What does "DoEvents" do in vb6?

Question

What does "DoEvents" do in vb6 ? Why do I get the error message "Out of stack space" ? What does it mean ?

Answer 1

DoEvents() allows other Windows messages to be processed.

The reason you get an out of stack space error is probably because DoEvents() is allowing events to occur that call your code again, which again calls DoEvents(), and so on until the stack space, which tracks the return addresses for all these calls, has run out.

In general, I do not recommend using DoEvents() due to problems like these and the fact that it violates the overall event-driven design of Windows.

Answer 2

A slightly different way of looking at DoEvents is that it flushes the events in the event queue. If your sub or function triggers an event, that event handler becomes a sub that is in line to run as soon as your sub/function is finished. DoEvents says to run that event handler sub now, instead of waiting till the end of your sub.

While I agree in spirit with Jonathon about not using DoEvents, I would temper his statement by saying I only recommend using it if you know exactly why, and know all of the repercussions of changing the order of the event queue this way. Most often, DoEvents is indicated when you want to update your screen in some way from within the context of a subroutine, before the subroutine is finished executing.

An example of this is when you are using the ProgressBar control. Suppose you are iterating through several thousand records, and want to provide feedback to the user as to how far along you are by updating a progress bar. You might interrupt your loop every hundred records and change the value on the progressbar control. However (unless you do something about it) you won't see the change on the screen until after the progressbar's change event handler runs, and that handler won't run until your sub is done executing. It will just get put in the event queue. The way to force the change event to run immediately, suspending your sub, is to call DoEvents. This will flush all existing events from the queue--in this case your progressbar's change event--and will update the progressbar control on the screen.

Now, "out of stack space" basically means that you've been caught in an endless loop of function calls. The most basic way to cause that is this:

Public sub MySub()     MySub End Sub 

And then call MySub from somewhere. You'll get an out of stack space error. If you look at the Call Stack, you'll see a very long line of calls to MySub.

A well-known real-world example of this would happen in older versions of VB:

Public Sub TextBoxArray_LostFocus(index as Integer)     If TextBoxArray(index) = "" Then         TextBoxArray(index).SetFocus         MsgBox "Please enter a value"     End If End Sub 

This situation assumes two members of a TextBox control array called TextBoxArray. Now, if the user starts with the first one (index 0) and moves to the second one (index 1) then index 0's LostFocus event will fire. However, VB would also internally set the focus to the index 1 box. Then the code would set the focus back to index 0, firing index 1's LostFocus event! You're caught in a loop. They fixed that in VB5 or 6 by waiting to set the focus until the LostFocus event was done executing.