how to implement Country-State-city using Dropdownlist in asp.net with ajax

for design page of your aspx page paste the below code

<table>
        <tr>
            <td>
                Select Country:
            </td>
            <td>
                <asp:DropDownList runat="server" ID="ddlcountry">
                </asp:DropDownList>
                <asp:CascadingDropDown ID="CascadingDropDown1" TargetControlID="ddlcountry" Category="country"
                    PromptText="Select Country" LoadingText="Please Wait..." ServiceMethod="GETCOUNTRY"
                    runat="server">
                </asp:CascadingDropDown>
            </td>
        </tr>
        <tr>
            <td>
                Select State:
            </td>
            <td>
                <asp:DropDownList runat="server" ID="ddlstate">
                </asp:DropDownList>
                <asp:CascadingDropDown ID="CascadingDropDown2" TargetControlID="ddlstate" Category="state"
                    PromptText="Select State" LoadingText="Please Wait..." ServiceMethod="GETSTATE"
                    runat="server" ParentControlID="ddlcountry">
                </asp:CascadingDropDown>
            </td>
        </tr>
        <tr>
            <td>
                Select City:
            </td>
            <td>
                <asp:DropDownList runat="server" ID="ddlcity">
                </asp:DropDownList>
                <asp:CascadingDropDown ID="CascadingDropDown3" TargetControlID="ddlcity" Category="city"
                    PromptText="Select City" LoadingText="Please Wait..." ServiceMethod="GETCITY"
                    runat="server" ParentControlID="ddlstate">
                </asp:CascadingDropDown>
            </td>
        </tr>
    </table>

& code behind this page paste below code:

 [System.Web.Services.WebMethod]
    public static CascadingDropDownNameValue[] GETCOUNTRY(string knownCategoryValues, string category)
    {
      //Fill your dt from database
        dt = //your dt

        List<CascadingDropDownNameValue> oq = new List<CascadingDropDownNameValue>();
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            oq.Add(new CascadingDropDownNameValue(dt.Rows[i]["country_name"].ToString(), dt.Rows[i]["country_id"].ToString()));
        }
        return oq.ToArray();
    }

    [System.Web.Services.WebMethod]
    public static CascadingDropDownNameValue[] GETSTATE(string knownCategoryValues, string category)
    {
   
        StringDictionary countrydetails = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        int countryID = Convert.ToInt32(countrydetails["country"]);
          //Fill your dt from database
        dt = //your dt

        List<CascadingDropDownNameValue> oq = new List<CascadingDropDownNameValue>();
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            oq.Add(new CascadingDropDownNameValue(dt.Rows[i]["state_name"].ToString(), dt.Rows[i]["state_id"].ToString()));
        }
        return oq.ToArray();
    }
    [System.Web.Services.WebMethod]
    public static CascadingDropDownNameValue[] GETCITY(string knownCategoryValues, string category)
    {
   

        StringDictionary countrydetails = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        int stateID = Convert.ToInt32(countrydetails["state"]);
           //Fill your dt from database
        dt = //your dt

        List<CascadingDropDownNameValue> oq = new List<CascadingDropDownNameValue>();
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            oq.Add(new CascadingDropDownNameValue(dt.Rows[i]["city_name"].ToString(), dt.Rows[i]["state_id"].ToString()));
        }
        return oq.ToArray();
    }

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