Question

First, I have function to flatten all controls on a Control:

Protected Function GetAllControls(Optional ownerControl As Control = Nothing) As IEnumerable(Of Control)

    Dim ret = New List(Of Control)()
    For Each child As Control In If(ownerControl, Me).Controls
        ret.AddRange(GetAllControls(child))
    Next
    ret.Add(ownerControl)
    Return ret

End Function

Then, I want to hide certain buttons on a control using this code:

Dim buttons = GetAllControls().Where(Function(c) c.Name.StartsWith("subButton"))
For Each ctrl As Control In buttons
    ctrl.Visible = False
    Debug.WriteLine("Hid button " & ctrl.Name)
Next

Yet, after four buttons – the correct count – have been hidden, I get a NullReferenceException, with VS2012 highlighting the lambda expression.

What could possibly cause this?

Answer 1

The last line in your first function adds ownerControl, which is null the first time you call it, so its adding a Nothing to the list. In your lambda you’re doing a c.Name which will throw an exception when c is Nothing.