I have managed to make a complete demo program that shows my suggestion in the 2nd comment above. Create a new VCL application. Rename the form to MainForm, place a TListBox on it, align it to client, rename it to ListBox, then make empty events for the form's OnCreate and OnDestroy.
Then copy/paste this PASCAL source into your main form's PAS file from right after "interface", overwriting the code already there:
uses
Winapi.Windows, Winapi.Messages,
System.SysUtils, System.Variants, System.Classes,
Vcl.Graphics, Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
CONST
WM_PEEK = WM_USER+1234;
type
TMainForm = class(TForm)
ListBox: TListBox;
procedure FormDestroy(Sender: TObject);
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
Running : HWND;
PROCEDURE PEEK(VAR MSG : TMessage); MESSAGE WM_PEEK;
PROCEDURE CopyData(VAR MSG : TMessage); MESSAGE WM_COPYDATA;
PROCEDURE BringForward(Sender : TObject);
PROCEDURE SendString(H : HWND ; CONST S : STRING ; E : TEncoding);
FUNCTION CommandLine : STRING;
FUNCTION MakeAtomName(H : HWND) : STRING;
FUNCTION FindGlobalAtom(CONST S : STRING) : ATOM;
FUNCTION AddGlobalAtom(CONST S : STRING) : ATOM;
FUNCTION GetGlobalAtomName(H : ATOM) : STRING;
FUNCTION AtomNameToHandle(CONST S : STRING) : HWND;
FUNCTION DeleteGlobalAtom(A : ATOM) : DWORD;
public
{ Public declarations }
PROCEDURE LOG(CONST S : STRING);
end;
var
MainForm: TMainForm;
implementation
USES System.Character;
{$R *.dfm}
PROCEDURE TMainForm.FormDestroy(Sender : TObject);
VAR
S : STRING;
A : ATOM;
BEGIN
S:=MakeAtomName(0);
REPEAT
A:=FindGlobalAtom(S);
IF A=0 THEN BREAK;
UNTIL DeleteGlobalAtom(A)<>ERROR_SUCCESS
END;
FUNCTION TMainForm.AddGlobalAtom(CONST S : STRING) : ATOM;
BEGIN
Result:=WinAPI.Windows.GlobalAddAtom(PChar(S))
END;
FUNCTION TMainForm.MakeAtomName(H : HWND) : STRING;
CONST
L = 8*SizeOf(POINTER); // 32 or 64 (number of bits in a handle)
VAR
S : STRING;
I : Cardinal;
C : CHAR;
BEGIN
Result:=ChangeFileExt(ExtractFileName(ParamStr(0)),''); S:='';
FOR C IN Result DO IF CharInSet(C,['A'..'Z','a'..'z']) THEN S:=S+C;
WHILE LENGTH(S)<L DO S:=S+S;
SetLength(S,L);
Result:='';
FOR I:=1 TO L DO BEGIN
IF H AND $01<>0 THEN C:=S[I].ToUpper ELSE C:=S[I].ToLower;
Result:=C+Result; H:=H SHR 1
END
END;
FUNCTION TMainForm.AtomNameToHandle(CONST S : STRING) : HWND;
VAR
C : CHAR;
BEGIN
Result:=0;
FOR C IN S DO BEGIN
Result:=Result SHL 1;
IF CharInSet(C,['A'..'Z']) THEN Result:=Result OR 1
END
END;
PROCEDURE TMainForm.BringForward(Sender : TObject);
BEGIN
SetForegroundWindow(Running);
SendString(Running,CommandLine,TEncoding.UTF8);
ExitProcess(0)
END;
FUNCTION TMainForm.CommandLine : STRING;
BEGIN
Result:=GetCommandLine
END;
PROCEDURE TMainForm.CopyData(VAR MSG : TMessage);
VAR
CDS : PCopyDataStruct;
S : STRING;
B : TBytes;
BEGIN
CDS:=PCopyDataStruct(MSG.LParam);
SetLength(B,CDS.cbData);
MOVE(CDS.lpData^,POINTER(B)^,LENGTH(B));
S:=TEncoding.UTF8.GetString(B);
LOG('Child['+IntToHex(MSG.WParam)+']: '+S)
END;
FUNCTION TMainForm.DeleteGlobalAtom(A : ATOM) : DWORD;
BEGIN
SetLastError(ERROR_SUCCESS);
WinAPI.Windows.GlobalDeleteAtom(A);
Result:=GetLastError
END;
FUNCTION TMainForm.FindGlobalAtom(CONST S : STRING) : ATOM;
BEGIN
Result:=WinAPI.Windows.GlobalFindAtom(PChar(S))
END;
PROCEDURE TMainForm.FormCreate(Sender : TObject);
VAR
A : ATOM;
H : HWND;
S,T : STRING;
BEGIN
S:=MakeAtomName(Handle);
REPEAT
A:=FindGlobalAtom(S);
IF A=0 THEN BREAK;
T:=GetGlobalAtomName(A); H:=AtomNameToHandle(T);
IF H<>Handle THEN
IF SendMessage(H,WM_PEEK,NativeInt(A),NativeInt(H))=NativeInt(A)+NativeInt(H) THEN BREAK
UNTIL DeleteGlobalAtom(A)<>ERROR_SUCCESS;
IF A=0 THEN BEGIN
A:=AddGlobalAtom(S);
LOG('Main['+IntToHex(Handle)+'] : '+CommandLine)
END ELSE BEGIN
Running:=H; OnDestroy:=NIL; OnActivate:=BringForward;
BorderStyle:=TFormBorderStyle.bsNone;
SetBounds(-10000,-10000,10,10)
END
END;
FUNCTION TMainForm.GetGlobalAtomName(H : ATOM) : STRING;
BEGIN
SetLength(Result,255);
SetLength(Result,WinAPI.Windows.GlobalGetAtomName(H,@Result[LOW(Result)],LENGTH(Result)))
END;
PROCEDURE TMainForm.LOG(CONST S : STRING);
BEGIN
ListBox.ItemIndex:=ListBox.Items.Add(S)
END;
PROCEDURE TMainForm.PEEK(VAR MSG : TMessage);
BEGIN
MSG.Result:=NativeInt(MSG.WParam)+MSG.LParam
END;
PROCEDURE TMainForm.SendString(H : HWND ; CONST S : STRING ; E : TEncoding);
VAR
B : TBytes;
CDS : TCopyDataStruct;
BEGIN
B:=E.GetBytes(S);
CDS.dwData:=1;
CDS.cbData:=LENGTH(B);
CDS.lpData:=POINTER(B);
SendMessage(H,WM_COPYDATA,Handle,NativeInt(@CDS));
END;
end.
When you initially run the application, it'll show the command line in the ListBox. If you then run it again, it'll detect the other window already exists (using a bit-encoded Global Atom to signify the initial instance's main form Handle) and move it to the foreground (after placing its own window out-of-screen, and thus being an invisible foreground window). It'll then use WM_COPYDATA to send the new instance's command line to the initial instance, and the initial instance will then log the received command line to the listbox.
Caveats:
- It's the MAIN form that is brought to front, receives and processes the command line. If you have child forms open, the behaviour is undefined (as in: I haven't tested this).
- The Atom name is a 32- (or 64-) character long name, consisting of a repeated pattern of the program executable's A-Z characters. If your application doesn't have A-Z character in its name, this will fail.
- To test if the Window decoded from the Global Atom is one we recognize, I call a WM_PEEK message on that window. This could lead to an unexpected message call into a foreign application, if your main instance is allowed to start (and create the Atom) and then not terminate properly (so that the Atom is deleted in FormDestroy).
