How to make MessageDlg centered on owner form

You can do

function MessageDlg(const AOwner: TForm; const Msg: string; DlgType: TMsgDlgType;
  Buttons: TMsgDlgButtons; HelpCtx: Integer = 0): Integer;
begin
  with CreateMessageDialog(Msg, DlgType, Buttons) do
    try
      Left := AOwner.Left + (AOwner.Width - Width) div 2;
      Top := AOwner.Top + (AOwner.Height - Height) div 2;
      Result := ShowModal;
    finally
      Free;
    end
end;

and call it like

procedure TForm1.FormClick(Sender: TObject);
begin
  MessageDlg(Self, 'This is a test', mtInformation, [mbOK]);
end;

However, I would personally not do this, because the dialog shown by CreateMessageDialog is not a native Windows dialog. Compare the visual result with the native stuff:

procedure TForm1.FormClick(Sender: TObject);
begin
  case MessageBox(Handle, PChar('This is a test. Do you wish to do something?'), PChar('A Silly Example'), MB_ICONQUESTION or MB_YESNO) of
    ID_YES:
      MessageBox(Handle, PChar('Great!'), PChar('A Silly Example'), MB_ICONINFORMATION or MB_OK);
    ID_NO:
      MessageBox(Handle, PChar('OK, well, I cannot force you...'), PChar('A Silly Example'), MB_ICONINFORMATION or MB_OK);
  end;
end;

At least in Windows 7 with the Aero theme enabled, the native dialog looks much better. However, it seems, this cannot be centered over any particular form. Instead, the dialog is centered on the current monitor. But this is also the default behaviour in Windows (try Notepad, WordPad, or Paint), so why do you need this new behaviour?

Why limit this desire to message dialogs? Like David Heffernan commented:

Native dialogs always win!

With the following unit(s), you can center any native dialog, such as: MessageBox, TFindDialog, TOpenDialog, TFontDialog, TPrinterSetupDialog, etc... The main unit provides two routines, both with some optional parameters:

function ExecuteCentered(Dialog: TCommonDialog;
  WindowToCenterIn: HWND = 0): Boolean;
function MsgBox(const Text: String; Flags: Cardinal = DefFlags;
  const Caption: String = DefCaption;
  WindowToCenterIn: HWND = 0): Integer;

Wherelse you would use OpenDialog1.Execute and let Windows decide where to show the dialog, you now use ExecuteCentered(OpenDialog1) and the dialog is centered in the screen's active form:

To show message dialogs, use MsgBox, a wrapper around Application.MessageBox (which in turn is a wrapper around Windows.MessageBox). Some examples:

• MsgBox('Hello world!');
• MsgBox('Cancel saving?', MB_YESNO or MB_ICONQUESTION or MB_DEFBUTTON2);
• MsgBox('Please try again.', MB_OK, 'Error');
• MsgBox('I''m centered in the toolbar.', MB_OK, 'Fun!', Toolbar1.Handle);

The units:

unit AwDialogs;

interface

uses
  Dialogs, Forms, Windows, Controls, Messages, AwHookInstance, Math, MultiMon;

const
  DefCaption = 'Application.Title';
  DefFlags = MB_OK;

procedure CenterWindow(WindowToStay, WindowToCenter: HWND);
function GetTopWindow: HWND;

function ExecuteCentered(Dialog: TCommonDialog;
  WindowToCenterIn: HWND = 0): Boolean;
function MsgBox(const Text: String; Flags: Cardinal = DefFlags;
  const Caption: String = DefCaption;
  WindowToCenterIn: HWND = 0): Integer;

implementation

procedure CenterWindow(WindowToStay, WindowToCenter: HWND);
var
  R1: TRect;
  R2: TRect;
  Monitor: HMonitor;
  MonInfo: TMonitorInfo;
  MonRect: TRect;
  X: Integer;
  Y: Integer;
begin
  GetWindowRect(WindowToStay, R1);
  GetWindowRect(WindowToCenter, R2);
  Monitor := MonitorFromWindow(WindowToStay, MONITOR_DEFAULTTONEAREST);
  MonInfo.cbSize := SizeOf(MonInfo);
  GetMonitorInfo(Monitor, @MonInfo);
  MonRect := MonInfo.rcWork;
  with R1 do
  begin
    X := (Right - Left - R2.Right + R2.Left) div 2 + Left;
    Y := (Bottom - Top - R2.Bottom + R2.Top) div 2 + Top;
  end;
  X := Max(MonRect.Left, Min(X, MonRect.Right - R2.Right + R2.Left));
  Y := Max(MonRect.Top, Min(Y, MonRect.Bottom - R2.Bottom + R2.Top));
  SetWindowPos(WindowToCenter, 0, X, Y, 0, 0, SWP_NOACTIVATE or
    SWP_NOOWNERZORDER or SWP_NOSIZE or SWP_NOZORDER);
end;

function GetTopWindow: HWND;
begin
  Result := GetLastActivePopup(Application.Handle);
  if (Result = Application.Handle) or not IsWindowVisible(Result) then
    Result := Screen.ActiveCustomForm.Handle;
end;

