asp.net custom onfocus event for textbox

There's nothing built-in, because focusing a textbox doesn't cause the page to submit itself. You could always attach an event to the focus on the client side, and then submit some AJAX request to the server, but I'm curious to know what exactly you're trying to achieve here.

I would strongly recommend using client side validation for this. You can use the ASP.NET Validators for this. One of the properties on the validator is to set focus on error. This should be more than enough for what you are trying to do and it also limits the number of server side roundtrips.

If you're really feeling creative, you can look at some of the other validation options and AJAX Control Panel extenders. There are some pretty snazzy validator controls out there. Another option would be to consider using some jQuery validation.

You should still validate on the server, but what you are describing sounds like a perfect it for client validation first.

Ok, i got a web form and on submission validation takes place, if there are errors on the page then these errors get stored in state and a warning sign gets displayed.

Now, when the user goes through the errors, i wanted a popup box to be displayed by the side of the textbox they are currently correcting.

for this to happen i wanted to have an event attached to an onfocus type handle.

Is a server-side onFocus really the right solution for that? If you want validation popups and such, you're probably better off with client-side solution. Having a form submit when you simply tab to a textbox would not be good usability.

You could do this using a custom validator hooked up to some AJAX call, however you only need to do a server trip if there is data on the server that you current data needs to be validated against.