Modify posted input values on onbegin of ajax.beginform

Answer 1

You're having this issue because funBegin is called after the form data has been serialized. From MSDN:

AjaxOptions.OnBegin Property: Gets or sets the name of the JavaScript function to call immediately before the page is updated.

I suggest you write your own submit handler:

<form id="myform" action="/Home/MyAction">
    <input type="text" id="txtName" name="txtName" value="gaurav" />
    <input type="submit" value="Submit" />
</form>

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.min.js"&gt;&lt;/script&gt;
<script type="text/javascript">
    $(document).ready(function() {
        $('#myform').submit(function() {
            $("#txtName").val("gaurav pandey");

            var form = $(this);
            var url = form.attr('action');
            var formData = form.serialize();
            $.post(url, formData, function(result) {
                // Do something with result
            });

            return false;
        });
    });
</script>