I don't see the good point having textbox containing the file content. Textboxes can't contain any nullchars (chr(0)) so you will probably have it corrupted if you put something that's not formatted as plain text. Here's the fast way to get your file as a string property:

Code:
Property Get File(Filename As String) As String
    Dim fnum As Byte
    fnum = FreeFile
    Open Filename For Binary As fnum
        File = Space(LOF(fnum))
        Get #fnum, , File
    Close fnum
End Property
Property Let File(Filename As String, textstring As String)
    Dim fnum As Byte
    If Dir(Filename) <> "" Then Kill Filename
    fnum = FreeFile
    Open Filename For Binary As fnum
        Put #fnum, , textstring
    Close fnum
End Property