title: Calculate Values in label
body: <p><code>label1</code> is ok,
  but <code>label2</code> is not ok and get error ,</p>
  
  <pre><code>private void button1_Click(object sender, EventArgs e)
  {
      double a = Convert.ToDouble(textBox1.Text);
      double b = Convert.ToDouble(textBox2.Text);
      double z = Convert.ToDouble(label1.Text);
  
      label1.Text = (a + b).ToString(); // it is ok
      label2.Text = (z + a).ToString(); // not ok and get error    
  }
  </code></pre>