Private Declare Function GetOpenFileName Lib "comdlg32.dll" Alias "GetOpenFileNameA" (pOpenfilename As OPENFILENAME) As Long
Private Type OPENFILENAME
    lStructSize As Long
    hwndOwner As Long
    hInstance As Long
    lpstrFilter As String
    lpstrCustomFilter As String
    nMaxCustFilter As Long
    nFilterIndex As Long
    lpstrFile As String
    nMaxFile As Long
    lpstrFileTitle As String
    nMaxFileTitle As Long
    lpstrInitialDir As String
    lpstrTitle As String
    flags As Long
    nFileOffset As Integer
    nFileExtension As Integer
    lpstrDefExt As String
    lCustData As Long
    lpfnHook As Long
    lpTemplateName As String
End Type
Private Function Prueba() As String
    
    Dim OFName As OPENFILENAME
    OFName.lStructSize = Len(OFName)
    OFName.lpstrFilter = "Ficheros de Texto (*.txt)" + Chr$(0) + "*.txt" + Chr$(0) + "All Files (*.*)" + Chr$(0) + "*.*" + Chr$(0)
    OFName.lpstrFile = Space$(254)
    OFName.nMaxFile = 255
    OFName.lpstrFileTitle = Space$(254)
    OFName.nMaxFileTitle = 255
    OFName.lpstrInitialDir = "C:\"
    OFName.lpstrTitle = "Abrir Fichero"
    OFName.flags = 0

    'Muestra el dialogo
    If GetOpenFileName(OFName) Then
        Prueba = Trim$(OFName.lpstrFile)
    Else
        Prueba = ""
    End If
End Function