When handling an ASP.NET server-side button’s Click event, you may want to do some client-side checking and possibly prevent the posting of a page back to the server.
btnClickMe.Attributes.Add("onclick", "return ClickMe();");
The function ClickMe() should be present on the client-side:
Do your validations in this function and, if your condition is false, it will prevent the page postback with this single line:
Returning a false value prevents further execution and, thus, the page from posting back to the server. If your conditions are right, the page will be posted back to the server and fire the button’s server-side Click event.