By default, the RadInput controls do not cause a postback when the user finishes editing. You can change this default behavior by setting the AutoPostBack property to True.

All four types of RadInput controls have the AutoPostBack property. When AutoPostBack is True, a postback occurs when the user changes the value in the control and the control then loses focus.

You can conditionally prevent the postback that occurs when AutoPostBack is True by using the client-side OnValueChanged event. To prevent the postback from occurring, call set_cancel(true) on the eventArgs argument of the event handler:

Copy[ASP.NET] Preventing postbacks

<script type="text/javascript">
    function PreventPostBack(sender, eventArgs)
    {
        if (eventArgs.get_newValue() == "")
            eventArgs.set_cancel(true);
    }
</script>
<telerik:RadTextBox ID="RadTextBox1" runat="server" AutoPostBack="True">
    <ClientEvents OnValueChanged="PreventPostBack" />
</telerik:RadTextBox>