vb.net中线程怎么使用_VB.NET中的线程介绍

曾经终败给现在 2022-12-06 15:46 121阅读 0赞

vb.net中线程怎么使用

To understand threading in VB.NET, it helps to understand some of the foundation concepts. First up is that threading is something that happens because the operating system supports it. Microsoft Windows is a pre-emptive multitasking operating system. A part of Windows called the task scheduler parcels out processor time to all the running programs. These small chunks of processor time are called time slices. Programs aren't in charge of how much processor time they get, the task scheduler is. Because these time slices are so small, you get the illusion that the computer is doing several things at once.

要了解VB.NET中的线程，它有助于了解一些基础概念。 首先是线程是会发生的事情，因为操作系统支持它。 Microsoft Windows是抢先式多任务操作系统。 Windows的一部分称为任务计划程序，将处理器时间分配给所有正在运行的程序。 这些小的处理器时间块称为时间片。 程序不负责获得多少处理器时间，任务调度程序负责。 由于这些时间片非常小，您会感到计算机正在一次执行多项操作。

## 线程的定义 **(** Definition of Thread **)** ##

A thread is a single sequential flow of control.

线程是控制的单个顺序流。

Some qualifiers:

一些预选赛：

*  A thread is a "path of execution" through that body of code.
    
    线程是通过那段代码的“执行路径”。
 *  Threads share memory so they have to cooperate to produce the correct result.
    
    线程共享内存，因此它们必须合作以产生正确的结果。
 *  A thread has thread-specific data such as registers, a stack pointer, and a program counter.
    
    线程具有特定于线程的数据，例如寄存器，堆栈指针和程序计数器。
 *  A process is a single body of code that can have many threads, but it has at least one and it has a single context (address space).
    
    进程是可以包含多个线程的单个代码主体，但是它至少具有一个线程，并且具有单个上下文(地址空间)。

This is assembly level stuff, but that's what you get into when you start thinking about threads.

这是程序集级别的东西，但这就是您开始考虑线程时要涉及的内容。

## 多线程与多处理 **(** Multithreading vs. Multiprocessing **)** ##

[Multithreading][] is not the same as multicore parallel processing, but multithreading and multiprocessing do work together. Most PCs today have processors that have at least two cores, and ordinary home machines sometimes have up to eight cores. Each core is a separate processor, capable of running programs by itself. You get a performance boost when the OS assigns a different process to different cores. Using multiple threads and multiple processors for even greater performance is called thread-level parallelism.

[多线程][Multithreading]与多核并行处理不同，但是多线程和多处理可以协同工作。 如今，大多数PC的处理器至少具有两个内核，而普通家用计算机有时最多具有八个内核。 每个内核是一个单独的处理器，能够自己运行程序。 当操作系统将不同的进程分配给不同的内核时，您将获得性能提升。 使用多个线程和多个处理器以获得更高的性能称为线程级并行性。

A lot of what can be done depends on what the operating system and the processor hardware can do, not always what you can do in your program, and you shouldn't expect to be able to use multiple threads on everything. In fact, you might not find many problems that benefit from multiple threads. So, don't implement multithreading just because it's there. You can easily reduce your program's performance if it's not a good candidate for multithreading. Just as examples, video codecs may be the worst programs to multithread because the data is inherently [serial][]. Server programs that handle web pages might be among the best because the different clients are inherently independent.

可以完成的很多工作取决于操作系统和处理器硬件的工作能力，而并不总是取决于程序中可以执行的工作，并且您不应该期望能够在所有程序上使用多个线程。 实际上，您可能找不到许多受益于多个线程的问题。 因此，不要仅仅因为多线程就在其中而实现多线程。 如果它不是多线程的理想选择，则可以轻松降低程序的性能。 就像示例一样，视频编解码器对于多线程来说可能是最差的程序，因为数据本质上是[串行的][serial] 。 处理网页的服务器程序可能是最好的，因为不同的客户端本质上是独立的。

## 练习线程安全 **(** Practicing Thread Safety **)** ##

Multithreaded code often requires complex coordination of threads. Subtle and difficult-to-find bugs are common because different threads often have to share the same data so data can be changed by one thread when another isn't expecting it. The general term for this problem is "race condition." In other words, the two threads can get into a "race" to update the same data and the result can be different depending on which thread "wins". As a trivial example, suppose you're coding a loop:

多线程代码通常需要复杂的线程协调。 细微且难以发现的错误是常见的，因为不同的线程通常必须共享相同的数据，因此当一个线程不期望使用另一个线程时就可以更改数据。 此问题的总称是“竞赛条件”。 换句话说，两个线程可以进入“竞赛”以更新相同的数据，并且结果取决于哪个线程“获胜”而有所不同。 作为一个简单的示例，假设您正在编写一个循环：

If the loop counter "I" unexpectedly misses the number 7 and goes from 6 to 8—but only some of the time—it would have disastrous effects on whatever the loop is doing. Preventing problems like this is called thread safety. If the program needs the result of one operation in a later operation, then it can be impossible to code parallel processes or threads to do it.

