关于treeview 中分级加载展开"+"问题

[rkdrc]

public Function NodeLevel(ByVal n As TreeNode) As Byte
'* 找出树中当前节点的级数
Dim i As Byte = 1
'Dim m As String
Do Until n.Parent Is Nothing
n = n.Parent
i = i + 1
Loop
Return i
End Function
Private Sub TreeView1_AfterSelect(sender As Object, e As TreeViewEventArgs) Handles TreeView1.AfterSelect
Dim SQLDB As New SQLDataBase(strConnect)
Dim strTable As New DataTable
Dim strProjectID As String = Split(SelectNodeID, "_")(1)

Select Case NodeLevel(e.Node).ToString
Case "1"
If e.Node.GetNodeCount(False) = 0 Then
strTable = SQLDB.ExecuteSQLToDataTable("Select * from tb_Matching_Region where Project_ID='" & strProjectID & "'")
For i As Integer = 0 To strTable.Rows.Count - 1
Dim tree_leaf As New TreeNode()
tree_leaf.Tag = strTable.Rows(i)("Matching_Region_ID").ToString()
tree_leaf.Text = strTable.Rows(i)("Matching_Name").ToString()
TreeView1.SelectedNode.Nodes.Add(tree_leaf)
Next
End If
Case "2"
If e.Node.GetNodeCount(False) = 0 Then
strTable = SQLDB.ExecuteSQLToDataTable("Select * from tb_Matching_Part where Matching_Region_ID='" & e.Node.Tag & "'")
For i As Integer = 0 To strTable.Rows.Count - 1
Dim tree_leaf As New TreeNode()
tree_leaf.Tag = strTable.Rows(i)("Matching_Part_ID").ToString()
tree_leaf.Text = strTable.Rows(i)("Part_ID").ToString()
TreeView1.SelectedNode.Nodes.Add(tree_leaf)
Next
End If
End Select
End Sub
以上属于分层加载功能，但存在一个问题，就是不显示“+”，只有单击某个节点后才能知道到底有没有下一级内容，如何自动加载子层级加号

[良朋]

this.treeView1.Nodes.Clear();
this.treeView1.BeginUpdate();

[rkdrc]

这好像不解决