<dx:ASPxTreeList ID="treeList" runat="server" Width="100%" KeyFieldName="ID" ParentFieldName="Parent_ID"
    AutoGenerateColumns="False" OnDataBound="treeList_DataBound">
    <Columns>
        <dx:TreeListDataColumn FieldName="Title" VisibleIndex="0" />
    </Columns>
</dx:ASPxTreeList>

using DevExpress.Web.ASPxTreeList;
using System;
using System.Collections.Generic;
using System.Data;

protected void Page_Load(object sender, EventArgs e) {
    DataTable dt = new DataTable();
    dt.Columns.Add("ID");
    dt.Columns.Add("Parent_ID");
    dt.Columns.Add("Title");

    dt.Rows.Add(dt.NewRow());
    dt.Rows[dt.Rows.Count - 1]["ID"] = 1;
    dt.Rows[dt.Rows.Count - 1]["Parent_ID"] = null;
    dt.Rows[dt.Rows.Count - 1]["Title"] = "One";

    dt.Rows.Add(dt.NewRow());
    dt.Rows[dt.Rows.Count - 1]["ID"] = 4;
    dt.Rows[dt.Rows.Count - 1]["Parent_ID"] = 1;
    dt.Rows[dt.Rows.Count - 1]["Title"] = "One A";

    dt.Rows.Add(dt.NewRow());
    dt.Rows[dt.Rows.Count - 1]["ID"] = 5;
    dt.Rows[dt.Rows.Count - 1]["Parent_ID"] = 1;
    dt.Rows[dt.Rows.Count - 1]["Title"] = "One B";

    treeList.DataSource = dt;
    treeList.DataBind();
}

protected void treeList_DataBound(object sender, EventArgs e) {
    ASPxTreeList list = sender as ASPxTreeList;
    ProcessNodes(list.Nodes[0]);
}
List<string> nodeKeys;
void ProcessNodes(TreeListNode startNode) {
    if (startNode == null) return;
    TreeListNodeIterator iterator = new TreeListNodeIterator(startNode);
    nodeKeys = new List<string>();
    while (iterator.Current != null) {
        GetParentNodeKey(iterator.Current);
        iterator.GetNext();
    }
}

private void GetParentNodeKey(TreeListNode node) {
    if (node != treeList.RootNode && node.HasChildren)
        nodeKeys.Add(node.Key);
}

Imports DevExpress.Web.ASPxTreeList
Imports System
Imports System.Collections.Generic
Imports System.Data

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
    Dim dt As System.Data.DataTable = New System.Data.DataTable()
    dt.Columns.Add("ID")
    dt.Columns.Add("Parent_ID")
    dt.Columns.Add("Title")
    dt.Rows.Add(dt.NewRow())
    dt.Rows(dt.Rows.Count - 1)("ID") = 1
    dt.Rows(dt.Rows.Count - 1)("Parent_ID") = Nothing
    dt.Rows(dt.Rows.Count - 1)("Title") = "One"
    dt.Rows.Add(dt.NewRow())
    dt.Rows(dt.Rows.Count - 1)("ID") = 4
    dt.Rows(dt.Rows.Count - 1)("Parent_ID") = 1
    dt.Rows(dt.Rows.Count - 1)("Title") = "One A"
    dt.Rows.Add(dt.NewRow())
    dt.Rows(dt.Rows.Count - 1)("ID") = 5
    dt.Rows(dt.Rows.Count - 1)("Parent_ID") = 1
    dt.Rows(dt.Rows.Count - 1)("Title") = "One B"
    treeList.DataSource = dt
    treeList.DataBind()
End Sub

Protected Sub treeList_DataBound(ByVal sender As Object, ByVal e As EventArgs)
    Dim list As ASPxTreeList = TryCast(sender, ASPxTreeList)
    ProcessNodes(list.Nodes(0))
End Sub

Private nodeKeys As List(Of String)

Private Sub ProcessNodes(ByVal startNode As TreeListNode)
    If startNode Is Nothing Then Return
    Dim iterator As TreeListNodeIterator = New TreeListNodeIterator(startNode)
    nodeKeys = New List(Of String)()

    While iterator.Current IsNot Nothing
        GetParentNodeKey(iterator.Current)
        iterator.GetNext()
    End While
End Sub

Private Sub GetParentNodeKey(ByVal node As TreeListNode)
    If Not node Is treeList.RootNode AndAlso node.HasChildren Then
        nodeKeys.Add(node.Key)
    End If
End Sub