Show AjaxModalPopUp Programatically

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C#

VB

ASP.NET

I have a div say _dvTest and button _btnTest. I am trying to add an ajaxModalPopUpExtender where TargetControl is _btnTest and PopupControlID/OkControlID is _dv

If i set AjaxPopUpExtender as Enabled = True then it does not the button to postback a page.
incase i set it to false then for the first load it works fine. but once i call _modalPopUp.Show() then again back to point one. "it does not let page to post back"

I have some logic at code behind to either show modalPopUp or to pass through.

i.e

_btntest_Click(Object, EventArgs)
{
  IF IsValid() THEN
   _modalPopUp.Show()
  ELSE
    Response.Redirect("....")
}

Note: At first load it runs very well but if i close the modalpopup and again click on _btnTest, at this time it does not let the button to postback and check for the logic. it just display the modalPopUp

Please suggest, what shall i do?