Populate DropDownList on DropDownList Click using Generic Handler in ASP.Net

Hi nauna,

Check this example. Now please take its reference and correct your code.

Database

For this example I have used of Northwind database that you can download using the link given below.

Download Northwind Database

HTML

<asp:DropDownList ID="ddlCustomers" runat="server">
</asp:DropDownList>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    $(function () {
        var loadedDropDownList = false;
        $('[id*=ddlCustomers]').on('click', function () {
            if (loadedDropDownList) {
                return;
            } else {
                $.ajax({
                    type: "POST",
                    url: "Handler.ashx/GetCustomers",
                    data: '{}',
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    success: function (response) {
                        var ddlCustomers = $("[id*=ddlCustomers]");
                        ddlCustomers.empty().append('<option selected="selected" value="0">Please select</option>');
                        $.each(response, function () {
                            ddlCustomers.append($("<option></option>").val(this['Value']).html(this['Text']));
                        });
                        loadedDropDownList = true;
                    }
                });
            }
        });
    });
</script>

Handler

C#

<%@ WebHandler Language="C#" Class="Handler" %>

using System;
using System.Web;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Web.UI.WebControls;
using System.Web.Script.Serialization;

public class Handler : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        List<ListItem> customers = new List<ListItem>();
        string query = "SELECT TOP 5 CustomerID, ContactName FROM Customers";
        string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand(query))
            {
                cmd.CommandType = CommandType.Text;
                cmd.Connection = con;
                con.Open();
                using (SqlDataReader sdr = cmd.ExecuteReader())
                {
                    while (sdr.Read())
                    {
                        customers.Add(new ListItem
                        {
                            Value = sdr["CustomerID"].ToString(),
                            Text = sdr["ContactName"].ToString()
                        });
                    }
                }
                con.Close();
            }
        }

        JavaScriptSerializer serializer = new JavaScriptSerializer();
        context.Response.ContentType = "text/plain";
        context.Response.Write(serializer.Serialize(customers));
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}

VB.Net

<%@ WebHandler Language="VB" Class="Handler" %>

Imports System
Imports System.Web
Imports System.Collections.Generic
Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient
Imports System.Web.UI.WebControls
Imports System.Web.Script.Serialization

Public Class Handler : Implements IHttpHandler
    Public Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
        Dim customers As New List(Of ListItem)()
        Dim query As String = "SELECT TOP 5 CustomerID, ContactName FROM Customers"
        Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Using con As New SqlConnection(constr)
            Using cmd As New SqlCommand(query)
                cmd.CommandType = CommandType.Text
                cmd.Connection = con
                con.Open()
                Using sdr As SqlDataReader = cmd.ExecuteReader()
                    While sdr.Read()
                        customers.Add(New ListItem() With {
                          .Value = sdr("CustomerID").ToString(),
                          .Text = sdr("ContactName").ToString()
                        })
                    End While
                End Using
                con.Close()
            End Using
        End Using

        Dim serializer As JavaScriptSerializer = New JavaScriptSerializer()
        context.Response.ContentType = "text/plain"
        context.Response.Write(serializer.Serialize(customers))
    End Sub

    Public ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
        Get
            Return False
        End Get
    End Property
End Class

Screenshot