VBA: Conditional - Is Nothing

问题:

There is an If condition in a VBA application as seen below:

If Not My_Object Is Nothing Then
My_Object.Compute

When the code is run in debug mode, I found that the If condition returns a true even when My_Object has "No Variables".

Could somebody please explain this? I want My_Object.Compute to be executed only when My_Object exists.

回答1:

Based on your comment to Issun:

Thanks for the explanation. In my case, The object is declared and created prior to the If condition. So, How do I use If condition to check for < No Variables> ? In other words, I do not want to execute My_Object.Compute if My_Object has < No Variables>

You need to check one of the properties of the object. Without telling us what the object is, we cannot help you.

I did test several common objects and found that an instantiated Collection with no items added shows <No Variables> in the watch window. If your object is indeed a collection, you can check for the <No Variables> condition using the .Count property:

Sub TestObj()
Dim Obj As Object
    Set Obj = New Collection
    If Obj Is Nothing Then
        Debug.Print "Object not instantiated"
    Else
        If Obj.Count = 0 Then
            Debug.Print "<No Variables> (ie, no items added to the collection)"
        Else
            Debug.Print "Object instantiated and at least one item added"
        End If
    End If
End Sub

---

It is also worth noting that if you declare any object As New then the Is Nothing check becomes useless. The reason is that when you declare an object As New then it gets created automatically when it is first called, even if the first time you call it is to see if it exists!

Dim MyObject As New Collection
If MyObject Is Nothing Then  ' <--- This check always returns False

This does not seem to be the cause of your specific problem. But, since others may find this question through a Google search, I wanted to include it because it is a common beginner mistake.

回答2:

Just becuase your class object has no variables does not mean that it is nothing. Declaring and object and creating an object are two different things. Look and see if you are setting/creating the object.

Take for instance the dictionary object - just because it contains no variables does not mean it has not been created.

Sub test()

Dim dict As Object
Set dict = CreateObject("scripting.dictionary")

If Not dict Is Nothing Then
    MsgBox "Dict is something!"  '<--- This shows
Else
    MsgBox "Dict is nothing!"
End If

End Sub

However if you declare an object but never create it, it's nothing.

Sub test()

Dim temp As Object

If Not temp Is Nothing Then
    MsgBox "Temp is something!"
Else
    MsgBox "Temp is nothing!" '<---- This shows
End If

End Sub

回答3:

In my sample code, I was setting my object to nothing, and I couldn't get the "not" part of the if statement to work with the object. I tried if My_Object is not nothing and also if not My_Object is nothing. It may be just a syntax thing I can't figure out but I didn't have time to mess around, so I did a little workaround like this:

if My_Object is Nothing Then
    'do nothing
Else
    'Do something
End if