Question

I like to add a validation on a label based on its visibility, in that a submit button will raise a validation message or error if the label is not visible.

I am used to the validation controls in the Toolbox, which wont allow this functionality!

Answer 1

It seems as though if an asp:Label’s visibility is set to false, the asp.net engine will not even put it in the DOM. So you can check in javascript, using the onclick property of the (html) button to check if the label is in the DOM or not, and use asp.net’s __doPostBack() javascript function to post back to the server if it is there:

      <script type="text/javascript">
        function testMe()
        {
          var lbl = document.getElementById('lblTest');
          if(lbl == null)
            document.getElementById('msg').innerHTML = "Error";
          else
            __doPostBack('testButton');
        }
      </script>
      <asp:Label ID="lblTest" runat="server" Visible="false" Text="Hello"></asp:Label>
      <button onclick="testMe();">test</button>

To be completely honest, I thought the lbl would be undefined if the label did not exist in the DOM, but Firebug revealed it is actually null. Anyway, a couple things to note is that in order for asp.net to define the __doPostBack() method, I believe you need some control in the form that has autopostback="true", and in the code-behind you can check what caused the postback in the Page_Load method like so:

if(Request.Form["__EVENTTARGET"] == "testButton") {}