为什么会出现索引和长度必须引用该字符串内的位置这样的错误

Public Class WebForm1
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not Page.IsPostBack Then
            Call Me.initTree()
        End If
    End Sub
    Sub initTree()
        Dim valuePath As String
        Dim i As Integer
        Dim o As New myData.clsTree
        Dim dr As SqlClient.SqlDataReader
        Dim dv As DataView
        dv = o.getall2ds(" User_id='admin'").Tables(0).DefaultView
        dr = o.getall2dr("")
        Dim myNode As TreeNode
        While dr.Read
            myNode = New TreeNode
            myNode.Expanded = False

            myNode.Value = dr.Item("Typeid").ToString
            dv.RowFilter = " Typeid='" & myNode.Value & "'"
            If dv.Count = 0 Then
                Continue While
            End If
            myNode.Text = dr.Item("FullName").ToString
            'myNode.ImageUrl = dr.Item("imgUrl").ToString
            If dr.Item("isGroup") = "0" Then
                myNode.Target = "WebForm1"
                myNode.NavigateUrl = "WebForm1.aspx"
            Else
                myNode.NavigateUrl = ""
            End If
            If dr.Item("Parid") = "0" Then
                Me.TreeView1.Nodes.Add(myNode)
            Else
                valuePath = ""
                For i = 1 To dr.Item("Parid").ToString.Length / 2
                    If valuePath = "" Then
                        valuePath = dr.Item("Parid").ToString.Substring(0, i * 2)
                    Else
                        valuePath &= "/" & dr.Item("Parid").ToString.Substring(0, i * 2)
                    End If
                Next
                If Not Me.TreeView1.FindNode(valuePath) Is Nothing Then
                    Me.TreeView1.FindNode(valuePath).ChildNodes.Add(myNode)
                End If

            End If
        End While
        dr.Close()
    End Sub
   
End Class