gistfile1.txt

// Suppose we want to update something based on Ajax Call
UI: HTML Dropdown
Step 1: // ***here we have the change event of dropdown which will call ajax	    
 <select id="idselect" name="classselect" onchange="makeajaxcall()"> 
         <option value="2">Two</option>	     
         <option value="3">Three</option>	      
 </select>
 Step 2: Make the actual ajax call on change of dropdown
         <script>
    function makeajaxcall()
    {
        //get the selected value from the dropdown
        var idselect = $('#idselect').val() 
        $.ajax({
            type: 'POST',
            url: '/DropDown/hello', // THIS IS THE URL WHICH WILL BE CALLED UPON THE CONTROLLER
            data: { id: idselect }, // HERE WE ARE SENDING THE JSON VALUE AS INPUT TO POST
            success: function (data) {
                //now here we are getting the json data, we will create a dropdown here dynamically

                $("#idtest").html(markup); // Here bascailly add the result to the UI element which you want updated
            },
            error: function () {
               // Error handling
            }
        });
    }
</script> 

Step 3: BACK END 

        [HttpPost]
        public JsonResult hello(int id)
        {
            if (id == 2)
            {
                l1.Add(new SelectListItem { Text = "Dropdown value for Two", Value = "1" });
                l1.Add(new SelectListItem { Text = "2A", Value = "2" });
            }
            var d = Json(l1);
            return Json(l1); / sending data to the front end.
        }