public class TreeViewVirtualNode :
    TreeViewNode,
    ITreeViewVirtualNode

Public Class TreeViewVirtualNode
    Inherits TreeViewNode
    Implements ITreeViewVirtualNode

<dx:ASPxTreeView ID="treeView" runat="server" EnableCallBacks="true" 
    OnVirtualModeCreateChildren="treeView_VirtualModeCreateChildren" />

using System.Collections.Generic;
using DevExpress.Web.ASPxTreeView;
using System.IO;

public partial class _Default : System.Web.UI.Page {
    const string FileImageUrl = "~/Images/file.png";
    const string DirImageUrl = "~/Images/directory.png";

    protected void Page_Load(object sender, EventArgs e) {
    }

    protected void treeView_VirtualModeCreateChildren(object source, TreeViewVirtualModeCreateChildrenEventArgs e) {
        string parentNodePath = string.IsNullOrEmpty(e.NodeName) ? Page.MapPath("~/") : e.NodeName;
        List<TreeViewVirtualNode> children = new List<TreeViewVirtualNode>();
        if (Directory.Exists(parentNodePath)) {
            foreach (string childPath in Directory.GetDirectories(parentNodePath)) {
                string childDirName = Path.GetFileName(childPath);
                if (IsSystemName(childDirName))
                    continue;
                TreeViewVirtualNode childNode = new TreeViewVirtualNode(childPath, childDirName);
                childNode.Image.Url = DirImageUrl;
                children.Add(childNode);
            }
            foreach (string childPath in Directory.GetFiles(parentNodePath)) {
                string childFileName = Path.GetFileName(childPath);
                if (IsSystemName(childFileName))
                    continue;
                TreeViewVirtualNode childNode = new TreeViewVirtualNode(childPath, childFileName);
                childNode.IsLeaf = true;
                childNode.Image.Url = FileImageUrl;
                children.Add(childNode);
            }
        }
        e.Children = children;
    }

    protected bool IsSystemName(string name) {
        name = name.ToLower();
        return name.StartsWith("app_") || name == "bin"
            || name.EndsWith(".aspx.cs") || name.EndsWith(".aspx.vb");
    }
}

Imports System.Collections.Generic
Imports DevExpress.Web.ASPxTreeView
Imports System.IO

Partial Public Class _Default
    Inherits System.Web.UI.Page
    Private Const FileImageUrl As String = "~/Images/file.png"
    Private Const DirImageUrl As String = "~/Images/directory.png"

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

    Protected Sub treeView_VirtualModeCreateChildren(ByVal source As Object, ByVal e As TreeViewVirtualModeCreateChildrenEventArgs)
        Dim parentNodePath As String
        If String.IsNullOrEmpty(e.NodeName) Then
            parentNodePath = Page.MapPath("~/")
        Else
            parentNodePath = e.NodeName
        End If
        Dim children As New List(Of TreeViewVirtualNode)()
        If Directory.Exists(parentNodePath) Then
            For Each childPath As String In Directory.GetDirectories(parentNodePath)
                Dim childDirName As String = Path.GetFileName(childPath)
                If IsSystemName(childDirName) Then
                    Continue For
                End If
                Dim childNode As New TreeViewVirtualNode(childPath, childDirName)
                childNode.Image.Url = DirImageUrl
                children.Add(childNode)
            Next childPath
            For Each childPath As String In Directory.GetFiles(parentNodePath)
                Dim childFileName As String = Path.GetFileName(childPath)
                If IsSystemName(childFileName) Then
                    Continue For
                End If
                Dim childNode As New TreeViewVirtualNode(childPath, childFileName)
                childNode.IsLeaf = True
                childNode.Image.Url = FileImageUrl
                children.Add(childNode)
            Next childPath
        End If
        e.Children = children
    End Sub

    Protected Function IsSystemName(ByVal name As String) As Boolean
        name = name.ToLower()
        Return name.StartsWith("app_") OrElse name = "bin" OrElse name.EndsWith(".aspx.cs") OrElse name.EndsWith(".aspx.vb")
    End Function
End Class