如果循环计数器“ I”意外地漏掉了数字7，并且从6变为8(但仅在某些时间)，则对循环所做的任何事情都会造成灾难性的影响。 防止这样的问题称为线程安全。 如果程序在下一个操作中需要一个操作的结果，则可能无法编写并行进程或线程的代码来执行该操作。

## 基本多线程操作 **(** Basic Multithreading Operations **)** ##

It's time to push this precautionary talk to the background and write some multithreading code. This article uses a Console Application for simplicity right now. If you want to follow along, start Visual Studio with a new Console Application project.

现在该把这种预防性的讨论推向后台并编写一些多线程代码了。 为了简便起见，本文现在使用控制台应用程序。 如果要继续，请使用新的Console Application项目启动Visual Studio。

The primary namespace used by multithreading is the System.Threading namespace and the Thread class will create, start, and stop new threads. In the example below, notice that TestMultiThreading is a delegate. That is, you have to use the name of a method that the Thread method can call.

多线程使用的主要命名空间是System.Threading命名空间，并且Thread类将创建，启动和停止新线程。 在下面的示例中，请注意TestMultiThreading是一个委托。 也就是说，您必须使用Thread方法可以调用的方法的名称。

In this app, we could have executed the second Sub by simply calling it:

在这个应用程序中，我们可以通过简单地调用它来执行第二个Sub：

This would have executed the entire application in serial fashion. The first code example above, however, kicks off the TestMultiThreading subroutine and then continues.

这将以串行方式执行整个应用程序。 但是，上面的第一个代码示例启动了TestMultiThreading子例程，然后继续。

## 递归算法示例 **(** A Recursive Algorithm Example **)** ##

Here's a multithreaded application involving calculating permutations of an array using a recursive algorithm. Not all of the code is shown here. The array of characters being permuted is simply "1," "2," "3," "4," and "5." Here's the pertinent part of the code.

这是一个多线程应用程序，涉及使用递归算法计算数组的排列。 此处未显示所有代码。 排列的字符数组只是“ 1，”，“ 2”，“ 3”，“ 4”和“ 5”。 这是代码的相关部分。

Notice that there are two ways to call the Permute sub (both commented out in the code above). One kicks off a thread and the other calls it directly. If you call it directly, you get:

请注意，有两种方法可以调用Permute子项(在上面的代码中都注释掉了)。 一个启动一个线程，另一个直接调用它。 如果直接调用它，则会得到：

However, if you kick off a thread and Start the Permute sub instead, you get:

但是，如果您启动一个线程并启动Permute子，则会得到：

This clearly shows that at least one permutation is generated, then the Main sub moves ahead and finishes, displaying "Finished Main," while the rest of the permutations are being generated. Since the display comes from a second sub called by the Permute sub, you know that is part of the new thread as well. This illustrates the concept that a thread is "a path of execution" as mentioned earlier.

这清楚地表明，至少生成了一个排列，然后Main子项继续前进并完成，显示“ Finished Main”，而其余的排列正在生成。 由于显示来自Permute子程序调用的第二个子程序，因此您知道这也是新线程的一部分。 如前所述，这说明了线程是“执行路径”的概念。

## 比赛条件示例 **(** Race Condition Example **)** ##

The first part of this article mentioned a race condition. Here's an example that shows it directly:

本文的第一部分提到了竞争条件。 这是一个直接显示它的示例：

The Immediate window showed this result in one trial. Other trials were different. That's the essence of a race condition.

立即窗口在一次试验中显示了此结果。 其他试验是不同的。 那是比赛条件的本质。

> 翻译自: [https://www.thoughtco.com/an-introduction-to-threading-in-vbnet-3424476][https_www.thoughtco.com_an-introduction-to-threading-in-vbnet-3424476]

vb.net中线程怎么使用

[Multithreading]: https://www.thoughtco.com/multi-threading-in-c-with-tasks-958372
[serial]: https://www.thoughtco.com/all-about-serializing-in-visual-basic-3424466
[https_www.thoughtco.com_an-introduction-to-threading-in-vbnet-3424476]: https://www.thoughtco.com/an-introduction-to-threading-in-vbnet-3424476