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In Ajax dropdown cascading i got these error.

System.NullReferenceException: Object reference not set to an instance of an object.
at DropdownWebService.BindCountrydropdown(String knownCategoryValues, String category)

My code is :

My code in webservice page is:
[WebService(Namespace = "")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
public class DropdownWebService : System.Web.Services.WebService {

    public CascadingDropDownNameValue[] BindCountrydropdown(string knownCategoryValues, string category)
        SqlConnection concountry = new SqlConnection(ConfigurationManager.ConnectionStrings["cnnstring"].ConnectionString);
        SqlCommand cmdcountry = new SqlCommand("select * from Country", concountry);
        SqlDataAdapter dacountry = new SqlDataAdapter(cmdcountry);
        DataSet dscountry = new DataSet();
        List<cascadingdropdownnamevalue> countrydetails = new List<cascadingdropdownnamevalue>();
        foreach (DataRow dtrow in dscountry.Tables[0].Rows)
            string CountryID = dtrow["CountryID"].ToString();
            string CountryName = dtrow["CountryName"].ToString();
            countrydetails.Add(new CascadingDropDownNameValue(CountryName, CountryID));
        return countrydetails.ToArray();
    public CascadingDropDownNameValue[] BindStatedropdown(string knownCategoryValues, string category)
        int CountryID;
        StringDictionary countrydetails = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        CountryID = Convert.ToInt32(countrydetails["Country"]);
        SqlConnection constate = new SqlConnection(ConfigurationManager.ConnectionStrings["cnnstring"].ConnectionString);
        SqlCommand cmdstate = new SqlCommand("select * from State where CountryID=@CountryID", constate);
        cmdstate.Parameters.AddWithValue("@CountryID", CountryID);
        SqlDataAdapter dastate = new SqlDataAdapter(cmdstate);
        DataSet dsstate = new DataSet();
        List<cascadingdropdownnamevalue> statedetails = new List<cascadingdropdownnamevalue>();
        foreach (DataRow dtstaterow in dsstate.Tables[0].Rows)
            string stateID = dtstaterow["StateID"].ToString();
            string statename = dtstaterow["StateName"].ToString();
            statedetails.Add(new CascadingDropDownNameValue(statename, stateID));
        return statedetails.ToArray();
    public CascadingDropDownNameValue[] BindRegiondropdown(string knownCategoryValues, string category)
        int stateID;
        StringDictionary statedetails = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        stateID = Convert.ToInt32(statedetails["State"]);
        SqlConnection conregion = new SqlConnection(ConfigurationManager.ConnectionStrings["cnnstring"].ConnectionString);
        SqlCommand cmdregion = new SqlCommand("Select * from Region where StateID=@StateID", conregion);
        cmdregion.Parameters.AddWithValue("@StateID", stateID);
        SqlDataAdapter daregion = new SqlDataAdapter(cmdregion);
        DataSet dsregion = new DataSet();
        List<cascadingdropdownnamevalue> regiondetails = new List<cascadingdropdownnamevalue>();
        foreach (DataRow dtregionrow in dsregion.Tables[0].Rows)
            string regionID = dtregionrow["RegionID"].ToString();
            string regionname = dtregionrow["RegionName"].ToString();
            regiondetails.Add(new CascadingDropDownNameValue(regionname, regionID));

        return regiondetails.ToArray();

so plz help me out to solve this error.
Updated 19-May-12 3:41am

When the error is 500, that means it's an internal error, meaning internal to the service - the service threw an exception that was not caught.
Look in the Windows event logs on the service to see what went wrong.

Also, try:
- Go to menu Tools/Internet Options in your IE.
- Click on the Advanced tab, uncheck "Show friendly HTTP error messages" option and click Ok.
- Try accessing your web page again. Now you will see much more meaningful error message which will help you to troubleshoot the problem.

Other possibility:
There might be an issue in the way you have hosted your service. May be multiple config files?
VJ Reddy 12-May-12 23:16pm
Good suggestion. 5!
Sandeep Mewara 13-May-12 1:06am
Since error 500 is a generic error and doesn't tell much about the actual problem, I suggest that you use try..catch blocks in your web services the get information abuot the root cause of the problem.

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