Inspection for use of variable Is Null

[ThunderFrame]

Runtime errors when code uses:

If varX Is Null Then

'or

If varX = Null Then

Should almost always be:

If IsNull(varX) Then

[retailcoder]

When are Is Null and = Null ever valid? Can we somehow skip the legit cases?

[comintern]

AFAIK, Is Null should always result in an Object required error. Any comparison with Null will always result in Null. I'd say it really should be 2 inspections though. The first is a problem because Null isn't an object. The second is a problem because the conditional expression is meaningless in the same way as If foo = foo is.

[ThunderFrame]

Same applies for:

If varX Is Empty Then

'but this isn't a runtime error (unless varX Is Nothing), but it is a logical error:

If varX = Empty Then

[retailcoder]

I can totally see this happening:

Condition 'varX = Null' is always false

[ThunderFrame]

A StackOverflow user got tripped on this:

https://stackoverflow.com/questions/47429509/excel-vba-check-if-number-is-null-doesnt-work-as-expected

Sub testNull()
    Dim a As Variant
    Dim b As Double

    a = Null
    b = 0

    If a = Null Then
        MsgBox (a & "null") 'doesn't print
    ElseIf a <> Null Then
        MsgBox (a & " not null") 'doesn't print
    Else
        MsgBox (b & " don't know") 'prints
    End If

    If b = Null Then
        MsgBox (b & "null") 'doesn't print
    ElseIf b <> Null Then
        MsgBox (b & " not null") 'doesn't print
    Else
        MsgBox (b & " don't know") 'prints
    End If

End Sub