VB.NET - problème de string

MonFichier = Chemin.FileName  as string

Private Sub Serialisation()
        Try
            'On crée le fichier et récupère ses données
            Dim Fichier As FileStream = File.Create("bibliotequeContact.xml")
            Dim Serialiseur As New XmlSerializer(GetType(List(Of Contact)))

            'Enregistrement et écriture
            Serialiseur.Serialize(Fichier, _ListeContact)
            'Fermeture du fichier
            Fichier.Close()
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

    MonFichier = Chemin.FileName.ToString

Private _ListeContact As List(Of Contact) 

Public Property ListeContact As List(Of Contact) 
        Get 
            Return _ListeContact 
        End Get 
        Set(ByVal value As List(Of Contact)) 
            _ListeContact = value 
        End Set 
    End Property

 
Dim chemin as string...

Public Sub New(ByVal surnom As String, ByVal chemin As String, ByVal nom As String, ByVal prenom As String, ByVal DateNaissance As Date, ByVal groupe As String, ByVal adresse As String, ByVal ville As String, ByVal fixe As String, ByVal port As String, ByVal pro As String, ByVal mail As String) 
        _surnom = surnom 
        _chemin = chemin 
        _nom = nom 
        _prenom = prenom 
        _dateNaissance = DateNaissance 
        _groupe = groupe 
        _adresse = adresse 
        _ville = ville 
        _mail = mail 
        _fixe = fixe 
        _port = port 
        _pro = pro 
    End Sub

FichierSelect = Chemin.FileName
        If FichierSelect IsNot Nothing Then
            Me.lbl_chemin.Text = FichierSelect
            Me.lbl_chemin.Text = CStr(Me.lbl_chemin.Text)
        End If

        Me.txt_chemin.Text = Me.lbl_chemin.Text

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        Dim chemin As New OpenFileDialog
        Dim fichier As String

        chemin.ShowDialog()
        fichier = chemin.FileName

        Me.TextBox1.Text = fichier

    End Sub
Private Sub TextBox1_MouseDoubleClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseDoubleClick
        Dim maliste As New List(Of String)

        maliste.Add(TextBox1.Text)
        MessageBox.Show(maliste.Item(0).ToString())

    End Sub

   <?xml version="1.0" ?> 
  <string>C:\Documents and Settings\All Users\test.jpg</string>  

XmlSerializer xmls = new XmlSerializer(typeof(string));
            FileStream fichier = new FileStream(@"C:\test.xml", FileMode.CreateNew);

            OpenFileDialog ofd = new OpenFileDialog();
            ofd.ShowDialog();
            string test = ofd.FileName.ToString();

            textBox1.Text = test;

            List<string> liste = new List<string>();

            liste.Add(textBox1.Text);

            xmls.Serialize(fichier, liste[0]);