c# - WinForms programming - Modal and Non-Modal forms problem

Question

I have a problem with modality of the forms under C#.NET. Let's say I have main form #0 (see the image below). This form represents main application form, where user can perform various operations. However, from time to time, there is a need to open additional non-modal form to perform additional main application functionality supporting tasks. Let's say this is form #1 in the image. On this #1 form there might be opened few additional modal forms on top of each other (#2 form in the image), and at the end, there is a progress dialog showing a long operation progress and status, which might take from few minutes up to few hours. The problem is that the main form #0 is not responsive until you close all modal forms (#2 in the image). I need that the main form #0 would be operational in this situation. However, if you open a non-modal form in form #2, you can operate with both modal #2 form and newly created non modal form. I need the same behavior between the main form #0 and form #1 with all its child forms. Is it possible? Or am I doing something wrong? Maybe there is some kind of workaround, I really would not like to change all ShowDialog calls to Show...

Image http://img225.imageshack.us/img225/1075/modalnonmodalproblem.png

Answer 1

Modal forms do exactly what "modal" means, they disable all other windows in the app. That's rather important, your program is in a somewhat perilous state. You've got a chunk of code that is waiting for the dialog to close. Really Bad Things could happen if those other windows were not disabled. Like the user could start the modal dialog again, now your code is nested twice. Or she could close the owner window of the dialog, now it suddenly disappears.

These are the exact kind of problems you'd run into if you call Application.DoEvents() inside a loop. Which is one way to get a form to behave modal without disabling other windows. For example:

    Form2 mDialog;

    private void button1_Click(object sender, EventArgs e) {
        mDialog = new Form2();
        mDialog.FormClosed += (o, ea) => mDialog = null;
        mDialog.Show(this);
        while (mDialog != null) Application.DoEvents();
    }

This is dangerous.

It is certainly best to use modal forms the way they were designed to stay out of trouble. If you don't want a modal form then simply don't make it modal, use the Show() method. Subscribe to its FormClosing event to know that it is about to close:

    private void button1_Click(object sender, EventArgs e) {
        var frm = new Form2();
        frm.FormClosing += new FormClosingEventHandler(frm_FormClosing);
        frm.Show();
    }

    void frm_FormClosing(object sender, FormClosingEventArgs e) {
        var frm = sender as Form2;
        // Do something with <frm>
        //...
    }