title: Input string was not in the correct format : Unhandled exception
body: <p>I want to calculate sum of five textboxes in 1 text box....
  But when ever I remain empty one text box out of five textboxes it is giving an error
  Input string was not in the correct format
   below is my code</p>
  
  <pre><code>        if (textBox6.Text.Length == 0)
  
          // textBox12.Text = (Convert.ToInt32(textBox5.Text) - Convert.ToInt32(textBox11.Text)).ToString();
          {
              textBox11.Text = (Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox9.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox12.Text = (Convert.ToInt32(textBox5.Text) - Convert.ToInt32(textBox11.Text)).ToString();
          }
          else if (textBox7.Text.Length == 0)
          {
              //textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox9.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox9.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox12.Text = (Convert.ToInt32(textBox5.Text) - Convert.ToInt32(textBox11.Text)).ToString();
          }
          else if (textBox8.Text.Length == 0)
          {
              //textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox9.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox9.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox12.Text = (Convert.ToInt32(textBox5.Text) - Convert.ToInt32(textBox11.Text)).ToString();
          }
          else if (textBox9.Text.Length == 0)
          {
              //textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox9.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox12.Text = (Convert.ToInt32(textBox5.Text) - Convert.ToInt32(textBox11.Text)).ToString();
          }
          else if (textBox10.Text.Length == 0)
          {
              //textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox9.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox9.Text)).ToString();
              textBox12.Text = (Convert.ToInt32(textBox5.Text) - Convert.ToInt32(textBox11.Text)).ToString();
  
          }
          else
              textBox11.Text = (Convert.ToInt32(textBox6.Text) + Convert.ToInt32(textBox7.Text) + Convert.ToInt32(textBox8.Text) + Convert.ToInt32(textBox9.Text) + Convert.ToInt32(textBox10.Text)).ToString();
              textBox12.Text = (Convert.ToInt32(textBox5.Text) - Convert.ToInt32(textBox11.Text)).ToString();
  
  
  
         // (Convert.ToInt32(textBox6.Text) +
      }
  </code></pre>