How can I assign a Variant to a Variant in VBA? - vba

Question

(Warning: Although it might look like one at first glance, this is not a beginner-level question. If you are familiar with the phrase "Let coercion" or you have ever looked into the VBA spec, please keep on reading.)
Let's say I have an expression of type Variant, and I want to assign it to a variable. Sounds easy, right?
Dim v As Variant
v = SomeMethod() ' SomeMethod has return type Variant
Unfortunately, if SomeMethod returns an Object (i.e., a Variant with a VarType of vbObject), Let coercion kicks in and v contains the "Simple data value" of the object. In other words, if SomeMethod returns a reference to a TextBox, v will contain a string.
Obviously, the solution is to use Set:
Dim v As Variant
Set v = SomeMethod()
This, unfortunately, fails if SomeMethod does not return an object, e.g. a string, yielding a Type Mismatch error.
So far, the only solution I have found is:
Dim v As Variant
If IsObject(SomeMethod()) Then
Set v = SomeMethod()
Else
v = SomeMethod()
End If
which has the unfortunate side effect of calling SomeMethod twice.
Is there a solution which does not require calling SomeMethod twice?

Answer 1

In VBA, the only way to assign a Variant to a variable where you don't know if it is an object or a primitive, is by passing it as a parameter.
If you cannot refactor your code so that the v is passed as a parameter to a Sub, Function or Let Property (despite the Let this also works on objects), you could always declare v in module scope and have a dedicated Sub solely for the purpose of save-assigning that variable:
Private v As Variant
Private Sub SetV(ByVal var As Variant)
If IsObject(var) Then
Set v = var
Else
v = var
End If
End Sub
with somewhere else calling SetV SomeMethod().
Not pretty, but it's the only way without calling SomeMethod() twice or touching its inner workings.
Edit
Ok, I mulled over this and I think I found a better solution that comes closer to what you had in mind:
Public Sub LetSet(ByRef variable As Variant, ByVal value As Variant)
If IsObject(value) Then
Set variable = value
Else
variable = value
End If
End Sub
[...] I guess there just is no LetSet v = ... statement in VBA
Now there is: LetSet v, SomeMethod()
You don't have a return value that you need to Let or Set to a variable depending of its type, instead you pass the variable that should hold the return value as first parameter by reference so that the Sub can change its value.

Answer 2

Dim v As Variant
For Each v In Array(SomeMethod())
Exit For 'Needed for v to retain it's value
Next v
'Use v here - v is now holding a value or a reference

Answer 3

You could use error trapping to reduce the expected number of method calls. First try to set. If that succeeds -- no problem. Otherwise, just assign:
Public counter As Long
Function Ambiguous(b As Boolean) As Variant
counter = counter + 1
If b Then
Set Ambiguous = ActiveSheet
Else
Ambiguous = 1
End If
End Function
Sub test()
Dim v As Variant
Dim i As Long, b As Boolean
Randomize
counter = 0
For i = 1 To 100
b = Rnd() < 0.5
On Error Resume Next
Set v = Ambiguous(b)
If Err.Number > 0 Then
Err.Clear
v = Ambiguous(b)
End If
On Error GoTo 0
Next i
Debug.Print counter / 100
End Sub
When I ran the code, the first time I got 1.55, which is less than the 2.00 you would get if you repeated the experiment but with the error-handling approach replaced by the naïve if-then-else approach you discussed in your question.
Note that the more often the function returns an object, the less function calls on average. If it almost always returns an object (e.g. that is what it is supposed to return but returns a string describing an error condition in certain cases) then this way of doing things will approach 1 call per setting/ assigning the variable. On the other hand -- if it almost always returns a primitive value then you will approach 2 calls per assignment -- in which case perhaps you should refactor your code.

Answer 4

It appears that I wasn't the only one with this issue.
The solution was given to me here.
In short:
Public Declare Sub VariantCopy Lib "oleaut32.dll" (ByRef pvargDest As Variant, ByRef pvargSrc As Variant)
Sub Main()
Dim v as Variant
VariantCopy v, SomeMethod()
end sub
It seems this is similar to the LetSet() function described in the answer, but I figured this'd be useful anyway.

Answer 5

Dim v As Variant
Dim a As Variant
a = Array(SomeMethod())
If IsObject(a(0)) Then
Set v = a(0)
Else
v = a(0)
End If