Question

I’m writing a simple application with several controls on a Windows form. I need to monitor the state of buttons (enabled/disabled) according to the state of a textbox and a listbox.

For example, when the listbox is empty, buttons Delete, Delete All and Edit are to be disabled, or when either the textbox or the listbox is empty button Forward is disabled, and so on.

So, I put the change of these properties on Application.Idle event, so it goes something like this:

    private void MainForm_Load(object sender, EventArgs e)
    {
        Application.Idle += new EventHandler(Application_Idle);
    }

    public void Application_Idle(object sender, EventArgs e)
    {
        CheckFillingFields(forwardBtn);
        CheckFillingList(deleteBtn);
        CheckFillingList(deleteAllBtn);
        CheckFillingList(editBtn);
    }

    private void CheckFillingFields(object sender)
    {
        if (questionTxt.Text == "" || answersLst.Items.Count == 0)
            (sender as Button).Enabled = false;
        else
            (sender as Button).Enabled = true;
    }

    private void CheckFillingList(object sender)
    {
        if (answersLst.Items.Count == 0)
            (sender as Button).Enabled = false;
        else
            (sender as Button).Enabled = true;
    }

So, the question is – is it acceptable to use Application.Idle in this case? Or should I make these properties dependable on user actions? (For example, when the user deletes an item from the listbox, I should check if it was the last one, and disable the corresponding buttons.)

Thanks a lot in advance, I really appreciate your help!

Answer 1

The simple answer is that, yes, the idle checking is bad and you should re-check the state of your controls on their change events, not “whenever possible”.