Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" ( _
     ByVal hwnd As Long, _
     ByVal wMsg As Long, _
     ByVal wParam As Long, _
     lParam As Any) As Long
Const WM_USER = &H400
Const EM_GETLINECOUNT = &HBA


Private Sub Command1_Click()
 
Dim Zeilen As Integer
Dim Zeichen As String
' Den Pfad müssen Sie ggf anpassen !
Open "C:\test.txt" For Input As #1
    Do While Not EOF(1)
        Line Input #1, Zeichen
        Text1.Text = Text1.Text & Zeichen & vbCrLf
    Loop
Close #1
 
    Zeilen = SendMessage(Text1.hwnd, EM_GETLINECOUNT, 0, 0&)
    MsgBox "Die Textbox hat " & Zeilen & " Zeilen."
 
End Sub