When must I set a variable to "Nothing" in VB6?

@Matt Dillard - Did setting these to nothing fix your memory leak?

VB6 doesn't have a formal garbage collector, more along the lines of what @Konrad Rudolph said.

Actually calling unload on your forms seems to me to be the best way to ensure that the main form is cleaned up and that each subform cleans up their actions.

I tested this with a blank project and two blank forms.

Private Sub Form_Load()
  Dim frm As Form2
  Set frm = New Form2
  frm.Show
  Set frm = Nothing
End Sub

After running both forms are left visible. setting frm to nothing did well... nothing.

After settign frm to nothing, the only handle open to this form is via the reference.

Unload Forms(1)

Am I seeing the problem correctly?

• Josh

Actually, VB6 implements RAII just like C++ meaning that locally declared references automatically get set to Nothing at the end of a block. Similarly, it should automatically reset member class variables after executing Class_Terminate. However, there have been several reports that this is not done reliably. I don't remember any rigorous test but it has always been best practice to reset member variables manually.

Objects in VB have reference counting. This means that an object keeps a count of how many other object variables hold a reference to it. When there are no references to the object, the object is garbage collected (eventually). This process is part of the COM specification.

Usually, when a locally instantiated object goes out of scope (i.e. exits the sub), its reference count goes down by one, in other words the variable referencing the object is destroyed. So in most instances you won't need to explicitly set an object equal to Nothing on exiting a Sub.

In all other instances you must explicitly set an object variable to Nothing, in order to decrease its reference count (by one). Setting an object variable to Nothing, will not necessarily destroy the object, you must set ALL references to Nothing. This problem can become particularly acute with recursive data structures.

Another gotcha, is when using the New keyword in an object variable declaration. An object is only created on first use, not at the point where the New keyword is used. Using the New keyword in the declaration will re-create the object on first use every time its reference count goes to zero. So setting an object to Nothing may destroy it, but the object will automatically be recreated if referenced again. Ideally you should not declare using the New keyword, but by using the New operator which doesn't have this resurrection behaviour.