SelectedIndexChanged doesn't work!

My code:

*.aspx:

<asp:DropDownList ID开发者_C百科="CountryList" CssClass="CountryList" runat="server" 
           OnSelectedIndexChanged="CountryList_SelectedIndexChanged" />

*.aspx.cs:

protected void Page_Load(object sender, EventArgs e)
{
   CountryList.SelectedIndexChanged += 
                          new EventHandler(CountryList_SelectedIndexChanged);
   ...  
}

protected void CountryList_SelectedIndexChanged(object sender, EventArgs e)
{
   LoadCityList(CountryList, CityList);
}

But this doesn't work.

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Try setting AutoPostBack="true" on this dropdown list:

<asp:DropDownList 
    ID="CountryList" 
    CssClass="CountryList" 
    runat="server" 
    OnSelectedIndexChanged="CountryList_SelectedIndexChanged"
    AutoPostBack="true"  
/>

Also you don't need to manually wire-up the event handler in the Page_Load method. It will be automatically done by ASP.NET when it compiles the webform:

protected void Page_Load(object sender, EventArgs e)
{
    ... 
}

protected void CountryList_SelectedIndexChanged(object sender, EventArgs e)
{
    LoadCityList(CountryList, CityList);
}

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I think you missed AutoPostBack="true" Property in aspx file

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Add AutoPostBack="true" in ur aspx code and everything will work as you thought.