Data transfer between different Delphi version apps via WM_COPYDATA - delphi

Question

I'm trying to make to Delphi applications communicate with each other via WM_COPYDATA. The problem I'm having though is is that the sender app is written in Delphi 7 and the receiver is written in Delphi 10.2 . I copied my Delphi 7 program's code into Delphi 10 and the communication worked perfectly. Using the exact same code in Delphi 7 however caused my string being passed to the receiver app to get corrupted. The codes I use are as follows:
One the sending side I have:
procedure TSenderApp.SendString(ToSend: string);
var
copyDataStruct : TCopyDataStruct;
receiverHandle : THandle;
res : integer;
begin
copyDataStruct.dwData := 140500; //use it to identify the message contents
copyDataStruct.cbData := (1+ Length(ToSend))* SizeOf(Char) ;
copyDataStruct.lpData := pchar(ToSend) ;
receiverHandle := FindWindow(PChar('TRecieverApp'),PChar('RecieverApp')) ;
if receiverHandle = 0 then
begin
ShowMessage('CopyData Receiver NOT found!') ;
Exit;
end;
res := SendMessage(receiverHandle, WM_COPYDATA, Integer(Handle),
LPARAM(#copyDataStruct)) ;
end;
And on the receiving side I have:
procedure TRecieverApp.WMCopyData(var Message: TMessage);
var
p : PCopyDataStruct;
l : Integer;
s : string;
begin
p := PCopyDataStruct( Message.lParam );
if (p <> nil) then
begin
ShowMessage('New Message Recieved!');
l := p^.cbData;
SetLength( s, (l+1) );
StrLCopy( PChar(s), PChar(p^.lpData), l );
Edit1.Text := s;
end
else
Edit1.Text := 'ERROR';
end;
What am I doing wrong? Or why is the message string being corrupted when sent from the Delphi 7 written SenderApp and not from the Delphi 10 written SenderApp?

Answer 1

You are sending and processing data using the native encoding of Char, which is AnsiChar in Delphi 7 but is WideChar in Delphi 10.2 Tokyo (Delphi switched everything to Unicode in D2009). When Delphi 7 sends the data as ANSI, Delphi 10.2 misinterprets it as UTF-16. And vice versa. So you end up with corruption either way.
You need to convert the data to an agreed-upon character encoding before sending it, and convert it from that encoding after receiving it.
Try something more like this:
{$IF CompilerVersion >= 24} // XE3+
{$LEGACYIFEND ON}
{$IFEND}
var
MyDataID: UINT = 0;
procedure TSenderApp.SendString(const ToSend: string);
var
copyDataStruct : TCopyDataStruct;
receiverHandle : HWND;
res : LRESULT;
s : UTF8String;
begin
if MyDataID = 0 then
begin
ShowMessage('CopyData ID NOT registered!');
Exit;
end;
receiverHandle := FindWindow('TRecieverApp', 'RecieverApp');
if receiverHandle = 0 then
begin
ShowMessage('CopyData Receiver NOT found!');
Exit;
end;
{$IF CompilerVersion >= 20} // D2009+
s := UTF8String(ToSend);
{$ELSE}
s := UTF8Encode(ToSend);
{$IFEND}
copyDataStruct.dwData := MyDataID; //use it to identify the message contents
copyDataStruct.cbData := Length(s) * SizeOf(AnsiChar);
copyDataStruct.lpData := PAnsiChar(s);
res := SendMessage(receiverHandle, WM_COPYDATA, WPARAM(Handle), LPARAM(#copyDataStruct));
end;
initialization
MyDataID := RegisterWindowMessage('MyDataID');
{$IF CompilerVersion >= 24} // XE3+
{$LEGACYIFEND ON}
{$IFEND}
var
MyDataID: UINT = 0;
procedure TRecieverApp.WMCopyData(var Message: TMessage);
var
p : PCopyDataStruct;
s : UTF8String;
begin
p := PCopyDataStruct(Message.lParam);
if (p <> nil) and (MyDataID <> 0) and (p^.dwData = MyDataID) then
begin
SetString(s, PAnsiChar(p^.lpData), p^.cbData);
{$IF CompilerVersion >= 20} // D2009+
Edit1.Text := String(s);
{$ELSE}
Edit1.Text := UTF8Decode(s);
{$IFEND}
ShowMessage('New Message Received!');
end else
inherited;
end;
initialization
MyDataID := RegisterWindowMessage('MyDataID');

Answer 2

The difference between the two Delphi versions is string format. In Delphi 2007 and earlier, string uses 1-byte AnsiChar characters in ANSI format. In Delphi 2009 and later, string uses 2-byte WideChar characters in UTF-16 format. You need to convert the data to a common character encoding when sending it.