There’re situations when you need access ASP.NET web controls very early in page lifecycle, more specifically – in Page
PreInit event – and you can, but only top-level controls. But what if you need to access child/nested controls? The example below uses Infragistics WebHierarchicalDataGrid as a child of Infragistics WebSplitter, but this pretty much applies to any such scenario.
Let’s say you have following layout
<ig:WebSplitter ID="WSP" runat="server"> <Panes> <ig:SplitterPane runat="server"> <Template> <ig:WebHierarchicalDataGrid ID="WHG" runat="server"> </ig:WebHierarchicalDataGrid> </Template> </ig:SplitterPane> <ig:SplitterPane runat="server"></ig:SplitterPane> </Panes> </ig:WebSplitter>
As you can see grid “WHG” is nested withing first pane of splittet “WSP. Let’s see what happens if you try access the controls in PagePreInit event:
Protected Sub Page_PreInit(sender As Object, e As EventArgs) Handles Me.PreInit 'accessing some basic properties Dim sID1 As String = WSP.ID 'this works Dim sID2 As String = WHG.ID 'this throws NullReference exception End Sub
If you run this code, you will realize that top-level/container control
WSP is available along with all its properties, so Line 4 executes fine. But but variable
WHG that should hold reference to nested/child control has value of
null, for all you, C#-heads) so “Object Reference Not Set” error is thrown in Line 5.
I have seen similar scenarios were Master Page is involved and you can Google numerous solutions to this one. But in this case there is no Master Page. But, there’s a way to make the child control available. Let’s change the code a little bit:
Protected Sub Page_PreInit(sender As Object, e As EventArgs) Handles Me.PreInit 'accessing some basic properties Dim sID1 As String = WSP.ID 'this works WSP.FindControl("WHG") '*** force page to look for the control Dim sID2 As String = WHG.ID 'now this also works End Sub
As you can see we added one more line, asking parent control to do a lookup for child control. We’re not assigning result to anything (and it can even return
Nothing) but after that line is executed – variable representing child control will actually have reference to child control and the code runs without an exception.