The task is to add a confirmation dialog whenever someone clicks a Delete or Add or whatever button/link/image on your GridView or Repeater. To accomplish this will require a bit of JavaScript, but it's really not hard at all.

The Repeater has the ItemDataBound and the GridView has the RowDataBound events, and in order to attach your JavaScript to an object within EACH row or item, you will need to hook this event (my samples will be with the GridView, but the Repeater uses the same methodology).

protected void GridView_Views_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
   ImageButton button =
      (ImageButton)e.Row.Cells[0].Controls[0];
   button.Attributes.Add(
      "onclick", "if(!confirm('Delete this item?'))return false;");

   return;
}
return;
}

Many people become confused on how to get at a particular control once rendered into the browser DOM with their script because the name changes for each row or item of the parent control (GridView or Repeater). They become curious how to attach their script to something seemingly random such as:

ctl00$ContentPlaceHolder1$Repeater1$ctl00$btnDelete

The answer is, you don't. You attach it to the object as it's being rendered into the newly bound row like above. Here, nothing has been scoped and named out for the client DOM, it's still just an object in memory.

The way the script works is simple: the event model of the browser will prevent something from bubbling if you return a false or 0 value from the on* handler (such as onclick above). If you return a success or 1 value, then the event will continue to trickle up the call chain to whomever else is registered to handle it (which, in this case, would be the ASP.Net client page framework which initiates the __doPostback() call).