Delay loading of combobox when form loads

Question

I've got a Windows Forms (C#) project with multiple comboboxes/listboxes etc that are populated when the form loads.

The problem is that the loading of the comboboxes/listboxes is slow, and since the loading is done when the form is trying to display the entire form isn't shown until all the controls have been populated. This can in some circumstances be 20+ seconds.

Had there been a Form_finished_loaded type of event I could have put my code in there, but I can't find an event that is fired after the form is done drawing the basic controls.

I have one requirement though - the loading has to be done in the main thread (since I get the items from a non-threading friendly COM-application).

I have found one potential solution, but perhaps there is a better way? I can create a System.Timer.Timer when creating the form, and have the first Tick be called about 1 second later, and then populate the lists from that tick. That gives the form enough time to be displayed before it starts filling the lists.

Does anyone have any other tips on how to delay the loading of the controls?

Answer 1

Can you get your data from a web service that calls the COM component?

That way, you can display empty controls on a Locked form at the start, make Asynchronous calls to get the data, and on return populate the respective combos, and once all of them are loaded, you can unlock the form for the user to use.

Answer 2

You could listen for the VisibleChanged event and the first time it's value is true you put your initialization code.

Answer 3

It has that certain smell of workaround, but this approach should fulfil your needs:

private bool _hasInitialized = false;
private void Form1_Shown(object sender, EventArgs e)
{    
    if (!_hasInitialized)
    {
        ThreadPool.QueueUserWorkItem(state =>
        {
            Thread.Sleep(200); // brief sleep to allow the main thread
                               // to paint the form nicely
            this.Invoke((Action)delegate { LoadData(); });    
        });
    }
}

private void LoadData()
{
    // do the data loading
    _hasInitialized = true;
}

What it does is that it reacts when the form is shown, checks if it has already been initialized before, and if not it spawns a thread that will wait for a brief moment before calling the LoadData method on the main thread. This will allow for the form to get painted properly. The samething could perhaps be achieve by simply calling this.Refresh() but I like the idea of letting the system decide how to do the work.

I would still try to push the data loading onto a worker thread, invoking back on the main thread for populating the UI (if it is at all possible with the COM component).

Answer 4

Isn't FormShown the event you're looking for?

Answer 5

When you say that you cannot use a background thread because of COM what do you mean? I am using many COM components within my apps and running them on background threads.

If you create a new thread as an STAThread you can probably load the ComboBox/ListBox on a Non-UI thread. IIRC the ThreadPool allocates worker threads as MTAThread so you'll need to actually create a thread manually instead of using ThreadPool.QueueUserWorkItem.

Answer 6

There is the Shown event that "occurs whenever the form is first displayed.". Also you may want to use the BeginUpdate and EndUpdate functions to make the populating of your combobox faster.