NOTE: I have been receiving a lot of traffic on this posting and don't see (m)any other solutions out there so I decided to port the code to Visual Basic and repost the entry. Most of the writeup is in C#, but a VB solution is included in the download. Sorry for the VB gripes, but I am trying to learn a bit here.... On another note, is reposting a social faux pas in the blog world?

I frequently use an ASP.NET TreeView as a navigation menu. This works well on a single page but if you include the control on a Master Page, it looses its expansion state as you move from one content page to another.

After populating my TreeView, either expand or collapse all of the nodes to insure that there is a consistent default behavior. Then restore your TreeView expansion state to a previously saved state by invoking the RestoreTreeView method of the TreeViewState class:

// set the default state of all nodes.

TreeViewMain.CollapseAll();

// get the saved state of all nodes.

new TreeViewState().RestoreTreeView(TreeViewMain, this.GetType().ToString());

Before navigating to a new page, save the existing state for future use but first you need to overcome the urge to use the NavigateUrl property on a TreeNode. Using this property will generate an HTML hyperlink tag on your TreeView which will cause the browser to initiate the transfer to the new Url. Control is not returned to the server and you will never have the opportunity to save the existing state. Instead, wire up the TreeView_SelectedNodeChanged event for the TreeView, put your page address into the Node.Value property and use a Response.Redirect():

protected void TreeViewMain_SelectedNodeChanged(object sender, EventArgs e)

{

    if (TreeViewMain.SelectedNode.Value != string.Empty)

    {

        Response.Redirect(TreeViewMain.SelectedNode.Value);

    }

}

Next, save the TreeView state by subscribing to the TreeView_Unload event. This event is fired just before the control is unloaded from memory. Pass your TreeView to the SaveTreeView method of the TreeViewState class:

protected void TreeViewMain_Unload(object sender, EventArgs e)

{

    // save the state of all nodes.

    new TreeViewState().SaveTreeView(TreeViewMain, this.GetType().ToString());

}

Within the SaveTreeView and RestoreTreeView methods of the class, I recursively walk the nodes collection and either save or restore the TreeNode.Expanded property in/from a generic list of type <bool?>. The TreeNode uses a nullable boolean value to store this state so I used the same type in a List.

The expansion state is saved in a Session variable. It is possible that you will want to save the state for two identically named TreeView controls on different master pages within your application. This would cause an issue with the name of the Session variable so I included a 'key' parameter that is concatenated with the ID of the TreeView to form the name of the session variable. You can pass any string value as the key, but using the name of the invoking master page class should eliminate most conflicts. I coded this as 'this.GetType().ToString()' which will return a string of the class type.

Lastly, if the number of nodes exceeds the element count in the List<> object, I simply return from the method. This might happen if the data source for your TreeView changes on the fly. If your data source if routinely changing, you will need to tweak the class to better handle this scenario.

As I indicated earlier, I have now included a VB.NET version of the solution. This is the most VB that I have coded since moving from VB6 to C# when I made the jump to .NET in the early beta days sometime in 2001. I love the simple syntax of C# compared to the wordiness of VB. Take a look as how the two handle generic collections:

VB.NET:

Public Sub RestoreTreeView(ByVal treeView As TreeView, ByVal key As String)

    Dim list As New List(Of Nullable(Of Boolean))

    If HttpContext.Current.Session(key + treeView.ID) IsNot Nothing Then

        list = CType(HttpContext.Current.Session(key + treeView.ID), List(Of Nullable(Of Boolean)))

    End If

    RestoreTreeViewIndex = 0

    RestoreTreeViewExpandedState(treeView.Nodes, list)

End Sub

C#:

public void RestoreTreeView(TreeView treeView, string key)

{

    RestoreTreeViewIndex = 0;

    RestoreTreeViewExpandedState(treeView.Nodes,

        (List<bool?>)HttpContext.Current.Session[key + treeView.ID] ?? new List<bool?>());

}

Is there a cleaner way of doing this in VB? Please let me know. Also is there a way of instantiating a class and invoking a method in one statement in VB.NET?

VB.NET:

Dim treeViewState As New TreeViewState()

treeViewState.SaveTreeView(TreeViewMain, Me.GetType.ToString())

C#:

new TreeViewState().SaveTreeView(TreeViewMain, this.GetType().ToString());

I am the first to admit that VB.NET has its place. I know that when it is time to work with Office automation, I will be using VB.NET so please, no holy wars.

Download the complete CS and VB solutions:
http://download.binaryocean.com/TreeViewSaveStateSolution.zip

Here is the complete CS class. The VB version is included in the download:

using System;

using System.Collections.Generic;

using System.Web;

using System.Web.UI.WebControls;

public class TreeViewState

{

    public void SaveTreeView(TreeView treeView, string key)

    {

        List<bool?> list = new List<bool?>();

        SaveTreeViewExpandedState(treeView.Nodes, list);

        HttpContext.Current.Session[key + treeView.ID] = list;

    }

    private int RestoreTreeViewIndex;

    public void RestoreTreeView(TreeView treeView, string key)

    {

        RestoreTreeViewIndex = 0;

        RestoreTreeViewExpandedState(treeView.Nodes,

            (List<bool?>)HttpContext.Current.Session[key + treeView.ID] ?? new List<bool?>());

    }

    private void SaveTreeViewExpandedState(TreeNodeCollection nodes, List<bool?> list)

    {

        foreach (TreeNode node in nodes)

        {

            list.Add(node.Expanded);

            if (node.ChildNodes.Count > 0)

            {

                SaveTreeViewExpandedState(node.ChildNodes, list);

            }

        }

    }

    private void RestoreTreeViewExpandedState(TreeNodeCollection nodes, List<bool?> list)

    {

        foreach (TreeNode node in nodes)

        {

            if (RestoreTreeViewIndex >= list.Count) break;

            node.Expanded = list[RestoreTreeViewIndex++];

            if (node.ChildNodes.Count > 0)

            {

                RestoreTreeViewExpandedState(node.ChildNodes, list);

            }

        }

    }

}