{ TAwCommonDialog }

type
  TAwCommonDialog = class(TObject)
  private
    FCenterWnd: HWND;
    FDialog: TCommonDialog;
    FHookProc: TFarProc;
    FWndHook: HHOOK;
    procedure HookProc(var Message: THookMessage);
    function Execute: Boolean;
  end;

function TAwCommonDialog.Execute: Boolean;
begin
  try
    Application.NormalizeAllTopMosts;
    FHookProc := MakeHookInstance(HookProc);
    FWndHook := SetWindowsHookEx(WH_CALLWNDPROCRET, FHookProc, 0,
      GetCurrentThreadID);
    Result := FDialog.Execute;
  finally
    if FWndHook <> 0 then
      UnhookWindowsHookEx(FWndHook);
    if FHookProc <> nil then
      FreeHookInstance(FHookProc);
    Application.RestoreTopMosts;
  end;
end;

procedure TAwCommonDialog.HookProc(var Message: THookMessage);
var
  Data: PCWPRetStruct;
  Parent: HWND;
begin
  with Message do
    if nCode < 0 then
      Result := CallNextHookEx(FWndHook, nCode, wParam, lParam)
    else
      Result := 0;
  if Message.nCode = HC_ACTION then
  begin
    Data := PCWPRetStruct(Message.lParam);
    if (FDialog.Handle <> 0) and (Data.message = WM_SHOWWINDOW) then
    begin
      Parent := GetWindowLong(FDialog.Handle, GWL_HWNDPARENT);
      if ((Data.hwnd = FDialog.Handle) and (Parent = Application.Handle)) or
        ((Data.hwnd = FDialog.Handle) and (FDialog is TFindDialog)) or
        (Data.hwnd = Parent) then
      begin
        CenterWindow(FCenterWnd, Data.hwnd);
        SetWindowPos(Data.hwnd, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE or
          SWP_NOSIZE or SWP_NOACTIVATE or SWP_NOOWNERZORDER);
        UnhookWindowsHookEx(FWndHook);
        FWndHook := 0;
        FreeHookInstance(FHookProc);
        FHookProc := nil;
      end;
    end;
  end;
end;

function ExecuteCentered(Dialog: TCommonDialog;
  WindowToCenterIn: HWND = 0): Boolean;
begin
  with TAwCommonDialog.Create do
  try
    if WindowToCenterIn = 0 then
      FCenterWnd := GetTopWindow
    else
      FCenterWnd := WindowToCenterIn;
    FDialog := Dialog;
    Result := Execute;
  finally
    Free;
  end;
end;

{ TAwMessageBox }

type
  TAwMessageBox = class(TObject)
  private
    FCaption: String;
    FCenterWnd: HWND;
    FFlags: Cardinal;
    FHookProc: TFarProc;
    FText: String;
    FWndHook: HHOOK;
    function Execute: Integer;
    procedure HookProc(var Message: THookMessage);
  end;

function TAwMessageBox.Execute: Integer;
begin
  try
    try
      Application.NormalizeAllTopMosts;
      FHookProc := MakeHookInstance(HookProc);
      FWndHook := SetWindowsHookEx(WH_CALLWNDPROCRET, FHookProc, 0,
        GetCurrentThreadID);
      Result := Application.MessageBox(PChar(FText), PChar(FCaption), FFlags);
    finally
      if FWndHook <> 0 then
        UnhookWindowsHookEx(FWndHook);
      if FHookProc <> nil then
        FreeHookInstance(FHookProc);
      Application.RestoreTopMosts;
    end;
  except
    Result := 0;
  end;
end;

procedure TAwMessageBox.HookProc(var Message: THookMessage);
var
  Data: PCWPRetStruct;
  Title: array[0..255] of Char;
begin
  with Message do
    if nCode < 0 then
      Result := CallNextHookEx(FWndHook, nCode, wParam, lParam)
    else
      Result := 0;
  if Message.nCode = HC_ACTION then
  begin
    Data := PCWPRetStruct(Message.lParam);
    if Data.message = WM_INITDIALOG then
    begin
      FillChar(Title, SizeOf(Title), 0);
      GetWindowText(Data.hwnd, @Title, SizeOf(Title));
      if String(Title) = FCaption then
      begin
        CenterWindow(FCenterWnd, Data.hwnd);
        SetWindowPos(Data.hwnd, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE or
          SWP_NOSIZE or SWP_NOACTIVATE or SWP_NOOWNERZORDER);
        UnhookWindowsHookEx(FWndHook);
        FWndHook := 0;
        FreeHookInstance(FHookProc);
        FHookProc := nil;
      end;
    end;
  end;
end;

function MsgBox(const Text: String; Flags: Cardinal = DefFlags;
  const Caption: String = DefCaption;
  WindowToCenterIn: HWND = 0): Integer;
begin
  with TAwMessageBox.Create do
  try
    if Caption = DefCaption then
      FCaption := Application.Title
    else
      FCaption := Caption;
    if WindowToCenterIn = 0 then
      FCenterWnd := GetTopWindow
    else
      FCenterWnd := WindowToCenterIn;
    FFlags := Flags;
    FText := Text;
    Result := Execute;
  finally
    Free;
  end;
end;

end.

