at c.Enabled = b (Object reference not set to an instance of an object.)

but when i use checkbox1.enabled = b instead of c.enabled = b works fine.

so as i see the wrong not with b right ?

& how can i fix this ?

I'm guessing this falls over when i = 0 ? Do you have a CheckBox0 on your form - again I'm guessing not. Have a look at the line <br> Dim c As CheckBox = CType(Me.Controls("CheckBox" & i.ToString), CheckBox) ... c is probably Nothing — CHill60
– CHill60, Commented Jan 30, 2013 at 11:43
Just tried debugging it for you and it fails for me on the Dim c as Checkbox line... can you post the asp code for your web form too? — markp3rry
– markp3rry, Commented Jan 30, 2013 at 11:46
don't have checkbox0 but i tried with [for loop] 1 To 2 & same result — Zaid
– Zaid, Commented Jan 30, 2013 at 11:48
I mean the code from your web page - the html markup that has your controls on. — markp3rry
– markp3rry, Commented Jan 30, 2013 at 11:50

Kapil Khandelwal · Accepted Answer · 2013-01-30 11:56:33Z

2

Try this:

For Each ctl In Me.Controls
  If TypeOf ctl Is CheckBox Then
   ctl.Enabled = b
  End If
Next

answered Jan 30, 2013 at 11:56

Kapil Khandelwal

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1 Comment

Zaid Over a year ago

ops, it works the reason is the checkboxes was in group box thank u ! ^

Tim Schmelter · Accepted Answer · 2013-01-30 11:50:26Z

1

Two possible reasons. Your for-loop creates this control names:

"CheckBox0"
"CheckBox1"
"CheckBox2"

Maybe you want 1-3 or 0-1 instead.

Maybe you want to find your checkbox recursively, then you can use Find:

For i As Integer = 0 To 2
    Dim ctrl = Me.Controls.Find("CheckBox" & i.ToString, True)
    If ctrl.Length <> 0 Then
        ctrl(0).Enabled = b 'Find returns an aray' 
    End If
Next

Side-note: 2013 i would not use this VB6 style anymore:

Call AllEnabled(False)

but just

AllEnabled(False)

answered Jan 30, 2013 at 11:50

Tim Schmelter

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2 Comments

Zaid Over a year ago

Works ! , Thank You , But could u explain what does ctrl(0) mean ? why the zero

Tim Schmelter Over a year ago

@Zaid: I've commented it in my code snippet, ControlCollection.Find returns an array of controls. It is empty(Length=0) when no control was found with a given name as key. Since i assume that there is just one CheckBox with that name i've just taken the first. ctrl(0) gives me the first control in the control-array since arrays are zero based in .NET.

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