prompt save changes while leaving page asp net

You'll need using some client side scripting which does this check during the onbeforeunload event.

var showPrompt=true;
var isModified=false;
var hidePopup=false;

    function onBeforeUnload() {
        if (showPrompt) {
            if (isModified) {
                if (hidePopup || confirm("Click ok to save your changes or click cancel to discard your changes.")) {
                    doSave();
                }
            }
        }
        showPrompt = true;
        hidePopup = false;
    }

- showPrompt can be set to false when your clicking on an anchor tag which won't navigate you away from the page. For example

<a onclick="showPrompt=false" href="javascript:doX()" />

- isModified can be used to track when you need to save something. You could for example do something like

$("input").onchange(function(){isModified=true;});

To track undo's you might want to replace isModified with a function which checks the current state from the last saved state.

- hidePopup lets us force a save without confirming to the user.