VBA-我可以卸载堆栈调用以避免运行时28“堆栈空间不足”错误吗？

Question

I'm working on a project that uses VBA UserForms to assist users in creating different Word documents. One of the requirements is that users are able to go back and forth between the Userforms.

This latest document has more UserForms (31 or so) than previous ones, and what I've found is that I can trigger a runtime error 28 "Out of stack space" by populating most of the forms, then navigating back to the first one, then going forward again.

When going forward, I'm doing nothing different than the first time through, so I do believe that it truly is out of stack space due to a limit on the number of calls, rather than recursive or other problem listed in the Microsoft documentation here https://msdn.microsoft.com/en-us/library/aa264523(v=vs.60).aspx .

For the record, I am unloading the forms when going back or forward. Example:

Private Sub cb_back_Click() 'backward navigation 

    Unload Me
    showPreviousForm 

End Sub

My question is, in VBA is there a way I can I "unload" a call from the stack?

It's extremely unlikely that a user would do what I did, so I'm not too concerned for this part of the project, but I would like to know in case other parts of the project are much closer to the limit.

Thanks in advance.

Answer 1

The only way to "unload" a stack frame, is to return from that stack frame... or to blow everything up.

I don't think your Unload Me does what you think it does, and even if it does work as intended, it's not going to help you with your call stack.

The problem is with calling showPreviousForm instead of returning to the caller: while the "previous form" is being displayed and until it's closed and execution runs to End Sub , then your call stack keeps track of that position inside cb_back_Click() , and until the call stack unwinds, it'll stay right where it is.

You can observe this behavior using the Call Stack debug toolwindow.

There is no programmatic way to access the call stack in VBA. Anyone that ever tried to report a stack trace for error reporting wishes it was possible.

The solution is to NOT Unload Me and store some state on your form instance that tells the caller where you need to go next; when End Sub is hit, the caller can then look at that state value and determine what to do.