终于懂了：TControl.Perform是有返回值的，且看VCL框架如何利用消息的返回值（全部例子都在这里）——它的存在仅仅是为了方便复用消息的返回值

代码如下：

function TControl.Perform(Msg: Cardinal; WParam, LParam: Longint): Longint;
var
  Message: TMessage;
begin
  Message.Msg := Msg;
  Message.WParam := WParam;
  Message.LParam := LParam;
  Message.Result := 0;
  if Self <> nil then WindowProc(Message);
  Result := Message.Result;
end;

虽然函数本身有返回值，但是一般情况下，不使用函数的返回值，而是把返回值记录在消息结构体里面，举例：

procedure PerformEraseBackground(Control: TControl; DC: HDC);
var
  LastOrigin: TPoint;
begin
  GetWindowOrgEx(DC, LastOrigin);
  SetWindowOrgEx(DC, LastOrigin.X + Control.Left, LastOrigin.Y + Control.Top, nil);
  Control.Parent.Perform(WM_ERASEBKGND, Integer(DC), Integer(DC));
  SetWindowOrgEx(DC, LastOrigin.X, LastOrigin.Y, nil);
end;

procedure TControl.ReadState(Reader: TReader);
begin
  Include(FControlState, csReadingState);
  if Reader.Parent is TWinControl then Parent := TWinControl(Reader.Parent);
  inherited ReadState(Reader);
  Exclude(FControlState, csReadingState);
  if Parent <> nil then
  begin
    Perform(CM_PARENTCOLORCHANGED, 0, 0);
    Perform(CM_PARENTFONTCHANGED, 0, 0);
    Perform(CM_PARENTSHOWHINTCHANGED, 0, 0);
    Perform(CM_SYSFONTCHANGED, 0, 0);
    Perform(CM_PARENTBIDIMODECHANGED, 0, 0);
  end;
end;

procedure TControl.Changed;
begin
  Perform(CM_CHANGED, 0, Longint(Self));
end;

procedure TControl.SetVisible(Value: Boolean);
begin
  if FVisible <> Value then
  begin
    VisibleChanging;
    FVisible := Value;
    Perform(CM_VISIBLECHANGED, Ord(Value), 0);
    RequestAlign;
  end;
end;

procedure TControl.SetEnabled(Value: Boolean);
begin
  if FEnabled <> Value then
  begin
    FEnabled := Value;
    Perform(CM_ENABLEDCHANGED, 0, 0);
  end;
end;

procedure TControl.SetTextBuf(Buffer: PChar);
begin
  Perform(WM_SETTEXT, 0, Longint(Buffer));
  Perform(CM_TEXTCHANGED, 0, 0);
end;

但是也有一些情况直接使用Perform函数的返回值，在Controls.pas单元里所有直接使用函数返回值的情况都摘录在这里了：

function TControl.GetTextLen: Integer;
begin
  Result := Perform(WM_GETTEXTLENGTH, 0, 0);
end;

function TControl.GetTextBuf(Buffer: PChar; BufSize: Integer): Integer;
begin
  Result := Perform(WM_GETTEXT, BufSize, Longint(Buffer));
end;

function TWinControl.IsControlMouseMsg(var Message: TWMMouse): Boolean;
var
  Control: TControl;
  P: TPoint;
begin
  if GetCapture = Handle then
  begin
    if (CaptureControl <> nil) and (CaptureControl.Parent = Self) then
      Control := CaptureControl
    else
      Control := nil;
  end
  else
    Control := ControlAtPos(SmallPointToPoint(Message.Pos), False);
  Result := False;
  if Control <> nil then
  begin
    P.X := Message.XPos - Control.Left;
    P.Y := Message.YPos - Control.Top;
    Message.Result := Control.Perform(Message.Msg, Message.Keys, Longint(PointToSmallPoint(P)));
    Result := True;
  end;
end;

procedure TWinControl.DefaultHandler(var Message);
begin
  if FHandle <> 0 then
  begin
    with TMessage(Message) do
    begin
      if (Msg = WM_CONTEXTMENU) and (Parent <> nil) then
      begin
        Result := Parent.Perform(Msg, WParam, LParam);
        if Result <> 0 then Exit;
      end;
      case Msg of
        WM_CTLCOLORMSGBOX..WM_CTLCOLORSTATIC:
          Result := SendMessage(LParam, CN_BASE + Msg, WParam, LParam);
        CN_CTLCOLORMSGBOX..CN_CTLCOLORSTATIC:
          begin
            SetTextColor(WParam, ColorToRGB(FFont.Color));
            SetBkColor(WParam, ColorToRGB(FBrush.Color));
            Result := FBrush.Handle;
          end;
      else
        if Msg = RM_GetObjectInstance then
          Result := Integer(Self)
        else
        begin
        if Msg <> WM_PAINT then
          Result := CallWindowProc(FDefWndProc, FHandle, Msg, WParam, LParam);
        end;
      end;
      if Msg = WM_SETTEXT then
        SendDockNotification(Msg, WParam, LParam);
    end;
  end
  else
    inherited DefaultHandler(Message);
end;

procedure TWinControl.CNKeyUp(var Message: TWMKeyUp);
begin
  if not (csDesigning in ComponentState) then
    with Message do
      case CharCode of
        VK_TAB, VK_LEFT, VK_RIGHT, VK_UP, VK_DOWN,
        VK_RETURN, VK_EXECUTE, VK_ESCAPE, VK_CANCEL:
          Result := Perform(CM_WANTSPECIALKEY, CharCode, 0);
      end;
end;

procedure TWinControl.CNSysChar(var Message: TWMChar);
begin
  if not (csDesigning in ComponentState) then
    with Message do
      if CharCode <> VK_SPACE then
        Result := GetParentForm(Self).Perform(CM_DIALOGCHAR,
          CharCode, KeyData);
end;

procedure TWinControl.WMContextMenu(var Message: TWMContextMenu);
var
  Ctrl: TControl;
begin
  if Message.Result <> 0 then Exit;
  Ctrl := ControlAtPos(ScreenToClient(SmallPointToPoint(Message.Pos)), False);
  if Ctrl <> nil then
    Message.Result := Ctrl.Perform(WM_CONTEXTMENU, 0, Integer(Message.Pos));

  if Message.Result = 0 then
    inherited;
end;

