Public Class TextboxAutoAppend
    Inherits TextBox
 
    Public Property AutoAppendList As List(Of String)
    Public Property AutoAppendKeyString As String
 
    Protected Overrides Sub OnKeyUp _
       (e As System.Windows.Forms.KeyEventArgs)
 
        MyBase.OnKeyUp(e)
 
        If e.KeyCode = Keys.Back Then Exit Sub
        If e.KeyCode = Keys.Left Or e.KeyCode = Keys.Right Then Exit Sub
 
        If String.IsNullOrEmpty(AutoAppendKeyString) Then Exit Sub
        If AutoAppendList Is Nothing OrElse AutoAppendList.Count = 0 Then Exit _
          Sub
 
        Dim pos As Integer = _
              Me.Text.ToUpper.IndexOf(AutoAppendKeyString.ToUpper)
        Dim posl As Integer = _
              Me.Text.ToUpper.LastIndexOf(AutoAppendKeyString.ToUpper)
        If Not pos = posl Then Exit Sub
 
        If pos < 0 Or pos = Me.Text.Length - AutoAppendKeyString.Length Then _
          Exit Sub
 
        Dim startstring As String = _
            Me.Text.ToUpper.Substring(pos + AutoAppendKeyString.Length)
 
        Dim found As Boolean, i As Integer = -1, appendtext As String = ""
        Do
            i += 1
            If AutoAppendList(i).Length >= startstring.Length Then         
               found = AutoAppendList(i).ToLower.StartsWith(startstring.ToLower)
               AppendText = AutoAppendList(i).Substring(startstring.Length)
            End If
        Loop While i < AutoAppendList.Count - 1 And Not found
 
        If found Then
            Me.AppendText(appendtext)
            Me.SelectionStart = Me.Text.Length - appendtext.Length
            Me.SelectionLength = appendtext.Length
            e.Handled = True
        End If
    End Sub
End Class

    'ab diesem Zeichen startet der Vervollständigungsversuch
    TextboxAutoAppend1.AutoAppendKeyString = "@"
 
    'Liste der anzuhängenden Zeichenfolgen
    TextboxAutoAppend1.AutoAppendList = _
        New List(Of String)(New String() _
         {"outlook.com", _
          "outback.de", _
          "hotmail.com", _
          "stofanet.dk", _
          "gmail.com", _
          "nuhme.dk"})