Refer the below sample code for your reference.
I have used Cascading DropDownList for Country/State/City in ASP.Net this article for reference.
Here i made SelectedIndexChanged Evemt as Public so it can accessible on child page too.
MasterPage
HTML
<head runat="server">
<title></title>
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<table>
<tr>
<td>
Country:
</td>
<td>
<asp:DropDownList ID="ddlCountries" runat="server" AutoPostBack="true" OnSelectedIndexChanged="Country_Changed">
</asp:DropDownList>
</td>
</tr>
<tr>
<td>
State:
</td>
<td>
<asp:DropDownList ID="ddlStates" runat="server" AutoPostBack="true" OnSelectedIndexChanged="State_Changed">
</asp:DropDownList>
</td>
</tr>
<tr>
<td>
City:
</td>
<td>
<asp:DropDownList ID="ddlCities" runat="server">
</asp:DropDownList>
</td>
</tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>
<br />
<br />
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</div>
</form>
</body>
</html>
C#
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
string query = "select CountryId, CountryName from Countries";
BindDropDownList(ddlCountries, query, "CountryName", "CountryId", "Select Country");
ddlStates.Enabled = false;
ddlCities.Enabled = false;
ddlStates.Items.Insert(0, new ListItem("Select State", "0"));
ddlCities.Items.Insert(0, new ListItem("Select City", "0"));
}
}
public void callEvent()
{
Country_Changed(null, null);
}
private void BindDropDownList(DropDownList ddl, string query, string text, string value, string defaultText)
{
string conString = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
SqlCommand cmd = new SqlCommand(query);
using (SqlConnection con = new SqlConnection(conString))
{
using (SqlDataAdapter sda = new SqlDataAdapter())
{
cmd.Connection = con;
con.Open();
ddl.DataSource = cmd.ExecuteReader();
ddl.DataTextField = text;
ddl.DataValueField = value;
ddl.DataBind();
con.Close();
}
}
ddl.Items.Insert(0, new ListItem(defaultText, "0"));
}
public void Country_Changed(object sender, EventArgs e)
{
ddlStates.Enabled = false;
ddlCities.Enabled = false;
ddlStates.Items.Clear();
ddlCities.Items.Clear();
ddlStates.Items.Insert(0, new ListItem("Select State", "0"));
ddlCities.Items.Insert(0, new ListItem("Select City", "0"));
int countryId = int.Parse(ddlCountries.SelectedItem.Value);
if (countryId > 0)
{
string query = string.Format("select StateId, StateName from States where CountryId = {0}", countryId);
BindDropDownList(ddlStates, query, "StateName", "StateId", "Select State");
ddlStates.Enabled = true;
}
}
protected void State_Changed(object sender, EventArgs e)
{
ddlCities.Enabled = false;
ddlCities.Items.Clear();
ddlCities.Items.Insert(0, new ListItem("Select City", "0"));
int stateId = int.Parse(ddlStates.SelectedItem.Value);
if (stateId > 0)
{
string query = string.Format("select CityId, CityName from Cities where StateId = {0}", stateId);
BindDropDownList(ddlCities, query, "CityName", "CityId", "Select City");
ddlCities.Enabled = true;
}
}
CS.aspx
HTML
<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="CS.aspx.cs" Inherits="CS" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
Default Page
<br />
Some content
<br />
<asp:Button Text="Refresh country dropdown" runat="server" OnClick="ChangeCountry" />
</asp:Content>
C#
protected void ChangeCountry(object sender, EventArgs e)
{
(((this.Master) as MasterPage).FindControl("ddlCountries") as DropDownList).ClearSelection();
((this.Master) as MasterPage).Country_Changed(null, null);
}
Screenshot