unit AwHookInstance;

interface

uses
  Windows;

type
  THookMessage = packed record
    nCode: Integer;
    wParam: WPARAM;
    lParam: LPARAM;
    Result: LRESULT;
  end;

  THookMethod = procedure(var Message: THookMessage) of object;

function MakeHookInstance(Method: THookMethod): Pointer;
procedure FreeHookInstance(HookInstance: Pointer);

implementation

const
  InstanceCount = 313;

type
  PHookInstance = ^THookInstance;
  THookInstance = packed record
    Code: Byte;
    Offset: Integer;
    case Integer of
      0: (Next: PHookInstance);
      1: (Method: THookMethod);
  end;

  PInstanceBlock = ^TInstanceBlock;
  TInstanceBlock = packed record
    Next: PInstanceBlock;
    Code: array[1..2] of Byte;
    HookProcPtr: Pointer;
    Instances: array[0..InstanceCount] of THookInstance;
  end;

var
  InstBlockList: PInstanceBlock;
  InstFreeList: PHookInstance;

function StdHookProc(nCode: Integer; wParam: WPARAM;
  lParam: LPARAM): LRESULT; stdcall; assembler;
{ In    ECX = Address of method pointer }
{ Out   EAX = Result }
asm
  XOR     EAX,EAX
  PUSH    EAX
  PUSH    LParam
  PUSH    WParam
  PUSH    nCode
  MOV     EDX,ESP
  MOV     EAX,[ECX].Longint[4]
  CALL    [ECX].Pointer
  ADD     ESP,12
  POP     EAX
end;

function CalcJmpOffset(Src, Dest: Pointer): Longint;
begin
  Result := Longint(Dest) - (Longint(Src) + 5);
end;

function MakeHookInstance(Method: THookMethod): Pointer;
const
  BlockCode: array[1..2] of Byte = ($59 { POP ECX }, $E9 { JMP StdHookProc });
  PageSize = 4096;
var
  Block: PInstanceBlock;
  Instance: PHookInstance;
begin
  if InstFreeList = nil then
  begin
    Block := VirtualAlloc(nil, PageSize, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
    Block^.Next := InstBlockList;
    Move(BlockCode, Block^.Code, SizeOf(BlockCode));
    Block^.HookProcPtr := Pointer(CalcJmpOffset(@Block^.Code[2], @StdHookProc));
    Instance := @Block^.Instances;
    repeat
      Instance^.Code := $E8;  { CALL NEAR PTR Offset }
      Instance^.Offset := CalcJmpOffset(Instance, @Block^.Code);
      Instance^.Next := InstFreeList;
      InstFreeList := Instance;
      Inc(Longint(Instance), SizeOf(THookInstance));
    until Longint(Instance) - Longint(Block) >= SizeOf(TInstanceBlock);
    InstBlockList := Block;
  end;
  Result := InstFreeList;
  Instance := InstFreeList;
  InstFreeList := Instance^.Next;
  Instance^.Method := Method;
end;

procedure FreeHookInstance(HookInstance: Pointer);
begin
  if HookInstance <> nil then
  begin
    PHookInstance(HookInstance)^.Next := InstFreeList;
    InstFreeList := HookInstance;
  end;
end;

end.

Legal notice: These units are written by me in this Dutch topic. The original versions are from Mark van Renswoude, see NLDMessageBox.

The dialog doesn't have a relationship with the instance of TForm1. It would not be hard to set the position of the form manually, but I bet someone who is more familiar with this area of the VCL will know how to do it a cleaner way.

Personally I never use the Position property and use my own code to position all my forms because I've never been satisfied with the performance of the Position property.

UPDATE: You can change the owner of the dialog using Self.InsertComponent(Dialog). You'd have to store your dialog into a local variable, say, Dialog, for this to work:

function TForm1.MessageDlg(const Msg: string; DlgType: TMsgDlgType;
  Buttons: TMsgDlgButtons; HelpCtx: Integer): Integer;
var
  Dialog: TForm;
begin
  Dialog := CreateMessageDialog(Msg, DlgType, Buttons);
  try
    Self.InsertComponent(Dialog);
    Dialog.Position := poOwnerFormCenter;
    Result := Dialog.ShowModal
  finally
    Dialog.Free
  end
end;