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 ?

Answer 1

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

1. “CheckBox0”
2. “CheckBox1”
3. “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)