Form.Load event triggered every time

Question

I was wondering if I am doing correctly.

I instantiate a Form (let's call this Form_B) within my class (also a form) and handle Form_B's Load event. Within this event I do some initialization.

Form_B can be displayed by the user multiple times, and I call ShowDialog on my instance variable.

The problem is that the Load is called each time I show the form. I tried debugging and also tried with Show() instead of ShowDialog(). Show() fails as I closed the window but ShowDialog() does not fail, but calls Load every time it is displayed.

Is it incorrect to continue using the instance once the form is closed?

Thanks, Stefan

Answer 1

Using the Load event to initialize a form is an anachronism from the VB6 days. It was really important back then, that unfortunately carried over in the design of the Winforms designer. It made Load the default event for a form.

That is however not the .NET way, you initialize a class object with the constructor. The only time you need to override OnLoad() (another .NET way, events are for code in other classes) is when you care about the size and position of the form. It won't be the design Size and Location when the user changed the Windows theme or runs the video adapter at a higher DPI setting. So you might want to use OnLoad to move the window or rearrange the controls. Not actually a very common thing to do.

So, fix your problem first by using the constructor instead. If you still need OnLoad then just use a bool flag that keeps track of whether or not it already ran.

    private bool initialized = false;

    protected override void OnLoad(EventArgs e) {
        if (!initialized) {
            initialized = true;
            // etc...
        }
        base.OnLoad(e);
    }

And yes, this only works if you use ShowDialog(). A form that's displayed with Show() automatically disposes itself when it is closed. That doesn't happen with ShowDialog() to avoid problems retrieving the dialog results. Re-creating the dialog instance is the better way, unless you really care about keeping the last entered values. That's however a really expensive way to do so, form objects take a lot of .NET and Windows resources.