how to find the code(or other thing) that close form automatically

To have control on form closing event you should just manage this event in your code, so you have to add an event handler for "FormClosing", then put your code for management the closing there.

So to not allow the clossing in some conditions you should do like this:

private void YourForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (yourDontCloseCondtion)
            {
                e.Cancel = true;
                return;
            }
}

Hi,

I'm assuming that you have two forms in your project MainForm and DialogForm. Main form shows DialogForm after some button is clicked:

private void btnOpenDialogForm_Click(object sender, EventArgs e)
{
    using (var dialogForm = new FrmDialogForm())
    {
        if (dialogForm.ShowDialog() == DialogResult.OK)
        {
            // Code if user clicks btnSave on DialogForm
        }                
    }
}

On your DialogForm you have two buttons (btnSave and btnCancel). Those buttons has set properties:

btnSave.DialogResult = DialogResult.OK
btnCancel.DialogResult = DialogResult.Cancel

And your DialogForm has set properties:

DialogForm.AcceptButton = btnSave
DialogForm.CancelButton = btnCancel

You have some code for btnSave ClickEvent:

private void btnSave_Click(object sender, EventArgs e)
{
    // Code for saving form data
}

DialogForm is automatically closed when one of buttons is clicked (btnSave or btnCancel) after buttons click code is executed, because your form is showed as Dialog window.

If you want to prevent auto closing form you should set properties of your DialogForm:

DialogForm.AcceptButton = (none)
DialogForm.CancelButton = (none)

And buttons properties:

btnSave.DialogResult = DialogResult.None
btnCancel.DialogResult = DialogResult.None

But your form will still close (and returns DialogResult.Cancel) when user click WindowClose button (upper right corner of your form 'X').

You can control your DialogForm close using FormClosing event:

private void FrmDialogForm_FormClosing(object sender, FormClosingEventArgs e)
{
    // if you want to prevent form closing set:
    e.Cancel = true;
}

Few links:
http://msdn.microsoft.com/en-us/library/39wcs2dh%28v=vs.110%29.aspx[^]
http://msdn.microsoft.com/en-us/library/c7ykbedk%28v=vs.110%29.aspx[^]
http://msdn.microsoft.com/library/system.windows.forms.form.closing%28v=vs.110%29.aspx[^]

I hope it helps you :)