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Most operations after creation of the Semaphore are just like the operations shown earlier for the Mutex class. The one major difference is that when you release the Semaphore you can specify how many slots you want to release, as shown in the following example:
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' VB theSemaphore.Release(5) // C# theSemaphore.Release(5);
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In addition, as with the Mutex class, you can specify a name that can be used to create and open shared semaphores across AppDomain and process boundaries. The following code snippet provides an example:
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' VB Dim theSemaphore As Semaphore = Nothing Try ' Try and open the Semaphore theSemaphore = Semaphore.OpenExisting("THESEMAPHORE") Catch ex as WaitHandleCannotBeOpenedException ' Cannot open the Semaphore because it doesn't exist End Try ' Create it if it doesn't exist If theSemaphore Is Nothing Then theSemaphore = New Semaphore(0, 10, "THESEMAPHORE") End If // C# Semaphore theSemaphore = null; try // Try and open the Semaphore { theSemaphore = Semaphore.OpenExisting("THESEMAPHORE"); } catch (WaitHandleCannotBeOpenedException) { // Cannot open the Semaphore because it doesn't exist } // Create it if it doesn't exist if (theSemaphore == null) { theSemaphore = new Semaphore(0, 10, "THESEMAPHORE"); }
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Event class Events are a type of kernel object that has two states, on and off. These states allow threads across an application to wait until an event is signaled to do something specific. There are two types of events: auto reset and manual reset. When an
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auto reset event is signaled, the first object waiting for the event turns it back to a nonsignaled state. This behavior is similar to that of a Mutex. Conversely, a manual reset event allows all threads that are waiting for it to become unblocked until something manually resets the event to a nonsignaled state. These events are represented as the AutoResetEvent and ManualResetEvent classes in the .NET Framework. Both of these classes inherit from a common EventWaitHandle class (which itself inherits from the WaitHandle class).
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EventWaitHandle, AutoResetEvent, and ManualResetEvent classes are new in the .NET Framework 2.0.
Creating a new instance of an event class allows you to specify the signal state of the event, as shown in the following example:
' VB Dim autoEvent As New AutoResetEvent(true) Dim manualEvent As New ManualResetEvent(false) // C# AutoResetEvent autoEvent = new AutoResetEvent(true); ManualResetEvent manualEvent = new ManualResetEvent(false);
The EventWaitHandle class supports two new methods that are specific to working with events: Set and Reset. These methods are used to switch the event on and off, as shown in the following example:
' VB autoEvent.Set() manualEvent.Reset() // C# autoEvent.Set(); manualEvent.Reset();
Like the other kernel objects, events allow you to specify a name that can be used to create and open them across AppDomain and process boundaries. The support for named events is at the EventWaitHandle level. When creating or opening a named event, you will need to deal with EventWaitHandles instead of the AutoResetEvent and ManualResetEvent classes. When creating a new EventWaitHandle object, you not only specify the signal state, but also the type of event needed. For example, you can use the following code to create or open a named event:
' VB Dim theEvent As EventWaitHandle = Nothing
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Try theEvent = EventWaitHandle.OpenExisting("THEEVENT") Catch ex as WaitHandleCannotBeOpenedException ' Cannot open the AutoResetEvent because it doesn't exist End Try ' Create it if it doesn't exist If theEvent Is Nothing Then theEvent = New EventWaitHandle(False, _ EventResetMode.AutoReset, "THEEVENT") End If // C# EventWaitHandle theEvent = null; try // Try and open the Event { theEvent = EventWaitHandle.OpenExisting("THEEVENT"); } catch (WaitHandleCannotBeOpenedException) { // Cannot open the AutoResetEvent because it doesn't exist } // Create it if it doesn't exist if (theEvent == null) { theEvent = new EventWaitHandle(false, EventResetMode.AutoReset, "THEEVENT"); }
Lab: Use a Mutex to Create a Single-Instance Application
In this lab, you create a simple console application in which you will use a Mutex to ensure there is only one instance of the application running at any point. If you encounter a problem completing an exercise, the completed projects are available on the companion CD in the Code folder. 1. Create a new console application called SingleInstance. 2. In the main code file, include (or import for Visual Basic) System.Threading. 3. In the main method of the console application, create a local Mutex variable and assign it a null (or Nothing in Visual Basic). 4. Create a constant string to hold the name of the shared Mutex. Make the value RUNMEONCE . 5. Create a try/catch block. 6. Inside the try section of the try/catch block, call the Mutex.OpenExisting method, using the constant string defined in step 4 as the name of the Mutex. Then assign the result to the Mutex variable created in step 2.
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