Properly handling a WinForms Timer event

Jared Parsons — Mon, 27 Oct 2008 15:00:07 GMT

The WinForms Timer class allows the user to perform a particular action at a set interval. Timer objects fire a Tick event at the set time which users can easily respond to. This is very useful if a developer wants to check for a particular condition say every 2 seconds ( for the remainder of this article I'm going to use 2 seconds as a practical example even though it's really any arbitrary time period).

Occasionally users are surprised to find that the Tick event will fire much faster than they are expecting. Instead of waiting for 2 seconds between calls, they event will fire almost immediately after one is finished processing.

What's going on here is a side effect of how this event works under the hood. The interval for the timer event is calculated in real world time. So quite literally every 2 seconds Windows will consider the internal reached and will issue a new tick message. The next time a WinForms event is not executing developer code a tick event is raised [1].

So imagine we had the following code.

Private Sub OnTimerTick() Handles m_timer.Tick
    RunSomeOperation()
End Sub

Consider what happens if RunSomeOperation takes longer than 2 seconds. The Tick event is fired in real time so while we're in the middle of RunSomeOperation, another Tick event is being queued up for processing. As soon as we leave OnTimerTick we're back in WinForms code which sees a Tick event and promptly raises it which puts us right back in OnTimerTick.

This is contrary to what most people expect. Most people expect the Tick event to fire 2 seconds after their code is finished executing.

To work around this developers should stop the timer when processing a timer event. Just before exiting the event handler, re-enable the timer. This will cause Windows to start calculating the interval from the start. This has the effect of making the timer event fire 2 seconds after developer code stops executing.

Private Sub OnTimerTick() Handles m_timer.Tick
    m_timer.Stop()
    Try
        RunSomeOperation()
    Finally
        m_timer.Start()
    End Try
End Sub

[1] This is not 100% true. It's really whenever the Application begins to pump messages again. Message pumping, more specifically when it does and does not occur, is too involved for this discussion.

AutoSize and DockStyle.Fill don't mix

Jared Parsons — Wed, 21 Nov 2007 08:29:57 GMT

The title of this post essentially says it all. AutoSize and DockStyle.Fill don't mix well together. Both properties exist to describe the size relationship relative to the rest of the control but they do so in conflicting ways.

AutoSize is a property describing the size of a control relative to it's contents. Setting this to True will generally cause a control to resize itself so that it takes up only enough room to display it's contents.

DockStyle.Fill is a property describing the size of a control relative to the size of it's container. A control will resize to fit all of the empty space in it's parent container with this property set.

In effect these properties represent opposite ways of describing size. Most controls will prevent you from setting conflicting settings in the designer. However it's difficult to prevent you from setting conflicting settings between a container and it's contents.

For instance, one combination is the following. This may seem a little odd at first. It commonly happens when you are trying to embed a non-autosizing control inside a Container that supports AutoSize. For instance if you are trying to place a TextBox inside of a FlowLayoutPanel

Container

AutoSize = true
AutoSizeMode = GrowAndShrink

Containee

Dock = DockStyle.Fill

If you try this code it will often cause the Container Control to shrink to nothing. Why? In affect both controls are asking each other how big they should be and with no one having the deciding factor they agree on ... nothing.

Instead for this scenario you should leave the Containee to DockStyle.None and manually set the size.

jaredpar's WebLog : WinForms

Properly handling a WinForms Timer event

AutoSize and DockStyle.Fill don't mix