Threading in Delphi for .NET - Apartment State and Thread Pooling Class
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System.Threading.ApartmentState Enumeration
The .NET Framework creates an apartment when it interacts with COM objects. This apartment is specified in one of two ways. For manually created threads, the ApartmentState property can be set to the appropriate ApartmentState enumeration. These values are listed in Table 14.3.
Table 14.3 Values of the System.Threading.ApartmentState Enumeration
Value |
Description |
STA | Single-threaded apartment |
MTA | Multi-threaded apartment |
Unknown | Currently mapped to MTA |
Once the ApartmentState property has been set, it cannot be changed again. No error or exception will result from attempting to set this property more than once.
An alternative method is to use either the [STAThread] or [MTAThread] attribute before the first line of code in the project's dpr file. Although it is possible to use the following code to set the apartment model,
Thread.CurrentThread.ApartmentState := ApartmentState.STA;
using the appropriate attribute guarantees that the apartment state will be set up before any startup code is executed.
The System.Threading.ThreadPool Class
One of the nicest features of threading in the .NET framework is the addition of a thread pooling class. Saving thousands of lines of code, this class provides the means necessary for using threads as easy as calling a single method.
Ideal for short-lived tasks, the ThreadPool class hides the underlying Thread class, taking away the flexibility and control that the Thread class provides.
Many features of the .NET platform use threads from the thread pool. Examples using the thread pool include asynchronous file I/O, timers, socket connections, and the asynchronous execution of delegates.
All AppDomains located within the same process share threads from the same thread pool. Figure 14.3 illustrates this relationship.
Applications request a thread by using one of the QueueUserWorkItem() methods of the ThreadPool class. These methods take a WaitCallback delegate that specifies which method to execute on a thread pool thread. This method is added to an internal queue and is executed when a thread is available.
Be careful when using any of the methods that begin with Unsafe because these methods bypass security checks in order to increase performance.
An example of using threads from the ThreadPool is shown in Listing 14.4.
Listing 14.4 Using ThreadPool Threads
1: program threadpool;
2: {$APPTYPE CONSOLE}
3: uses
4: System.Threading;
5:
6: type
7: TThreadPoolMe = class
8: public
9: // this method will be executed on a threadpool thread
10: procedure ThreadMePlease(state : System.Object);
11: end;
12:
13: procedure TThreadPoolMe.ThreadMePlease(state : System.Object);
14: var
15: id : string;
16: begin
17: id := 'n/a';
18: if assigned(state) then
19: id := state.ToString;
20:
21: writeln(id,') Hello from the thread pool. Thread ID is ',
22: AppDomain.GetCurrentThreadID,' IsThreadPool = ',
23: Thread.CurrentThread.IsThreadPoolThread);
24:
25: Thread.Sleep(2000);
26: end;
27:
28: const
29: MAX_THREADS = 10;
30: var
31: i : integer;
32: thrd : array[1..MAX_THREADS] of TThreadPoolMe;
33: begin
34: writeln('The main thread''s id is ', AppDomain.GetCurrentThreadID);
35: // queue up a bunch of threads to use the thread pool
36: for i:=1 to MAX_THREADS do
37: begin
38: // create another instance of our class
39: thrd[i] := TThreadPoolMe.Create;
40: writeln('Queueing thread ', i);
41: // now queue up the ThreadMePlease method, passing i in for the state
42: System.Threading.ThreadPool.QueueUserWorkItem(
43: @thrd[i].ThreadMePlease, System.Object(i));
44: end;
45: // watch the re-use of the thread id's when this application runs
46: readln;
47: writeln('Done');
48: end.
Note: Find the code on the CD: \Code\Chapter 14\Ex03\.
Listing 14.4 queues 10 threads to the thread pool for execution. Running this example will reveal how the CLR reuses existing threads by examining the reuse of thread IDs.
Note - In addition to manually created threads and threads within the thread pool, the .NET Framework must also keep track of other threads. These threads include the main thread and the finalizer (or the garbage collector) thread. Unmanaged threads can also be running from PInvoke or COM interoperability as well.
 This chapter is from Delphi for .NET Developer's Guide, by Xavier Pacheco (Sams, 2004, ISBN: 0-672-32443-1). Check it out at your favorite bookstore today.
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