.......                
                    con.Open();
                
                    string query = @"select DISTINCT nvarchar1,nvarchar3 
                                from UserData 
                                where nvarchar3 in (
                                select DISTINCT nvarchar34
                                from UserData 
                                where nvarchar34 IS NOT NULL 
                                order by nvarchar1";
                    
                SqlCommand cmd = new SqlCommand(query, con);
                SqlDataReader dr = cmd.ExecuteReader();

                while (dr.Read())
                {
                    query = dr[0].ToString();
                    DropDownList1.Items.Add(query);
                    DropDownList1.DataTextField = "nvarchar1";
                    DropDownList1.DataValueField = "nvarchar3";
                }
               
        
                dr.Close();
                con.Close();       

........string combox = DropDownList1.SelectedValue;........

while (dr.Read())
{
    DropDownList1.Items.Add(new ListItem(dr.GetString(0), dr.GetString(1)));
}