title: Open an existing form from the main form
body: <p>I designed two forms: <code>Form1</code> and <code>Form2</code>. <code>Form1</code> is the main form. There is a button in <code>Form1</code>, if I click the button, then <code>Form2</code> will pop out. I want to do something on <code>Form2</code>.</p>
  
  <pre><code>// click button in Form1.
  private void button1_Click(object sender, EventArgs e)
      {
          Form form2= new Form();
          form2.ShowDialog();
      }
  </code></pre>
  
  <p>But <code>Form2</code> is a new form rather than an existing form.</p>
  
  <p>It is wrong.</p>
  
  <p>How? Thanks.</p>