asp.net link button preventing postback - asp.net

Question

<asp:LinkButton ID="lbnImages" runat="server" CssClass="link"
PostBackUrl="AddImage.aspx" CausesValidation="True"
ValidationGroup="Save">Images</asp:LinkButton>
<asp:CustomValidator ID="cstval" runat="server" ControlToValidate="txtName"
ValidationGroup="Save" ErrorMessage ="Duplicate name"
OnServerValidate="Project_validation" />
I am checking for duplicates using the custom validator (server side code)
If the server side custom validator shows error the link button must not redirect to the page.
I need to stop the post back when the validator fires.

Answer 1

If you want to prevent the postback, you should do the validation client side by adding a custom javascript validation function to your validator control. This is described in this blog post.
If your validation logic can only run on the server, you will have to let the postback happen, but wrap all the code that you want not to run in e.g. an
if (Page.IsValid) { ... }

Answer 2

I see you are checking for duplicates, the only way to validate it without causing a postback is to make an ajax call.
Make a call to some service that will check if it's a duplicate or not, then if it's not valid stop it. Using jQuery syntax:
$("save_button").click(function(e){
$.ajax({
async: false, // will finish this request before executing anything else
url: "validation_page",
type: "POST",
data: ({name: "value from a field"}),
success: function(msg){
if (msg != "ERROR") // whatever msg is not valid
e.preventDefault(); // will not do the postback
}
})
});