Form gets disposed on Hide()

Question

I have a C# WinForms application running on .NET Framework 4.0.

When the user is inactive for a certain period of time, I want it to hide all the displayed forms and show an icon in the notification area. When the user clicks that icon, a login form appears and if the credentials are valid, it opens the exact forms that were open before.

To do this, I store the list of open forms in a List of Form objects and hide them, like this. This method is called by a Timer:

private void LogOut()
{
    foreach (Form form in Application.OpenForms)
        if (form.Visible)
        {
            GlobalVariables.formList.Add(form);
            form.Hide();
        }
}

When the credentials are validated, I try to make the forms visible again, like this:

//Show the previous forms.
foreach (Form form in GlobalVariables.formList)
    form.Visible = true;

//Clear the forms list.
GlobalVariables.formList.Clear();

If I only have the MainForm open when I hide the forms, it shows it back fine when logging back in. If I have any other forms open (which are opened using ShowDialog() from the MainForm), the program will crash on form.Visible = true; and give me the following error message:

ObjectDisposedException was unhandled
Cannot access a disposed object

How can I fix this problem? An alternative way of doing what I'm trying to achieve would also be great.

Please note that using a try - catch block to determine if the form has been disposed and just relaunch the form is not an option as the user may have unsaved input in the hidden forms.

I couldn't manage to find anything related online in over 3 hours of search so any help would be much appreciated!

EDIT: After trying various things, I have noted that the problem only occurs on forms I have opened forms using ShowDialog(). If I only have forms opened using Show(), everything works fine.

However in my case, using Show() is not an option because I cannot have the user click on things in the parent form. Hiding the parent form is not an option either as he needs to see information in the parent form.

How can you open two modal forms from your main? If you open the first you have to close it to open the next one — γηράσκω δ' αεί πολλά διδασκόμε, Feb 16 '14 at 15:18
@ispiro, I was setting the Opacity as mentioned in that post. However, after removing all the calls to .Opacity, the problem still occurs. I am not using any of the other properties. — Vlad Schnakovszki, Feb 16 '14 at 15:30
I am not opening two at a time. The user can open one and then launch another one from the one he opened and so on. — Vlad Schnakovszki, Feb 16 '14 at 15:31
When the user closes the modal form do you call .close method? — γηράσκω δ' αεί πολλά διδασκόμε, Feb 16 '14 at 16:01
@valter, either I do this.Close() or if he clicks the X box, the system does it automatically. — Vlad Schnakovszki, Feb 16 '14 at 16:08
Nop. When the user clicks the X box the form "goes" to hide mode. You have to call close to dispose the form. So a user opens a form. Timer hits amd because it is visible is added to list. Then the form is closed (calling .close), form disposed but you have it on the list. — γηράσκω δ' αεί πολλά διδασκόμε, Feb 16 '14 at 16:14
You are correct about this behavior in modal forms. However, this problem occurs even without clicking any X buttons or closing anything. — Vlad Schnakovszki, Feb 16 '14 at 16:22
I see you've solved your problem. But see my edit to my answer - You are actually also doing something I assume you don't mean to do. — ispiro, Feb 16 '14 at 17:59

score 6 · Accepted Answer · answered Feb 16 '14 at 18:08

Clearly hiding a form is more impactful than you counted on. Your code was involved in a security review that Microsoft conducted on Winforms. Very thorough, not often visible in the way it behaves but very visible in the source code. One rule is imposes is that a user should never lose control over the application.

A dialog is very troublesome that way. The core problem is that ShowDialog() creates a modal window that disables all the other windows. That creates an opportunity for malware, very easy to take advantage of, all it has to do is hide a dialog and you snookered the user. There isn't any way that the user can gain control of the app again. The one window that was enabled is hidden with no way for the user to re-activate it again. All the other windows are disabled so trying to click on them, or their taskbar button, will not have any effect. All that's left is for the user to use Task Manager to kill the app. And if the user account is locked down then that's not an option either.

