Dispose form after closing

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();
开发者_如何学编程        // 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!

This question turns out to be about Dispose.

Firstly, Dispose has nothing 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;

The simplest and most reliable way to dispose a Form after using is to put the usage inside of a using block

using (Form2 form = new Form2()) {
  form.ShowDialog();
}

The using block in C# is a construct that essentially expands the above into the following code.

Form2 form;
try {
  form = new Form2(); 
  ...
} finally {
  if ( form != null ) {
    form.Dispose();
  }
}

This is an old question but it touches some interesting points about how objects work. A form is, essentially, an object. All objects of the same class share the same methods but each one has their own data. What does this mean? This means that, closing or disposing an object does not free/delete/remove any code from the memory. Only data. All that was about objects in general, no matter the language.

Now, specifically about your code. Let us examine what the line Program.timer.Tick += timer_Tick; does. This gives a pointer to your function in the Form object to the timer object. So, now, no matter what you do to the Form object, the timer object will keep calling that function. The timer object does not care about your Form and is not even aware of the existence of the Form object. It only cares about the function you passed the pointer to. As far as the timer object is concerned, this function is a standalone function.

What does Form.Close() do? Form.Close() disposes the resources used by the form, aka, marks the form's controls for garbage collection unless the form is displayed using ShowDialog. In that case, Dispose() must be called manually. MSDN

Needless to say(or maybe not so needless) that if closing/disposing the form cleared the function from the memory, the timer object would have an invalid pointer and your program would crash after 5 seconds.

Perhaps I am reading the question wrong, but I think the gentlemen needs to know that, to close a form (say form2) opened as Form2.ShowDialog(), you need to set Form2.DialogResult within Form2. Just setting that member is all it takes to close the form and return the result.

form.ShowDialog() shows the form as a modal dialog. This means that the call doesn't return until the Form is closed.
Note that clicking the close X on a modal dialog doesn't close the form, it just hides it. I am guessing that this is what confuses you. If you want the code in form1 to continue executing rather than blocking, you should call Show() instead of ShowDialog(). Non-modal will close when the X is clicked.

If you do want a blocking modal dialog, you should surround the form with a using block as described in the other answers.
When building a modal dialog you would typically add an "OK" button or similar and set the AcceptButton property of the form to that button to allow the user to close the form by pressing enter. Similarly you can add a "Cancel" button and set the CancelButton property to capture the Esc key.
Add a click handler to the two buttons, set the DialogResult property of the form accordingly and call Close().