title: Validate TextBox based on DropDownList selection
body: <p>I  need to validate <code>TextBox</code> based on Value selected in <code>DropDownList</code> controls. I have <code>asp:TextBox</code> and <code>asp:DropDownList</code> controls.</p>
  
  <p>If user selects <code>Yes</code> option from the first dropdown he must type value in text box. How can I validate second box? Thanks for help.</p>