Bizarre behaviour in simple Delphi code - delphi

Question

I am passing TProcessItem structure as a return value from my function. While in GetProcessFromHandle function, the ExeFile property of the structure returned by FindByID, contains the expected executable name, but in the structure returned by GetProcessFromHandle it somehow becomes empty. I added two Messageboxes to demonstrate it. Is anyone able to explain what is happening here?
function GetProcessFromHandle(hWnd: HWND): TProcessItem;
var
ProcessInfo: TProcessInfo;
PID: Cardinal;
begin
Result := nil;
GetWindowThreadProcessId(hWnd, #PID);
ProcessInfo := TProcessInfo.Create(nil);
try
Result := ProcessInfo.RunningProcesses.FindByID(PID);
if Assigned(Result) then ShowMessage(Result.ExeFile); //first message
finally
ProcessInfo.Free;
end;
end;
procedure Test;
var
Process: TProcessItem;
begin
Process := GetProcessFromHandle(FindWindow(nil, 'My App'));
if Assigned(Process) ShowMessage(Process.ExeFile); //second message
end;

Answer 1

The TProcessInfo object owns the TProcessItem object that GetProcessFromHandle() is returning a pointer to. As such, when the TProcessInfo object is freed, so is the TProcessItem object, and thus the pointer being used by Test() is invalid. So you run into undefined behavior. You are lucky your code did not just crash instead.
If you want to return the path to the EXE file, return only that value, eg:
function GetProcessExeFileFromHandle(hWnd: HWND): string;
var
ProcessInfo: TProcessInfo;
ProcessItem: TProcessItem;
PID: DWORD;
begin
Result := '';
GetWindowThreadProcessId(hWnd, #PID);
ProcessInfo := TProcessInfo.Create(nil);
try
ProcessItem := ProcessInfo.RunningProcesses.FindByID(PID);
if Assigned(ProcessItem) then
Result := ProcessItem.ExeFile;
finally
ProcessInfo.Free;
end;
end;
procedure Test;
var
ProcessExeFile: string;
begin
ProcessExeFile := GetProcessExeFileFromHandle(FindWindow(nil, 'My App'));
ShowMessage(ProcessExeFile);
end;