Why I cannot find window? - api

Question

I use this example to send a string between two applications.
When I press the Send button for the first time, the string is sent to the Receiver, but only a part of the string is received.
When I press the Send button for the second time, I get "Window not found!".
The window is right there on screen. Why it works when I press the button the first time, but not the second time?
This is the sender:
procedure TfrmSender.SendString;
var
stringToSend : string;
copyDataStruct : TCopyDataStruct;
begin
Caption:= 'Sending';
stringToSend := 'About - Delphi - Programming';
copyDataStruct.dwData := 12821676; //use it to identify the message contents
copyDataStruct.cbData := 1 + Length(stringToSend) ;
copyDataStruct.lpData := PChar(stringToSend);
SendData(copyDataStruct) ;
end;
procedure TfrmSender.SendData(CONST copyDataStruct: TCopyDataStruct);
VAR
receiverHandle : THandle;
res : integer;
begin
receiverHandle := FindWindow(PChar('TfrmReceiver'), PChar('frmReceiver')) ;
if receiverHandle = 0 then
begin
Caption:= 'Receiver window NOT found!';
EXIT;
end;
res:= SendMessage(receiverHandle, WM_COPYDATA, Integer(Handle), Integer(#copyDataStruct));
if res= 0 then Caption:= 'Receiver window found but msg not hand';
end;
And this is the receiver:
procedure TfrmReceiver.WMCopyData(var Msg: TWMCopyData);
VAR
s : string;
begin
if Msg.CopyDataStruct.dwData = 12821676 then
begin
s := PChar(Msg.CopyDataStruct.lpData);
msg.Result := 2006; //Send something back
Winapi.Windows.Beep(800, 300);
Caption:= s;
end
end;

Answer 1

To summarize the comments there are two errors
1) (See #Tom Brunberg) is that the length is set incorrectly which is why you only get part (about half? of the string)
It should be
copyDataStruct.cbData := sizeof( Char )*(Length(stringToSend) + 1 );
2) The forms caption is being changed which invalidates the expression
FindWindow(PChar('TfrmReceiver'), PChar('frmReceiver'))
because the second parameter is the form's caption (in Delphi terminology)