Delphi and C++Builder 2009 - TApplication is Apartment Threaded

In Delphi 2009 and C++Builder 2009 (AKA Tiburon) by default the main thread of a VCL application will be apartment threaded. Take a look at TApplication.Create for the details. This isn't really a change from previous versions of VCL but the location of the call and how to override it is new. And now VCL calls OleInitialize instead of CoInitialize so there is a slight behavior change.

There may be reasons why a developer won't want this default behavior and the best way of doing this is to create this unit:

unit InitCom;

interface

implementation

uses
  ActiveX;

var
  NeedToUninitialize: Boolean;
initialization
  NeedToUninitialize := Succeeded(CoInitializeEx(nil, COINIT_MULTITHREADED));
finalization
  if NeedToUninitialize then
    CoUninitialize;
end.

And place it in the .dpr as the first unit (it must be before Forms or anything that brings in Controls).

It isn't recommended to run any UI under a multithreaded apartment but this will allow those who need it to change the default threading model.

Here is a blog post by Raymond Chen about why running the UI thread as multithreaded isn't a good idea.