有关vb.net 调用dll

Imports
Public Class TXTClass1
    Private SW As StreamWriter
      Public Sub New()

    End Sub
    '写入追加的形式写入一行
    Public Sub ZhuiJiaSingle(ByVal Parth As String, ByVal strline As String)
        SW = New StreamWriter(Parth, True, System.Text.Encoding.Default)
        SW.WriteLine(strline)
        SW.Flush()
        SW.Close()
        SW = Nothing
    End Sub
以上是dll内容，以下是调用总报错！请高手指教。谢谢！
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
      Dim bool As Boolean = False
        bool = ZhuiJiaSingle(Application.StartupPath & "\Wenben1.txt", TextBox1.Text)

        If bool Then
            MessageBox.Show("数据已写入")
        Else
            MessageBox.Show("写入数据时出错")
        End If
    End Sub