How can I implement a thread-safe list wrapper in Delphi?

Question

I have a list wrapper that maintains two Tstringlists and a TClassList

I need this to be thread safe, such that:

• Concurrent writes are not allowed (wait state of some sort should be entered)
• Reading while writing (or vice versa) is not allowed (wait state of some sort should be entered)
• Concurrent reads are allowed

Any ideas on how I can do this? My instinct tells me it needs more than just a critical section, perhaps a semaphore or "usage counter", perhaps one of these in conjunction with a CS.

I'm just not quite sure where to start - anything from an overall approach in english to psuedo-code, to delphi implementation or external link would be much appreciated.

Answer 1

You should have a look at the TMultiReadExclusiveWriteSynchronizer class declared in sysutils.pas...

Answer 2

Have a look at this tutorial. Threading the Delphi Way

Look at Chapter 11, but it's all good stuff.

Answer 3

You really should look at TThreadList.

The methods .Add, .Remove, .Clear automatically lock the list for you. If needed, you can also lock/unlock as needed:

x.LockList; 
try 
  //do whatever
finally  
  x.Unlocklist; 
end;

TMultiReadExclusiveWriteSynchronizer is a grand idea but I don't know if they ever ironed all the bugs out. It has always had issues under load.