Dispose form after closing

Question

I have got the new problem with opening and closing form in C#.

My problem is how to dispose the form after closing .

here is my code :

Program.cs:

static class Program
{
    public static Timer timer;

    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);

        timer = new Timer { Interval = 1000};
        timer.Start();

        Application.Run(new Form1());
    }
}

Form1.cs:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Form2 form = new Form2();
        form.ShowDialog();
       /// I've tried Dispose() method . but didn't work
    }
}

Form2.cs:

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
    }

    private void Form2_Load(object sender, EventArgs e)
    {
        Program.timer.Tick += timer_Tick;    
        Close();
        // I've tried Dispose() method instead of Close() but didn't work
    }

    private int count = 0; 
    void timer_Tick(object sender, EventArgs e)
    {
        count++;
        if (count == 5) MessageBox.Show("");
    }
}

Edited : My question is : why the message box shows after 5 seconds when the form2 has closed!

Answer 1

Edit: This question turns out to be about Dispose.

Firstly, Dispose has noting to do with garbage collection. The following happens:

1. You have a global Timer instance
2. You create form2
3. Form2 subscribes to the timer
4. Form2 is Closed and/or Disposed
5. The Timer event fires, increments the counter and shows a MessageBox
6. The Timer event keeps firing until the App closes.

The main point to understand is that Close/Dispose only change the status of the Form, they don't (can't) 'delete' the instance. So the (closed) form is there, the counter field is still there and the Event fires.

---

OK, part 1:

A using () {} block would be better but this should work:

    private void button1_Click(object sender, EventArgs e)
    {
        Form2 form = new Form2();
        form.ShowDialog();
       /// I've tried Dispose() method . but didn't work
        form.Dispose(); // should work
    }

If not, please describe "doesn't work".

---

    private void Form2_Load(object sender, EventArgs e)
    {
        Program.timer.Tick += timer_Tick;    
        Close();
       /// I've tried Dispose() method instead of Close() . but didn't work
    }

This is strange, but I'll assume that it is artifical code for the question.

Your global Program.Timer now stores a reference to your Form2 instance and will keep it from being collected. It does not prevent it from being Disposed/Close so your timer will keep firing for a Closed Form, and that will usually fail and cause other problems.

1. Don't do this (give Form2 it's own timer)
2. Use a FormClosed event to unsubscribe: Program.timer.Tick -= timer_Tick;