Tree List for ASP.NET Web Forms - How to restrict selection to a single leaf node

C#
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using DevExpress.Web.ASPxTreeList;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void ASPxTreeList1_DataBound(object sender, EventArgs e) {
        DisableParentNodeSelection();
    }

    protected void DisableParentNodeSelection() {
        // Iterates through all nodes and prevents the display of selection check boxes within parent nodes
        TreeListNodeIterator iterator = ASPxTreeList1.CreateNodeIterator();
        TreeListNode node = iterator.GetNext();
        while (node != null) {
            node.AllowSelect = !node.HasChildren;
            node = iterator.GetNext();
        }
    }
    public string LastSelectedNode
    {
        get
        {
            string key = Session["PrevSelectedNodeKey"] as string;
            if (String.IsNullOrEmpty(key))
                key = "-1";
            return key;
        }
        set {
            Session["PrevSelectedNodeKey"] = value;
        }
    }
    protected void ASPxTreeList1_SelectionChanged(object sender, EventArgs e) {
        ASPxTreeList treeList = sender as ASPxTreeList;

        if (treeList.SelectionCount == 1)  // One node is selected within the control
            LastSelectedNode = treeList.GetSelectedNodes()[0].Key;

        if (treeList.SelectionCount > 1) { // Applies selection to the last selected node, if two nodes are selected
            TreeListNode prevSelectedNode = treeList.FindNodeByKeyValue(LastSelectedNode);
            if (prevSelectedNode != null)
            {
                prevSelectedNode.Selected = false;
                LastSelectedNode = treeList.GetSelectedNodes()[0].Key;
            }
        }
    }

    protected void Button1_Click(object sender, EventArgs e) {
        if (ASPxTreeList1.SelectionCount == 0)
            Label2.Text = "None";
        else {
            TreeListNode node = ASPxTreeList1.GetSelectedNodes()[0];
            Label2.Text = node["Department"].ToString();
        }
    }
}

Visual Basic
Imports System
Imports System.Data
Imports System.Configuration
Imports System.Web
Imports System.Web.Security
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Web.UI.WebControls.WebParts
Imports System.Web.UI.HtmlControls
Imports DevExpress.Web.ASPxTreeList

Partial Public Class _Default
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)

    End Sub

    Protected Sub ASPxTreeList1_DataBound(ByVal sender As Object, ByVal e As EventArgs)
        DisableParentNodeSelection()
    End Sub

    Protected Sub DisableParentNodeSelection()
        ' Iterates through all nodes and prevents the display of selection check boxes within parent nodes
        Dim [iterator] As TreeListNodeIterator = ASPxTreeList1.CreateNodeIterator()
        Dim node As TreeListNode = [iterator].GetNext()
        Do While node IsNot Nothing
            node.AllowSelect = Not node.HasChildren
            node = [iterator].GetNext()
        Loop
    End Sub
    Public Property LastSelectedNode() As String
        Get
            Dim key As String = TryCast(Session("PrevSelectedNodeKey"), String)
            If String.IsNullOrEmpty(key) Then
                key = "-1"
            End If
            Return key
        End Get
        Set(ByVal value As String)
            Session("PrevSelectedNodeKey") = value
        End Set
    End Property
    Protected Sub ASPxTreeList1_SelectionChanged(ByVal sender As Object, ByVal e As EventArgs)
        Dim treeList As ASPxTreeList = TryCast(sender, ASPxTreeList)

        If treeList.SelectionCount = 1 Then ' One node is selected within the control
            LastSelectedNode = treeList.GetSelectedNodes()(0).Key
        End If

        If treeList.SelectionCount > 1 Then ' Applies selection to the last selected node, if two nodes are selected
            Dim prevSelectedNode As TreeListNode = treeList.FindNodeByKeyValue(LastSelectedNode)
            If prevSelectedNode IsNot Nothing Then
                prevSelectedNode.Selected = False
                LastSelectedNode = treeList.GetSelectedNodes()(0).Key
            End If
        End If
    End Sub

    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)
        If ASPxTreeList1.SelectionCount = 0 Then
            Label2.Text = "None"
        Else
            Dim node As TreeListNode = ASPxTreeList1.GetSelectedNodes()(0)
            Label2.Text = node("Department").ToString()
        End If
    End Sub
End Class