TThread Facts

Delphi RTL TThread class has been an object of criticism since its introduction. Some criticism was deserved, some not. TThread implementation was gradually improving with every Delphi version. Here I am neither criticizing nor advocating the TThread class, just listing some details of TThread implementation in modern Delphi versions that Delphi programmer should know.

1. TThread class does not allow setting a thread stack size – it is always equal to default value, usually 1 Mb. If you need a different value you can call BeginThread function directly, but that is a low level solution and not as handy as using TThread class;
2. TThread constructor has the only parameter (CreateSuspended: Boolean). Irrespective of the parameter value the underlying Windows thread object is always created in suspended state:
   

       FCreateSuspended := CreateSuspended {...};
       FHandle := BeginThread(nil, 0, @ThreadProc, Pointer(Self), CREATE_SUSPENDED, FThreadID);
   

   
   A thread can start execution in AfterConstruction method (which is called after all inherited constructors):
   

     if not FCreateSuspended {...} then Resume;
   

   
   or later;
3. Synchronization with the main thread does not work in console application. That means that you should not use Synchronize and Queue methods of TThreads class in console application. The same also applies to OnTerminate event which calls Synchronize method internally; you can override protected DoTerminate method instead;
4. If you set FreeOnTerminate = True for a TThread instance, the instance is destroyed in the context of underlying background thread. Usually the destruction context does not matter, but in some cases it does. For example, create a TTimer instance in the constructor of TThread descendant, destroy timer in the destructor and see what happens (hint: an invisible window of TTimer instance is created in the main thread and destroyed in a different thread);
5. TThread does not guarantee that Execute method will be called. I have bumped into this problem a little before.

Despite all issues we are better to use TThread class ‘as is’ in GUI applications. For my experiments I have written a simple alternative thread wrapper class that allows setting a thread stack size, always calls Execute method and does not contain synchronization methods; it is well suited for console applications.

Advertisement