这还不算，还得看看那些记录在消息结构体里的返回值是被如何使用的：

procedure TControl.MouseWheelHandler(var Message: TMessage);
var
  Form: TCustomForm;
begin
  Form := GetParentForm(Self);
  if (Form <> nil) and (Form <> Self) then Form.MouseWheelHandler(TMessage(Message))
  else with TMessage(Message) do
    Result := Perform(CM_MOUSEWHEEL, WParam, LParam);
end;

procedure TControl.DefaultHandler(var Message);
var
  P: PChar;
begin
  with TMessage(Message) do
    case Msg of
      WM_GETTEXT:
        begin
          if FText <> nil then P := FText else P := '';
          Result := StrLen(StrLCopy(PChar(LParam), P, WParam - 1));
        end;
      WM_GETTEXTLENGTH:
        if FText = nil then Result := 0 else Result := StrLen(FText);
      WM_SETTEXT:
        begin
          P := StrNew(PChar(LParam));
          StrDispose(FText);
          FText := P;
          SendDockNotification(Msg, WParam, LParam);
        end;
    end;
end;

procedure TControl.WMMouseWheel(var Message: TWMMouseWheel);
begin
  if not Mouse.WheelPresent then
  begin
    Mouse.FWheelPresent := True;
    Mouse.SettingChanged(SPI_GETWHEELSCROLLLINES);
  end;
  TCMMouseWheel(Message).ShiftState := KeysToShiftState(Message.Keys);
  MouseWheelHandler(TMessage(Message));
  if Message.Result = 0 then inherited; // 如果消息没有被处理，就要送到DefaultHandler里去
end;

procedure TControl.CMMouseWheel(var Message: TCMMouseWheel);
begin
  with Message do
  begin
    Result := 0;
    if DoMouseWheel(ShiftState, WheelDelta, SmallPointToPoint(Pos)) then
      Message.Result := 1
    else if Parent <> nil then
      with TMessage(Message) do
        Result := Parent.Perform(CM_MOUSEWHEEL, WParam, LParam);
  end;
end;

procedure TWinControl.Broadcast(var Message);
var
  I: Integer;
begin
  for I := 0 to ControlCount - 1 do
  begin
    Controls[I].WindowProc(TMessage(Message));
    if TMessage(Message).Result <> 0 then Exit; // 如果有一个子控件（图形和Win控件）处理过了，就退出广播
  end;
end;

procedure TWinControl.DefaultHandler(var Message);
begin
  if FHandle <> 0 then
  begin
    with TMessage(Message) do
    begin
      if (Msg = WM_CONTEXTMENU) and (Parent <> nil) then
      begin
        Result := Parent.Perform(Msg, WParam, LParam);
        if Result <> 0 then Exit; // 即使不退出，好像也没什么机会继续传递了
      end;
      case Msg of
        WM_CTLCOLORMSGBOX..WM_CTLCOLORSTATIC:
          Result := SendMessage(LParam, CN_BASE + Msg, WParam, LParam);
        CN_CTLCOLORMSGBOX..CN_CTLCOLORSTATIC:
          begin
            SetTextColor(WParam, ColorToRGB(FFont.Color));
            SetBkColor(WParam, ColorToRGB(FBrush.Color));
            Result := FBrush.Handle;
          end;
      else
        if Msg = RM_GetObjectInstance then
          Result := Integer(Self)
        else
        begin
        if Msg <> WM_PAINT then
          Result := CallWindowProc(FDefWndProc, FHandle, Msg, WParam, LParam);
        end;
      end;
      if Msg = WM_SETTEXT then
        SendDockNotification(Msg, WParam, LParam);
    end;
  end
  else
    inherited DefaultHandler(Message);
end;

function DoControlMsg(ControlHandle: HWnd; var Message): Boolean;
var
  Control: TWinControl;
begin
  DoControlMsg := False;
  Control := FindControl(ControlHandle);
  if Control <> nil then
    with TMessage(Message) do
    begin
      Result := Control.Perform(Msg + CN_BASE, WParam, LParam);
      DoControlMsg := True; // 不多见的函数返回值写法
    end;
end;

procedure TWinControl.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  with ThemeServices do
  if ThemesEnabled and Assigned(Parent) and (csParentBackground in FControlStyle) then
    begin
      { Get the parent to draw its background into the control's background. }
      DrawParentBackground(Handle, Message.DC, nil, False);
    end
    else
    begin
      { Only erase background if we're not doublebuffering or painting to memory. }
      if not FDoubleBuffered or
         (TMessage(Message).wParam = TMessage(Message).lParam) then
        FillRect(Message.DC, ClientRect, FBrush.Handle);
    end;

  Message.Result := 1;
end;

结论：它的存在仅仅是为了方便复用消息的返回值，至少官方提供的Perform函数清清楚楚、明明白白，就只有这个意思。

当然Perform作为一个函数提供返回值，还有2个好处：1.在处理的过程中偷梁换柱 2.覆盖Perform函数都可以（虽然一般没有必要这么做），但这两点几乎不用考虑。普通程序员怎么可能会做这种修改VCL核心代码的事情，根本没必要。