为什么我可以在没有跨线程异常的情况下从线程中的表单访问控件? [英] Why can I access controls from a form in a thread without cross thread exception?
问题描述
通常，当您访问线程中的控件时，最终会遇到一些跨线程异常.在我的C#WinForms应用程序中，我有一个图片框和一个工具条标签，它们不会导致该异常.我不明白为什么，有人可以向我解释一下吗?
Usually, when you access controls in a Thread you end up with some cross thread exceptions. In my C# WinForms Application I have a picture box and a toolstriplabel which do not cause that exception. I don't understand why, can anybody explain this to me?
这里有一些代码说明:
在主要形式中，我有一个图片框和一个工具条标签.另外，我还引用了另一个窗体，该窗体没有控件，也没有其他源代码.然后在主要形式中，还有另一个与线程一起工作的对象.该线程可以引发三个不同的事件，主要形式是预订这三个事件.
In the main form I have a picturebox and a toolstriplabel. Also I have a reference to another Form, which has no controls and no additional source code. And then in the main form there is another object which works with a thread. This thread can raise three different events and the main form is subscribed to these three events.
- Event1导致toolstriplabel更新(带有来自线程的一些信息).
- Event2导致图片框更新(带有线程中的新图片).
Event1和Event2可以正常工作.我不使用任何调用方法，我直接更改了Text和BackgroundImage属性，而没有跨线程异常.
Event1 and Event2 work perfectly fine. I do not use any invoke methods, I directly change Text and BackgroundImage properties without cross thread exception.
- 事件3会带来麻烦.它应该显示其他形式，但是我收到了交叉therad例外.仅当我使用BeginInvoke显示表单时，它才起作用.
那是为什么?
多线程处理是通过 MJPEGStream 对象完成的.我订阅了该MJPEGStream对象的 NewFrame 方法.
The multithreading is done by an MJPEGStream object. I subscribe the NewFrame method of that MJPEGStream object.
public partial class Form1 : Form
{
private CAM cam;
private PeekWindow frmPeekWindow;
public Form1()
{
InitializeComponent();
cam = new CAM();
cam.NewImageMessageEvent += new NewImageEventHandler(cam_NewImageMessageEvent);
cam.DetectionEvent += new DetectionEventHandler(cam_DetectionEvent);
cam.FpsChangedMessageEvent += new FpsChangedEventHandler(cam_FpsChangedMessageEvent);
cam.DetectionThreshold = (float)this.numDetectionThreshold.Value;
frmPeekWindow = new PeekWindow();
// without the next two lines, frmPeekwindow.Show() won't work if called in an event
frmPeekWindow.Show();
frmPeekWindow.Hide();
}
void cam_FpsChangedMessageEvent(object sender, FpsChangedEventArgs e)
{
lblFPS.Text = string.Format("fps: {0:0.0}", e.FPS);
}
void cam_DetectionEvent(object sender, DetectionEventArgs e)
{
if (chkEnablePeakWindow.Checked)
{
if (frmPeekWindow.InvokeRequired)
{
frmPeekWindow.Invoke((MethodInvoker)delegate()
{
frmPeekWindow.Show();
frmPeekWindow.setImage(e.Image);
});
}
else
{
frmPeekWindow.Show();
frmPeekWindow.setImage(e.Image);
}
}
}
void cam_NewImageMessageEvent(object sender, NewImageEventArgs e)
{
picStream.BackgroundImage = e.Image;
}
}
这是CAM类:
class CAM
{
private object lockScale = new object();
private MJPEGStream stream;
private Bitmap image;
public event NewImageEventHandler NewImageMessageEvent;
public event FpsChangedEventHandler FpsChangedMessageEvent;
public event DetectionEventHandler DetectionEvent;
// configure (login, pwd, source)
public CAM()
{
this.stream = new MJPEGStream("...");
this.stream.Login = "...";
this.stream.Password = "...";
this.stream.NewFrame += new NewFrameEventHandler(OnNewFrame)
}
private void OnNewFrame(object sender, NewFrameEventArgs ev)
{
try
{
FpsChangedMessageEvent(this, new FpsChangedEventArgs(10));
// get image
image = ev.Frame;
NewImageMessageEvent(this, new NewImageEventArgs(new Bitmap(image)));
DetectionEvent(this, new DetectionEventArgs(new Bitmap(image)));
}
catch (Exception ex)
{
Console.Out.WriteLine(ex.Message);
}
}
}
推荐答案
您不会遇到跨线程异常，但这并不意味着这是一个安全的操作.您的控件总是有可能变得不稳定.您只是不知道何时会发生.
You won't get cross thread exception, but it doesn't mean that this is a safe operation. There is always a possibility for your control to go unstable. You just don't know when it will happen.
请参阅Microsoft的以下说明. http://msdn.microsoft.com/en-us/library/ms171728.aspx
See the following explanation from Microsoft. http://msdn.microsoft.com/en-us/library/ms171728.aspx
访问Windows Forms控件本质上不是线程安全的.如果你 有两个或多个线程来控制控件的状态，它是 可能会迫使控件进入不一致状态.其他 与线程相关的错误是可能的，例如竞争条件和 僵局.确保访问控件很重要 以线程安全的方式执行.
Access to Windows Forms controls is not inherently thread safe. If you have two or more threads manipulating the state of a control, it is possible to force the control into an inconsistent state. Other thread-related bugs are possible, such as race conditions and deadlocks. It is important to make sure that access to your controls is performed in a thread-safe way.
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