Delphi TList in multithreading
That is not safe without synchronisation. The reading threads can be in the middle of a read at the same time as the writing thread modifies the list. And modifying the list can mean reallocating the underlying memory.
The RTL provides the TThreadList class for such a scenario. Each thread, both writing and reading threads, need to wrap all access to the list in LockList and UnlockList pairs.
var ThreadList: TThreadList;//declared in some shared location....//each thread accesses the list like this:var List: TList;....List := ThreadList.LockList;try .... do stuff with Listfinally ThreadList.UnlockList;end;If you are using a Delphi that supports generics there is a generic version, TThreadList<T>.
As others have stated, TList by itself is not thread-safe. If you are worried about the overhead of using TThreadList (which uses a critical section internally), then have a look at wrapping your existing TList code with a TMultiReadSingleWriteSynchronizer, or even a Win32 SRW lock.