How can I overcome memory leak in accessing TWebBrowser.Document in Delphi 10.3.3

Question

I'm using this code from StackOverflow (How can I get HTML source code from TWebBrowser) to get the full response from a webpage:

function TMain.GetWebBrowserHTML(const WebBrowser: TWebBrowser): String;
var
  LStream: TStringStream;
  Stream: IStream;
  LPersistStreamInit: IPersistStreamInit;
begin
  if not Assigned(WebBrowser.Document) then
    Exit;
  LStream := TStringStream.Create('');
  try
    LPersistStreamInit := WebBrowser.Document as
      IPersistStreamInit;
    Stream := TStreamAdapter.Create(LStream, soReference);
    LPersistStreamInit.Save(Stream, True);
    Result := LStream.DataString;
  finally
    LStream.Free();
  end;
end;

After a couple of hundred calls to the routine with some large web pages, I'm out of memory.

Apparently there is a known problem with the component's Document property, but the suggestion of replacing WebBrowser.Document with WebBrowser.DefaultInterface.Document doesn't help. I really don't want to try to fix the VCL, and the other suggestion of calling Release might work if I knew where and how to do it. And the leak could be something else entirely. This code is above my pay grade.

I can't use TIdHTTP because of some scripting that has to occur, and I need the visual anyway.

See also: TWebbrowser massive memory leaks : no solution so far

Answer 1

Apparently there is a known problem with the component's Document property

For reference to anyone seeing this:

RSP-32393: Reference leak in TOleControl.GetIDispatchProp and TOleControl.GetIUnknownProp

UPDATE: this issue was reportedly fixed in 10.0 Seattle, so it should not be happening anymore in 10.3.

I really don't want to try to fix the VCL, and the other suggestion of calling Release might work if I knew where and how to do it.

You would call it like this:

function TMain.GetWebBrowserHTML(const WebBrowser: TWebBrowser): String;
var
  Disp: IDispatch;
  LStream: TStringStream;
  Stream: IStream;
  LPersistStreamInit: IPersistStreamInit;
begin
  Disp := WebBrowser.Document;
  if not Assigned(Disp) then
    Exit;
  try
    LStream := TStringStream.Create('');
    try
      LPersistStreamInit := Disp as IPersistStreamInit;
      Stream := TStreamAdapter.Create(LStream, soReference);
      LPersistStreamInit.Save(Stream, True);
      Result := LStream.DataString;
    finally
      LStream.Free;
    end;
  finally
    Disp._Release;
  end;
end;

Thus:

• the TWebBrowser.Document property returns an IDispatch whose refcount has been erroneously incremented +2 instead of +1 due to a bug in TOleControl
• the assignment to Disp increments the refcount +1
• the cast+assignment to LPersistStreamInit increments the refcount +1.

When the function exits:

• the explicit _Release() decrements the refcount -1 to workaround the bug
• an implicit _Release() when LPersistStreamInit goes out of scope decrements the refcount -1
• an implicit _Release() when Disp goes out of scope decrements the refcount -1
• an implicit _Release() on the Document property's return value decrements the refcount -1.

The refcount is balanced properly.

Alternatively, you can do this instead:

function TMain.GetWebBrowserHTML(const WebBrowser: TWebBrowser): String;
var
  Disp: IDispatch;
  LStream: TStringStream;
  Stream: IStream;
  LPersistStreamInit: IPersistStreamInit;
begin
  Pointer(Disp) := WebBrowser.Document;
  if not Assigned(Disp) then
    Exit;
  LStream := TStringStream.Create('');
  try
    LPersistStreamInit := Disp as IPersistStreamInit;
    Stream := TStreamAdapter.Create(LStream, soReference);
    LPersistStreamInit.Save(Stream, True);
    Result := LStream.DataString;
  finally
    LStream.Free;
  end;
end;

This way, you don't need the explicit _Release() anymore:

• the TWebBrowser.Document property still returns an IDispatch whose refcount has been erroneously incremented +2 instead of +1
• the assignment to Disp WON'T increment the refcount +1
• the cast+assignment to LPersistStreamInit increments the refcount +1.

When the function exits:

• an implicit _Release() when LPersistStreamInit goes out of scope decrements the refcount -1
• an implicit _Release() when Disp goes out of scope decrements the refcount -1
• an implicit _Release() on the Document property's return value decrements the refcount -1.

The refcount is balanced properly.