I can hear you sputter by now: "But, but, it is my code that hides the dialog, not malware!" That's not the way it works in Windows, there's no way to tell that it actually was your code that did it. Not only because it could be injected code, it doesn't even have to be code that runs in your process. Any code can do it, it is part of the winapi.

So there's a specific counter-measure against this built into Winforms, it will automatically close a form if it is hidden while operating in dialog mode. Which of course has a big impact, code that was written after the ShowDialog() call will now run. Anything is possible, but a sure-fire mishap in your case is that this disposes another window and an attempt to revive it will die.

The rough guidance here is that you are doing it wrong. You are trying to build a security system on top of one that's already highly secure and heavily tested. And it is very risky, handling passwords yourself is a very good way to make the overall system much less secure. The average user will of course favor picking the same password as he used to login to Windows. Makes it much easier for an attacker to harvest that password.

Call LockWorkStation() instead.

score 2 · Answer 2 · edited May 23 '17 at 11:57

2

Testing, it seems that Hide()ing a modal dialog - closes it. It actually triggers the FormClosing event.

Tested like this: (Also, see this answer.)

private void button1_Click(object sender, EventArgs e)
{
    Form1 f1 = new Form1();
    f1.ShowDialog();
}

private void button2_Click(object sender, EventArgs e)
{
    Hide();
}

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    MessageBox.Show("Closing");
}

EDIT

I noticed this actually doesn't solve the mystery, just adds more information. Here's another piece of information: When you're setting Visible to true - you're not showing it modal again. This time it's equivalent to Show().

edited May 23 '17 at 11:57

Community

1
1

answered Feb 16 '14 at 17:27

ispiro

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A good observation which is on the right path to the actual problem so +1 for that! Also, I have noticed that making it visible again is the equivalent of Show() rather than ShowDialog() as before so I'll now have to figure out another way to implement everything. Even though you are right with both observations, I have already figured it out. Nevertheless, the fair thing would be to mark your answer as the accepted one since it would have definitely led me on the right path had I been busier today! :) – Vlad Schnakovszki Feb 16 '14 at 18:01
1

Thanks. But feel free to accept Hans' answer instead for the useful information (though it seems that `LockWorkStation` is not exactly what you're after) or your own answer (+1) which was the actual solution. I've learned new things today! – ispiro Feb 16 '14 at 21:17

score 2 · Answer 3 · answered Feb 16 '14 at 17:56

After many hours of trial and error, I found out what the problem was.

My understanding of modal forms was that code would continue executing in the parent form only after the modal form was closed. In fact, the specification found on MSDN states:

A modal form or dialog box must be closed or hidden before you can continue working with the rest of the application.

This introduced a subtle bug in the way I handled the forms. This is the code I used to display the forms:

using (var theForm = new CreateInvoice())
{
    theForm.ShowDialog();

    if (theForm.Updated)
    {
        GetInvoiceStatus();
    }
}

The using statement disposes of theForm as soon as the statement exits. Normally, this works perfectly fine as it would be called only when the user closes theForm. However, because ShowDialog() permits the parent form to continue its work when it is hidden, this meant that the code actually exited the using statement, which effectively disposed of theForm, resulting in my error.

score 0 · Answer 4 · edited May 23 '17 at 12:29

One suggestion to the design: instead of saving forms(views), you should save the data that the form holds(Model) and destroy the form. When you need the form again, create it back with the data(model). First, this can resolve this mysterious dispose problem, second, each form will need GDI resource, which are limited, if two many forms, you'll encounter the memory and GDI problems.

As how to do this, please refer the MVC or MVP design pattern.

BTW my guess on this problem: when you make the form visible, it will try find its parent, but its parent may be already disposed. I encountered this problem once, it throws the object disposed exception.

Form gets disposed on Hide()

4 Answers4