Many times I have seen a problem that many developers face binding a DropdownList using Ajax. There are many ways to do that but I just want to share a very simple way. .... Hope you people will like this!

Coding behind Default.aspx page

<form id="form1" runat="server">
 <asp:ScriptManager ID="ScriptManager1" runat="server">
 </asp:ScriptManager>
 <div>
 <asp:UpdatePanel ID="countrypanel" runat="server">
 <ContentTemplate >
 <asp:DropDownList ID="ddlcountry" AutoPostBack ="true" AppendDataBoundItems
="true" runat="server" Height="20px" Width="156px"
onselectedindexchanged="ddlcountry_SelectedIndexChanged">
 </asp:DropDownList>
 </ContentTemplate>

<Triggers>
 <asp:AsyncPostBackTrigger ControlID ="ddlcountry" />
 </Triggers>
 </asp:UpdatePanel>
 <br />

<asp:UpdatePanel ID="statepanel" runat="server">
<ContentTemplate >
 <asp:DropDownList ID="ddlstate" AutoPostBack ="true"
AppendDataBoundItems ="true" runat="server" Height="20px"
Width="155px" onselectedindexchanged="ddlstate_SelectedIndexChanged">
 </asp:DropDownList>
 </ContentTemplate>
 <Triggers >
 <asp:AsyncPostBackTrigger ControlID ="ddlstate" />
 </Triggers>
 </asp:UpdatePanel>
 <br />

<asp:UpdatePanel ID="citypanel" runat="server">
<ContentTemplate >
<asp:DropDownList ID="ddlcity" AutoPostBack ="true"
AppendDataBoundItems ="true" runat="server" Height="20px" Width="155px">
 </asp:DropDownList>
 </ContentTemplate>
 <Triggers >
 <asp:AsyncPostBackTrigger ControlID ="ddlcity" /> </Triggers>
</asp:UpdatePanel>
 </div>
</form>

C# Code:

Coding behind Default.aspx.cs page

public partial class _Default : System.Web.UI.Page
{

SqlConnection conn = new SqlConnection("Data Source=YOGENDRA-PC\\SQLEXPRESS;Initial Catalog=register;Integrated Security=True");
public void Bind_ddlCountry()
{
conn.Open();
SqlCommand cmd = new SqlCommand("select CountryID,CountryName from tblCountry",conn );
SqlDataReader dr = cmd.ExecuteReader();
ddlcountry.DataSource = dr;
ddlcountry.Items.Clear();
ddlcountry.Items.Add("--Select--");
ddlcountry.DataTextField = "CountryName";
ddlcountry.DataValueField = "CountryID";
ddlcountry.DataBind();
conn.Close();

}

public void Bind_ddlState()
{
conn.Open();
SqlCommand cmd = new SqlCommand("select StateID,StateName from tblState where Country_ID='"+ddlcountry .SelectedValue +"'",conn );
SqlDataReader dr = cmd.ExecuteReader();
ddlstate.DataSource = dr;
ddlstate.Items.Clear();
ddlstate.Items.Add("--Select--");
ddlstate.DataTextField = "StateName";
ddlstate.DataValueField = "StateID";
ddlstate.DataBind();
conn.Close();

}

public void Bind_ddlCity()
{
conn.Open();
SqlCommand cmd = new SqlCommand("select CityID,CityName from tblCity where State_ID ='"+ddlstate .SelectedValue +"'",conn );
SqlDataReader dr = cmd.ExecuteReader();
ddlcity.DataSource = dr;
ddlcity.Items.Clear();
ddlcity.Items.Add("--Select--");
ddlcity.DataTextField = "CityName";
ddlcity.DataValueField = "CityID";
ddlcity.DataBind();
conn.Close();
}

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
Bind_ddlCountry();
}
}
protected void ddlcountry_SelectedIndexChanged(object sender, EventArgs e)
{
Bind_ddlState();
}
protected void ddlstate_SelectedIndexChanged(object sender, EventArgs e)
{
Bind_ddlCity();
}
}