Option Explicit
 
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
Private Declare Function LockWindowUpdate Lib "user32" (ByVal hwndLock As Long) As Long
 
Private Const EM_SETSEL = &HB1
Private Const EM_GETLINECOUNT = &HBA
Private Const EM_LINEINDEX = &HBB
 
Private Sub BorrarLinea(ByVal pNumeroLinea As Long)
    Dim vComienzo As Long, vFin As Long, vCantLineas As Long
 
    With Text1
        vCantLineas = SendMessage(.hwnd, EM_GETLINECOUNT, 0, ByVal 0&)
 
        Select Case True
            Case pNumeroLinea > vCantLineas - 1
            Exit Sub
        Case pNumeroLinea = 0 And vCantLineas = 1
            vComienzo = 0
            vFin = Len(.Text)
        Case pNumeroLinea = vCantLineas - 1
            vComienzo = SendMessage(.hwnd, EM_LINEINDEX, pNumeroLinea, ByVal 0&) - 1
            vFin = Len(.Text) - vComienzo + 1
        Case Else
            vComienzo = SendMessage(.hwnd, EM_LINEINDEX, pNumeroLinea, ByVal 0&)
            vFin = SendMessage(.hwnd, EM_LINEINDEX, pNumeroLinea + 1, ByVal 0&) - vComienzo
        End Select
 
        .SetFocus
        Call SendMessage(.hwnd, EM_SETSEL, vComienzo, ByVal vFin)
 
        LockWindowUpdate .hwnd
        .SelStart = vComienzo
        .SelLength = vFin
        .SelText = vbNullString
        LockWindowUpdate False
    End With
End Sub
 
Private Sub Command1_Click()
    Call BorrarLinea(2) ' Borra la tercera linea del textbox
End Sub