Null check in VB

Question

All I want to do is check if an object is null  but no matter what I do  if it compiles  it throws a NullReferenceException just trying to check  Here s what I ve done       If   Not  comp Container Is Nothing   And  Not  comp Container Components Is Nothing    Then         For i As Integer   0 To comp Container Components Count   - 1 Step 1             fixUIIn comp Container Components Item i   style          Next     End If      If   Not IsDBNull comp Container   And  Not IsDBNull comp Container Components    Then         For i As Integer   0 To comp Container Components Count   - 1 Step 1             fixUIIn comp Container Components Item i   style          Next     End If      If   Not IsNothing comp Container   And  Not IsNothing comp Container Components    Then         For i As Integer   0 To comp Container Components Count   - 1 Step 1             fixUIIn comp Container Components Item i   style          Next     End If      If   Not  comp Container Is DBNull Value   And  Not  comp Container Components Is DBNull Value    Then         For i As Integer   0 To comp Container Components Count   Step 1             fixUIIn comp Container Components Item i   style          Next     End If   I ve looked through VB books  searched several forums  and everything that SHOULD work doesn t  Sorry for asking such a remedial question  but I just need to know   Just so you know  the debugger says that the null object is comp Container

User · Answer

Your code is way more cluttered than necessary.

Replace (Not (X Is Nothing)) with X IsNot Nothing and omit the outer parentheses:

If comp.Container IsNot Nothing AndAlso comp.Container.Components IsNot Nothing Then
    For i As Integer = 0 To comp.Container.Components.Count() - 1
        fixUIIn(comp.Container.Components(i), style)
    Next
End If

Much more readable. … Also notice that I’ve removed the redundant Step 1 and the probably redundant .Item.

But (as pointed out in the comments), index-based loops are out of vogue anyway. Don’t use them unless you absolutely have to. Use For Each instead:

If comp.Container IsNot Nothing AndAlso comp.Container.Components IsNot Nothing Then
    For Each component In comp.Container.Components
        fixUIIn(component, style)
    Next
End If

User · Answer

Change your Ands to AndAlsos A standard And will test both expressions   If comp Container is Nothing  then the second expression will raise a NullReferenceException because you re accessing a property on a null object  AndAlso will short-circuit the logical evaluation   If comp Container is Nothing  then the 2nd expression will not be evaluated

[vb.net] Null check in VB

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