VBA 检查是否设置了对象

If obj Is Nothing Then
' need to initialize obj: '
Set obj = ...
Else
' obj already set / initialized. '
End If

Or, if you prefer it the other way around:

If Not obj Is Nothing Then
' obj already set / initialized. '
Else
' need to initialize obj: '
Set obj = ...
End If

The (un)safe way to do this - if you are ok with not using option explicit - is...

Not TypeName(myObj) = "Empty"

This also handles the case if the object has not been declared. This is useful if you want to just comment out a declaration to switch off some behaviour...

Dim myObj as Object
Not TypeName(myObj) = "Empty"  '/ true, the object exists - TypeName is Object


'Dim myObj as Object
Not TypeName(myObj) = "Empty"  '/ false, the object has not been declared

This works because VBA will auto-instantiate an undeclared variable as an Empty Variant type. It eliminates the need for an auxiliary Boolean to manage the behaviour.

When using global variables it's possible to run into a situation in which the object is empty. So, the code:

If Not obj Is Nothing Then
'obj is already set
Else
'set obj
End If

produces an 'object required' error.

In this situation, the following works:

'First check it is initialized
If IsObject(obj) Then
'Then check if it is set
If Not obj Is Nothing Then
'obj is set
Else
'set obj
End If
End If