how can get value after addition if user left some textbox blank
<p>I am using following code:</p>

<pre><code>textbox5.text =  (Double.Parse(TextBox1.text) 
                + Double.Parse(TextBox2.text) 
                + Double.Parse(TextBox3.text) 
                + Double.Parse(TextBox4.text)).ToString();
</code></pre>

<p>The problem is if I left any textbox blank then it shows an exception. </p>

<p>Sometimes I want to use only 2 textboxes. Sometimes I want to use only 3 Textboxes and so on</p>
