How can I call a function in code behind file in ASP.NET?

Question

I have ShowValue working. Now, on the last line I need to call the function newFunction to repopulate the dropdownlist object on the onClick event.

I am getting errors when I click the dropdownlist down arrow.

How do I get the two to hand shake with each other?

My code:

Public Sub ShowValue(ByVal sender As Object, ByVal e As System.EventArgs)

    lblupdatePanel.Text = DropDownList1.SelectedValue.ToString
    Dim LocationDescription2 As DropDownList = CType(dvContact.FindControl("LocationDescription2"), DropDownList)

    Dim LocationLogic As New LocationBLL
    LocationDescription2.DataSource = LocationLogic.GetUnitByUnitID(DropDownList1.SelectedValue.ToString)
    LocationDescription2.DataTextField = "LocationDescription"
    LocationDescription2.DataValueField = "LocationCode"
    LocationDescription2.SelectedValue = DropDownList1.SelectedValue.ToString
    LocationDescription2.DataBind()

    LocationDescription2.Attributes.Add("onclick", "newFunction(what to put here?);")

End Sub

Public Sub newFunction(ByVal sender As Object, ByVal e As System.EventArgs)

    Dim LocationDescription2 As DropDownList = CType(dvContact.FindControl("LocationDescription2"), DropDownList)
    Dim DivisionDescription1 As DropDownList = CType(dvContact.FindControl("DivisionDescription1"), DropDownList)

    Dim LocationLogic As New LocationBLL
    LocationDescription2.DataSource = LocationLogic.GetLocationByDivisionCode(DivisionDescription1.SelectedValue.ToString)
    LocationDescription2.DataTextField = "LocationDescription"
    LocationDescription2.DataValueField = "LocationCode"
    LocationDescription2.DataBind()

End Sub

Answer 1

LocationDescription2.Attributes.Add("onclick", "newFunction(what to put here?);"

Adds a client side javascript event handler which can't call VB.NET codebehind functions. You need to wire the event up in VB.NET

So it will trigger a postback at which time ASP.NET can invoke the codebehind event handler.

Your best off defining event handlers in Page_Load though.

Answer 2

The Attributes.Add method you're calling only effects the rendered html for the control. The rendered HTML can only directly call javascript functions. You need this code to be called at the server. Remember to keep the HTML/DOM view of a page and server view of a page separate in your head.

To hook up an event in the server view, the vb.net syntax looks like this:

AddHandler LocationDescription2.[EventName], AddressOf newFunction

Unfortunately, the DropDownList control does not have a click event, so you will need to replace EventName from my sample with one of the events shown here:
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.dropdownlist_events.aspx

You'll have to settle for the events included in that last, or do some complicated javascript work to simulate a Click event.

Additionally, it's important to remember that when you handle server side events not only does your specific event handler run, but the entire page is rebuilt as well. It does a full postback (ajax aside for the moment). You need to make sure that's really what you intend.

Answer 3

ASP.Net Ajax Toolkit. Look for the Cascading Dropdown Lists and the associated examples. It'll look at act better too.

ASP.Net Ajax Toolkitt