如何在VB .Net中使用外部配置.ini文件？

Question

我在VB.Net中有使用Winsock的程序。我想在.ini文件中进行外部配置。我运行它时程序可以运行，但是当我想要使用端口时，我会收到错误。

这是读取.ini文件的函数：

Public Function GetSettingItem(ByVal File As String, ByVal Identifier As String) As String
    Dim S As New IO.StreamReader(File) : Dim Result As String = ""
    Do While (S.Peek <> -1)
        Dim Line As String = S.ReadLine
        If Line.ToLower.StartsWith(Identifier.ToLower & ":") Then
            Result = Line.Substring(Identifier.Length + 2)
        End If
    Loop
    Return Result
End Function

这是Winsock的监听端口：

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    AxWinsock1.LocalPort = GetSettingItem("D:\MainConfig.ini", "port")
    AxWinsock1.Listen()
End Sub

错误

An unhandled exception of type 'System.InvalidCastException' occurred in Microsoft.VisualBasic.dll

Additional information: Conversion from string "" to type 'Integer' is not valid.

问题出在哪里？如何在.ini文件中监听端口？