how to open only one instance of class not more than that ?

Please use the singleton pattern for that. Short summary:

Make class constructor private.

Have a public static function like GetInstance(...).
Function will return content of variable instance if not null
else it will instantiate the class and assign that to the variable instance.

Modification:
Sorry! Your're right of course, closing the form will dispose it and it can't be used anymore. So I added an event handler that just hides the form when you close it (also works by calling Close() method of form):

namespace Singleton
{
    public partial class Form1 : Form
    {
        private static Form1 instance;
        private Form1()
        {
            InitializeComponent();
            Form1.instance = this;
            this.FormClosing += new FormClosingEventHandler(Form1_FormClosing);
        }

        void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            e.Cancel = true; //This prevents form from really closing
            ((Form1)sender).Hide();
        }

        public static Form1 GetInstance()
        {
            if (instance == null)
                return new Form1();
            else
               return instance;
        }
    }
}

That works now. In another form I have a button with click event handler:

private void button1_Click(object sender, EventArgs e)
{
    Form1 form = Form1.GetInstance();
    form.Show();
}

Now you can "close" Form1 as often as you want and when button is pressed it reappears and it's always the same instance.


Cheers


Manfred