Q:

How to add value to a List Array

I am working on a cascading drop-down for Country and City, which is working fine for now but I need to add other city as last value to the second drop down (City DD) irrespective of what country user selects. i am not able to make it work like a normal add operation to  list.
public CascadingDropDownNameValue[] FetchCities(string knownCategoryValues, string category)
    {
        string CountryCode;
        StringDictionary strCountries = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        CountryCode = strCountries["Country"].ToString();
        con.Open();
        SqlCommand cmd = new SqlCommand("select * from Cities where Country=@Code Order by CityName ", con);
        cmd.Parameters.AddWithValue("@Code", CountryCode);
        cmd.ExecuteNonQuery();
        SqlDataAdapter dastate = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        dastate.Fill(ds);
        con.Close();
        List<CascadingDropDownNameValue> states = new List<CascadingDropDownNameValue>();
        foreach (DataRow dtRow in ds.Tables[0].Rows)
        {
            string StateID = dtRow["CityID"].ToString();
            string StateName = dtRow["CityName"].ToString();
            states.Add(new CascadingDropDownNameValue(StateName, StateID));
        }
        return states.ToArray();
    }

Above code is a sample code  I am following this tutorials for Cascading drop-down

A:

Why not pass it as a list to the function  (i don't like returning lists) as return values. This way you're in control what happens to the list after getting the database values.
public void Fill()
{
    List<CascadingDropDownNameValue> cities = new List<CascadingDropDownNameValue>();

    FetchCities(cities, knownCategoryValues, category);

    cities.Add(new CascadingDropDownNameValue("new city", "0"));
}

public void FetchCities(List<CascadingDropDownNameValue> values, string knownCategoryValues, string category)
{
    string CountryCode;
    StringDictionary strCountries = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
    CountryCode = strCountries["Country"].ToString();
    con.Open();
    SqlCommand cmd = new SqlCommand("select * from Cities where Country=@Code Order by CityName ", con);
    cmd.Parameters.AddWithValue("@Code", CountryCode);
    cmd.ExecuteNonQuery();
    SqlDataAdapter dastate = new SqlDataAdapter(cmd);
    DataSet ds = new DataSet();
    dastate.Fill(ds);
    con.Close();
    List<CascadingDropDownNameValue> states = new List<CascadingDropDownNameValue>();
    foreach (DataRow dtRow in ds.Tables[0].Rows)
    {
        string StateID = dtRow["CityID"].ToString();
        string StateName = dtRow["CityName"].ToString();
        values.Add(new CascadingDropDownNameValue(StateName, StateID));
    }
}

UPDATE: Another nice approach would be returning IEnumerable< CascadingDropDownNameValue>
    public void Fill()
    {
        List<CascadingDropDownNameValue> cities = new List<CascadingDropDownNameValue>();

        cities.AddRange(FetchCities(connectionString, knownCategoryValues, category));

        cities.Add(new CascadingDropDownNameValue("new city", "0"));
    }

    public IEnumerable<CascadingDropDownNameValue> FetchCities(string connectionString, string knownCategoryValues, string category)
    {
        StringDictionary strCountries = AjaxControlToolkit.CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
        string CountryCode = strCountries["Country"].ToString();

        using (SqlConnection con = new SqlConnection(connectionString))
        using (SqlCommand cmd = new SqlCommand("select * from Cities where Country=@Code Order by CityName ", con))
        {
            cmd.Parameters.Add("@Code", SqlDbType.VarChar).Value = CountryCode;
            using (SqlDataReader reader = cmd.ExecuteReader())
                while (reader.Read())
                {
                    string StateID = reader["CityID"].ToString();
                    string StateName = reader["CityName"].ToString();
                    yield return new CascadingDropDownNameValue(StateName, StateID);
                }
        }
    }

The advantage is that it will stream the rows and transform them to objects when needed. This will give the advantage that if you only want a top 10. It won't get more rows from the sql-server. (except for some row caching/block reads)  
DataSet.Fill would get all rows before returning objects. So if that table contains +1m rows. It will first create a DataTable with 1m rows and then returning them in objects.
The reason i pass the connectionstring to it, is to allow more open recordsets. (one open recordset per connection) 
In this case the cities.AddRange will iterate all rows. To add only a top 10 would be: 
var cities = FetchCities(connectionString, knownCategoryValues, category).Take(10);
cities.AddRange(cities);

Good luck.

