ASP NET C DropDownList SelectedIndexChanged in server control not firing

Question

I m creating a server control that basically binds two dropdown lists  one for country and one for state  and updates the state dropdown on the country s selectedindexchanged event  However  it s not posting back  Any ideas why  Bonus points for wrapping them in an UpdatePanel  having rendering issues  maybe because I don t have a Page to reference    Here s what I  have  with some extra data access stuff stripped out    public class StateProv   WebControl       public string SelectedCountry      public string SelectedState       private DropDownList ddlCountries   new DropDownList        private DropDownList ddlStates   new DropDownList         protected override void OnLoad EventArgs e                base OnLoad e            IList lt Country gt  countries   GetCountryList            IList lt State gt  states   new List lt State gt              if  SelectedCountry    null  amp  amp  SelectedCountry                              states   GetStateList GetCountryByShortName SelectedCountry  CountryShortName                     else                       states Add new State   CountryId   0  Id   0  StateLabelName    No states available   StateLongName    No states available   StateShortName                             ddlCountries DataSource   countries          ddlCountries DataTextField    CountryLongName           ddlCountries DataValueField    CountryShortName           ddlCountries SelectedIndexChanged    new EventHandler ddlCountry SelectedIndexChanged           ddlCountries AutoPostBack   true           ddlStates DataSource   states          ddlStates DataTextField    StateLongName           ddlStates DataTextField    StateShortName            ddlCountries DataBind            ddlStates DataBind             if   string IsNullOrEmpty SelectedCountry                         ddlCountries SelectedValue   SelectedCountry               if   string IsNullOrEmpty SelectedState                                 ddlStates SelectedValue   SelectedState                                                  protected override void RenderContents HtmlTextWriter output                ddlCountries RenderControl output           ddlStates RenderControl output              private IList lt Country gt  GetCountryList                   return stuff            private IList lt State gt  GetStateList Country country                  return stuff            private IList lt State gt  GetStateList string countryAbbrev                Country country   GetCountryByShortName countryAbbrev           return GetStateList country              private Country GetCountryByShortName string countryAbbrev                IList lt Country gt  list   dataAccess RetrieveQuery lt Country gt               return stuff            private IList lt State gt  GetAllStates                   return stuff            protected void ddlCountry SelectedIndexChanged object sender  EventArgs e                IList lt State gt  states   GetStateList GetCountryList     DropDownList sender  SelectedIndex            ddlStates DataSource   states          ddlStates DataBind              Edit  Viewstate is on the page  and other controls on the page perform postbacks correctly  just not this

User · Answer

You need to set AutoPostBack to true for the Country DropDownList   protected override void OnLoad EventArgs e           base stuff      ddlCountries AutoPostBack   true          other stuff     Edit  I missed that you had done this  In that case you need to check that ViewState is enabled

User · Answer

Is Viewstate turned on   Edit  Perhaps you should reconsider overriding the rendering function    protected override void RenderContents HtmlTextWriter output                ddlCountries RenderControl output           ddlStates RenderControl output           and instead add the dropdownlists to the control and render the control using the default RenderContents   Edit  See the answer from Dennis which I alluded to in my previous comment   Controls Add   ddlCountries    Controls Add   ddlStates

User · Answer

I can t see that you re adding these controls to the control hierarchy  Try   Controls Add   ddlCountries    Controls Add   ddlStates      Events won t be invoked unless the control is part of the control hierarchy

User · Answer

First  I would like to clarify something  Is this a post back  trip back to server  never occur  or is it the post back occurs  but it never gets into the ddlCountry SelectedIndexChanged event handler   I am not sure which case you are having  but if it is the second case  I can offer some suggestion  If it is the first case  then the following is FYI   For the second case  event handler never fires even though request made   you may want to try the following suggestions    Query the Request Params ddlCountries UniqueID  and see if it has value  If it has  manually fire the event handler  As long as view state is on  only bind the list data when it is not a post back  If view state has to be off  then put the list data bind in OnInit instead of OnLoad    Beware that when calling Control DataBind    view state and post back information would no longer be available from the control  In the case of view state is on  between post back  values of the DropDownList would be kept intact  the list does not to be rebound   If you issue another DataBind in OnLoad  it would clear out its view state data  and the SelectedIndexChanged event would never be fired   In the case of view state is turned off  you have no choice but to rebind the list every time  When a post back occurs  there are internal ASP NET calls to populate the value from Request Params to the appropriate controls  and I suspect happen at the time between OnInit and OnLoad  In this case  restoring the list values in OnInit will enable the system to fire events correctly   Thanks for your time reading this  and welcome everyone to correct if I am wrong

User · Answer

I had the same problem but got round it by setting AutoPostBack to true and in an update panel set the trigger to the dropdownlist control id and event name to SelectedIndexChanged e g        lt asp UpdatePanel ID  UpdatePanel1  runat  server  UpdateMode  Always  enableViewState  true  gt           lt Triggers gt               lt asp AsyncPostBackTrigger ControlID  ddl1  EventName  SelectedIndexChanged    gt           lt  Triggers gt           lt ContentTemplate gt               lt asp DropDownList ID  ddl1  runat  server  ClientIDMode  Static  OnSelectedIndexChanged  ddl1 SelectedIndexChanged  AutoPostBack  true  ViewStateMode  Enabled  gt                   lt asp ListItem Text  --Please select a item--  Value  0    gt               lt  asp DropDownList gt           lt  ContentTemplate gt       lt  asp UpdatePanel gt

[c#] ASP.NET / C#: DropDownList SelectedIndexChanged in server control not firing

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