Threading in Delphi for .NET - Mutex and Monitor Classes
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The System.Threading.Mutex Class
A mutex is a mutually exclusive object that acts similar to a lock. Locking the mutex is accomplished by calling one of the overloaded WaitOne() methods. Mutexes have the option of being named. Specifying a name allows for the mutex to be shared between AppDomains and processes. The mutex is unlocked by calling the ReleaseMutex() method. Listing 14.8 shows the methods declared in the Mutex class.
Listing 14.8 Declaration of the System.Threading.Mutex Class
System.Threading.Mutex = class (System.Threading.WaitHandle)
public
constructor Create(initiallyOwned: Boolean; name: String;
var createdNew: Boolean); overload;
constructor Create(initiallyOwned: Boolean; name: String); overload;
constructor Create(initiallyOwned: Boolean); overload;
constructor Create; overload;
procedure ReleaseMutex;
end;
In addition to the Wait methods inherited from WaitHandle, the Mutex class adds several constructors as well as the ReleaseMutex() method. These constructors provide the ability to create, optionally lock (own), and optionally name the mutex. A mutex is unlocked by using the ReleaseMutex() method.
The System.Threading.Monitor Class
The System.Threading.Monitor class has been designed to be more lightweight than the Mutex class. It should be used when a high-performance locking mechanism is needed.
Although the Monitor class looks similar to a mutex, there are some subtle differences. Take a look at Listing 14.9 for the definition of the Monitor class.
Listing 14.9 Declaration of the System.Threading.Monitor Class
System.Threading.Monitor = class (System.Object)
public
class procedure Enter(obj: System.Object); static;
class function TryEnter(obj: System.Object) : Boolean; overload; static;
class function TryEnter(obj: System.Object;
millisecondsTimeout: Integer) : Boolean; overload; static;
class function TryEnter(obj: System.Object;
timeout: TimeSpan) : Boolean; overload; static;
class function Wait(obj: System.Object; millisecondsTimeout: Integer;
exitContext: Boolean) : Boolean; overload; static;
class function Wait(obj: System.Object; timeout: TimeSpan;
exitContext: Boolean) : Boolean; overload; static;
class function Wait(obj: System.Object;
millisecondsTimeout: Integer) : Boolean; overload; static;
class function Wait(obj: System.Object;
timeout: TimeSpan) : Boolean; overload; static;
class function Wait(obj: System.Object) : Boolean; overload; static;
class procedure Pulse(obj: System.Object); static;
class procedure PulseAll(obj: System.Object); static;
class procedure Exit(obj: System.Object); static;
end;
Using the Enter() and Exit() methods of the Monitor has the same effect as locking with a Mutex's WaitOne() and ReleaseMutex() methods. The Monitor class also has TryEnter() methods that attempt to acquire a lock without waiting. An optional timeout parameter specifies how long to wait on the lock before giving up.
Notice that all the methods of the Monitor class are class methods. This provides the flexibility to specify any object to lock upon.
Finally, the Monitor class allows for signaling and waiting for a signal with the Wait() and Pulse() methods. Both Wait() and Pulse() methods require the Monitor to be locked—that is, surround the Wait() and Pulse() methods with an Enter() / Exit() pair.
Find the code on the CD: \Code\Chapter 14\Ex06\prodcons.dpr for an example that demonstrates a thread-safe queue using the Mutex and Monitor classes.
 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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