SelectedIndexChanged event doesn't fire

I have added the AutoPostBack = "true" attribute to the drop down list. It doesn't work.

<asp:DropDownList CssClass="dropDownList" ID="ddlBusinessUnit" 
                  AutoPostBack="true" runat="server" Width="250px"   
                  OnSelectedIndexChanged="ddlBusinessUnit_SelectedIndexChanged">
</asp:DropDownList>

code behind:

Protected Sub ddlBusinessUnit_Select开发者_开发知识库edIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
    Response.Write("Hello")
End

How can I ensure that the event method is called?

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Are you populating your Dropdownlist on every Postback?

Use IsPostBack to check whether the page is loaded for the first time or not.

If Not IsPostBack Then
   BinDdlBusinessUnit()
End If