How can I send data between 2 applications using SendMessage? - delphi

Question

I have 2 applications- Manager with this code:
procedure TForm1.CopyData(var Msg: TWMCopyData);
var sMsg: String;
begin
if IsIconic(Application.Handle) then Application.Restore;
sMsg := PWideChar(Msg.CopyDataStruct.lpData);
Caption := Caption+'#'+sMsg;
Msg.Result := 123;
end;
procedure TForm1.Button1Click(Sender: TObject);
const
WM_MY_MESSAGE = WM_USER + 1;
var
h: HWND;
begin
Caption := 'X';
h := FindWindow('TForm1', 'Client');
if not IsWindow(h) then Exit;
Caption := Caption+'#';
SendMessage(h, WM_MY_MESSAGE, 123, 321);
end;
And Client with:
procedure TForm1.WndProc(var Message: TMessage);
const
WM_MY_MESSAGE = WM_USER + 1;
var DataStruct: CopyDataStruct;
S: String;
h: HWND;
begin
inherited;
if Message.Msg <> WM_MY_MESSAGE then Exit;
h := FindWindow('TForm1', 'Manager');
if not IsWindow(h) then Exit;
Message.Result := 123;
S := Edit2.Text + '#' + Edit1.Text;
DataStruct.dwData := 0;
DataStruct.cbData := 2*Length(S)+1;
DataStruct.lpData := PWideChar(S);
Caption := Caption + '#';
PostMessage(h, WM_CopyData, Form1.handle, integer(#DataStruct));
end;
The code works- but only once.
Manager sends 2 integers: 123 and 321 as a "wake up" message to the Client.
Client responds by sending contents of Edit1 + Edit2.
Then Manager gets this data and shows on its caption.
Why does it work only once? After I click Button1 again it does nothing.

Answer 1

As noted in comments, you must use SendMessage with WM_COPYDATA. The primary reason for this is that the message sender is responsible for cleaning up the resources used for the transfer. As noted in the documentation :
The receiving application should consider the data read-only. The lParam parameter is valid only during the processing of the message. The receiving application should not free the memory referenced by lParam. If the receiving application must access the data after SendMessage returns, it must copy the data into a local buffer.
The only way this can work is if the message sender waits for the receiver to process the message and return a result. Otherwise the sender cannot know when it is safe to release those resources.
PostMessage is asynchronous and returns immediately so this is simply not viable. SendMessage will block until the receiver processes the message and assigns a return value.
Here you are passing a pointer to a stack allocated (local variable) record #DataStruct. Further, you are also passing a pointer to a string which is a local variable. If you use PostMessage, this method will return immediately - the stack locations (for value types like the record) will become invalid and susceptible to being overwritten. The string lives on the heap but is reference counted and, in this case, will be freed when the method returns.
The solution is to always be sure to use SendMessage with WM_COPYDATA.