Help with Progress Bar and tree view

Hi guys,

Im using the standard Progress Bar in Silverlight 3. Im returning a dataset using LINQ and then populating a treeview by looping through the returned data.

What i want to do is show the progress of this through a progress bar.

The problem is progress bar only updates (or at least shows it) when the process had finished. Im not seeing the progress as i thought i would.

Here is my code

Code:

---

PlotXMLData = XDocument.Parse(e.Result)

            TreeProgBar.Value = 0
            Dim LettingCount = From i In PlotXMLData...<Letting> Select i.@LettingsID
            TreeProgBar.Maximum = LettingCount.Count

            'Dim Letting As Object
            Dim n As Liquid.Node
            For Each SiteNode As Liquid.Node In LettingsTree.Nodes
                Dim intSiteID As Integer
                intSiteID = SiteNode.ID

                Dim Letting = From i In PlotXMLData...<Letting> Select i.@LettingsID, i.@PlotID, i.@SiteID Where SiteID.ToString = intSiteID.ToString

                For c = 0 To Letting.Count - 1
                    n = New Liquid.Node
                    n.Title = Letting(c).PlotID
                    n.ID = Letting(c).LettingsID
                    SiteNode.Nodes.Add(n)

                    TreeProgBar.Value = TreeProgBar.Value + 1
                    TreeProgBar.UpdateLayout()

                Next

            Next

            LettingsTree.UpdateLayout(

---

Any help would be great. Thanks