title: When not entering any value, my program crashes
body: <p>I have a problem in my windows form in c#. The program is simple;
  there are 3 textboxes and then it sums their values. however, when i click on the sum button without entering any values in the other 3 textboxes, the program crashes.</p>
  
  <p>How can i make these textboxes accept only positive numbers and zeros?
  this is what i did </p>
  
  <pre><code>    private void button1_Click(object sender, EventArgs e)
      {
          double FirstNumb = Convert.ToDouble(txtFirstValue.Text);
          double SecondNumb = Convert.ToDouble(txtSecondValue.Text);
          double ThirdNumb = Convert.ToDouble(txtThirdValue.Text);
  
          double m;
  
          m = FirstNumb + SecondNumb + ThirdNumb;
          listBox1.Items.Add(m);
  
  
      }
  </code></pre>