File Activation Delphi 2016

Title: File Activation (Handle file open/activation) in Delphi 2016 — best practice

Body: Hi all,

I'm building a Windows desktop app in Delphi 2016 (VCL). I need the app to respond when the user double-clicks an associated file (e.g., .mydoc) or opens a file from Explorer while the app is already running — i.e., receive the filename and bring the running instance to the foreground (single-instance behavior). I want a robust, recommended approach for Delphi 2016.

What I have in mind:

Questions:

Context:

Thanks!

--Example answer (concise) below--

Answer summary

Minimal code sketch (adapt to your project):

uses
  Windows, SysUtils, Messages, Forms, Classes;
const
  MUTEX_NAME = 'Global\MyApp_Mutex_12345';
  WM_MY_COPYDATA = WM_USER + 1;
var
  hMutex: THandle;
function SendArgsToRunningInstance(const Args: string): Boolean;
var
  hwnd: HWND;
  cds: TCOPYDATASTRUCT;
begin
  Result := False;
  hwnd := FindWindow(nil, PChar('MyMainFormCaption')); // better: register a unique class/name
  if hwnd = 0 then Exit;
  cds.dwData := 0;
  cds.cbData := (Length(Args) + 1) * SizeOf(Char);
  cds.lpData := PChar(Args);
  Result := SendMessageTimeout(hwnd, WM_COPYDATA, 0, LPARAM(@cds), SMTO_ABORTIFHUNG, 2000, PDWORD(nil)^) <> 0;
end;
begin
  hMutex := CreateMutex(nil, False, PChar(MUTEX_NAME));
  if (hMutex = 0) then Halt;
  if GetLastError = ERROR_ALREADY_EXISTS then
  begin
    // Send command-line to running instance
    if ParamCount > 0 then
      SendArgsToRunningInstance(ParamStr(1)); // extend to multiple args
    CloseHandle(hMutex);
    Halt;
  end;
Application.Initialize;
  Application.CreateForm(TMainForm, MainForm);
  // In TMainForm.WndProc handle WM_COPYDATA to open file and call SetForegroundWindow
  Application.Run;
  CloseHandle(hMutex);
end.

procedure TMainForm.WndProc(var Message: TMessage);
var
  pcd: PCOPYDATASTRUCT;
  s: string;
begin
  if Message.Msg = WM_COPYDATA then
  begin
    pcd := PCOPYDATASTRUCT(Message.LParam);
    s := PChar(pcd.lpData); // Unicode safe in Delphi 2016
    // handle possibly multiple filenames separated by a delimiter
    Show; BringToFront; SetForegroundWindow(Handle);
    OpenFileByName(s);
    Exit;
  end;
  inherited WndProc(Message);
end;

Pitfalls & tips

If you'd like, I can produce a complete copy-paste-ready example project (.dpr + unit) implementing this pattern.

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