WebBrowser TargetFrameName值

Question

像这样的WebBrowser示例：

Private Sub Form4_Load(sender As Object, e As EventArgs) Handles Me.Load
    WebBrowser1.Navigate("http://www.google.com")
End Sub

Public ReadOnly Property TargetFrameName As String

Private Sub WebBrowser1_Navigating(sender As Object, e As WebBrowserNavigatingEventArgs) Handles WebBrowser1.Navigating

    'Me.TargetFrameName = TargetFrameName
    Dim messageBoxVB As New System.Text.StringBuilder()
    'MsgBox(e.TargetFrameName.ToString)

    messageBoxVB.AppendFormat("{0} = {1}", "Url", e.Url)
    messageBoxVB.AppendLine()
    messageBoxVB.AppendFormat("{0} = {1}", "TargetFrameName", e.TargetFrameName)
    messageBoxVB.AppendLine()
    messageBoxVB.AppendFormat("{0} = {1}", "Cancel", e.Cancel)
    messageBoxVB.AppendLine()
    MessageBox.Show(messageBoxVB.ToString(), "Navigating Event")

End Sub

消息框显示：

Url= http://www.google.com
TargetFrameName=  
Cancel= False

我的问题是为什么TargetFrameName没有价值或名称？