net qr code reader open source Lesson 2: Managing Threads in Visual Basic .NET

Creator QR-Code in Visual Basic .NET Lesson 2: Managing Threads

Lesson 2: Managing Threads
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// The thread procedure performs the tasks (displaying // the greeting and multiplying the value by 2). public void ThreadProc() { Console.WriteLine(greeting); if (callback != null) callback(value * 2); } } // Delegate that defines the signature for the callback method. public delegate void ResultDelegate(int value);
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The previous code sample outputs the following:
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Main thread does some work, then waits. Hello, world! Returned value: 26 Thread completed.
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Synchronizing Access to Resources
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If an application needs to write to a file, it typically locks the file. Locking the file prevents other applications from accessing it. If another application needs to access the file, it either must wait for the lock to be released or cancel the action that requires the file. Multithreaded applications have similar challenges when accessing shared resources. To reduce problems, the .NET Framework provides synchronization objects that you can use to coordinate resources shared among multiple threads. Resources that require synchronization include the following: System resources (such as communications ports) Resources shared by multiple processes (such as file handles) The resources of a single application domain (such as global, static, and instance fields) accessed by multiple threads Object instances that are accessed by multiple threads To understand what can happen if you don t synchronize access to resources in a multithreaded application, consider the following Console application. The constructor for the Math class accepts two integer values and provides methods to perform calculations using those values. However, the calculations take a full second to complete therefore, the private result variable might be overwritten by other
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Threading
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threads between the time the calculation is performed and the time the result is displayed to the console:
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' VB Sub Main() Dim m As New Math(2, 3) Dim t1 As New Thread(New ThreadStart(AddressOf m.Add)) Dim t2 As New Thread(New ThreadStart(AddressOf m.Subtract)) Dim t3 As New Thread(New ThreadStart(AddressOf m.Multiply)) t1.Start() t2.Start() t3.Start() ' Wait for the user to press a key Console.ReadKey() End Sub Class Math Public value1 As Integer Public value2 As Integer Private result As Integer Public Sub New(ByVal _value1 As Integer, ByVal _value2 As Integer) value1 = _value1 value2 = _value2 End Sub Public Sub Add() result = value1 + value2 Thread.Sleep(1000) Console.WriteLine("Add: " + result.ToString) End Sub Public Sub Subtract() result = value1 - value2 Thread.Sleep(1000) Console.WriteLine("Subtract: " + result.ToString) End Sub Public Sub Multiply() result = value1 * value2 Thread.Sleep(1000) Console.WriteLine("Multiply: " + result.ToString) End Sub End Class // C# class Program { static void Main() { Math m = new Math(2, 3);
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Lesson 2: Managing Threads
Thread t1 = new Thread(new ThreadStart(m.Add)); Thread t2 = new Thread(new ThreadStart(m.Subtract)); Thread t3 = new Thread(new ThreadStart(m.Multiply)); t1.Start(); t2.Start(); t3.Start(); // Wait for the user to press a key Console.ReadKey(); } } class Math { public int value1; public int value2; private int result; public Math(int _value1, int _value2) { value1 = _value1; value2 = _value2; } public void Add() { result = value1 + value2; Thread.Sleep(1000); Console.WriteLine("Add: " + result); } public void Subtract() { result = value1 - value2; Thread.Sleep(1000); Console.WriteLine("Subtract: " + result); } public void Multiply() { result = value1 * value2; Thread.Sleep(1000); Console.WriteLine("Multiply: " + result); } }
At first glance, you might expect that this Console application should display the following output:
Add: 5 Subtract: -1 Multiply: 6
7
Threading
However, because the Math.Multiply method was called last and the first two threads were in a Sleep state, Math.Multiply overwrote the value of result before Math.Add and Math.Subtract could display the value. So, the actual output is as follows:
Add: 6 Subtract: 6 Multiply: 6
The sections that follow describe different techniques for synchronizing access to resources. Just as the file system uses file locks to prevent different applications from accessing the same file, you can use the Monitor class to lock objects and prevent a specific section of code from running until the object is unlocked. Although you can use the Monitor class correctly, it s much easier to use built-in keywords to call the Monitor.Enter and Monitor.Exit methods. In C#, use the keyword lock to specify the object to monitor. In Visual Basic, use the keyword SynLock.
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