Form2 f2; Form3 f3; Form99 f99;

        private void button1_Click(object sender, EventArgs e)
        {
            ShowForm(f2, "Form2");
        }

        private void button2_Click(object sender, EventArgs e)
        {
            ShowForm(f3, "Form3");
        }

        private void button3_Click(object sender, EventArgs e)
        {
            ShowForm(f99 , "Form99");
        }

        private void ShowForm(Form f, string t)
        {
            if (t == "Form2")
                f = new Form2();

            if (t == "Form3")
                f = new Form3();

            if (t == "Form99")
                f = new Form99();

            f.Show();

        }

        private void button1_Click(object sender, EventArgs e)
        {
           ShowForm<Form2>();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            ShowForm<Form3>();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            ShowForm<Form99>();
        }

        private void ShowForm<T>()
        where T: Form, new ()
        {
            var form = new T();
            form.Show();
        }