What's the difference between Dim As New vs Dim / Set

Question

In VBA, I can create objects one of two ways:

\'First way
Dim myCol1 As New Collection

\'Second way
Dim myCol2 As Col         


        

Answer 1

The As New construct has legitimate uses. In a class, with a module level variable when you do not know which method will be called first then it saves some lines of code. So some code snippet I have lying around in a class is given here

Option Explicit

Private mdicQueryStringParams As New Scripting.Dictionary

Function SafeItem(ByVal sKey As String, ByRef pvItem As Variant) As Boolean

    If mdicQueryStringParams.Exists(sKey) Then
        pvItem = mdicQueryStringParams.Item(sKey)
        SafeItem = True
    End If

End Function

Imagine very many more methods that rely upon an initialised mdicQueryStringParams. You'd have to write guard code to ensure that it was created in all of these methods.

Now at this point you're saying but you can use Sub Class_Initialize to New up upon class creation. Like this

Private Sub Class_Initialize()
    Set mdicQueryStringParams = New Scripting.Dictionary
End Sub

But suppose I want to recycle/reset part of the class's state then I could write a Clear method which sets mdicQueryStringParams to Nothing . In this case Sub Class_Initialise won't run again. Here on SO Mat's Mug has taught me that Static classes are possible in VBA (thanks!) so sometimes Sub Class_Initialise will only run once.

(^ Admittedly I could set it to a New instance in the Clear method, yes, yes, I know, I know)

The point is with the As New syntax you get a resurrecting variable as well as auto-initialisation. Surely as developers this is another technique/pattern in our toolbox that we should exploit and not ban.

In truth, I use it infrequently but I just don't like banning stuff.