在单独的线程c ++ / cli上运行Picturebox [英] Running Picturebox on separate thread c++/cli

问题描述

嗨
这是我在Code项目中的第一个问题



i是c ++ / cli的新手。



i在我的表单中有一个图片框并创建了一个计时器

private：System :: Timers :: Timer ^ aTimer;

i我在aTimer tick事件上更新图片框，但它显示错误



PicBox_Invoke.exe中发生类型'System.NullReferenceException'的例外，但未在用户代码中处理附加信息：未设置对象引用到一个对象的实例。



如何使用代表来解决这个问题？



谢谢

Hi This is my First question in Code project

i am new to c++/cli.

i have a picturebox in my form and created a timer
private: System::Timers::Timer^ aTimer;
i am updating picturebox on aTimer tick event, but its showing an error

An exception of type 'System.NullReferenceException' occurred in PicBox_Invoke.exe but was not handled in user code Additional information: Object reference not set to an instance of an object.

how to use delegates to solve this issue?

Thanks

推荐答案

会员11894340写道：

Member 11894340 wrote:

一切都很好，但问题是我我只能从imshow（）函数看到来自我的网络摄像头的视频

但我在winforms picturebox中看不到视频

everything is fine, but the problem is that i can see video from my webcam only from imshow() function
but i cannot see video in winforms picturebox

PictureBox 是非常低效的控制用于显示视频。为什么你需要一个显示图像的图片框（控件），视频数据和控件之间的冗余中间人，如果你需要自己渲染视频？您最好使用自己的自定义控件，将视频图形直接渲染到 Graphics 像素。请参阅： https://msdn.microsoft .com / zh-cn / library / system.drawing.bitmap.lockbits％28v = vs.110％29.aspx [ ^ ]。



另见我过去的答案：

在图片框中附加图片 [ ^ ]，

在C＃中绘制一个矩形 [ ^ ]，

如何从旧图纸中清除面板 [ ^ ]。



但是在你的表单中嵌入现有的媒体播放器要好得多。请参阅：

https ：//msdn.microsoft.com/en-us/library/windows/desktop/dd562851%28v=vs.85%29.aspx [ ^ ]，

< a href =https://msdn.microsoft.com/en-us/library/bb383953%28v=vs.90%29.aspx> https://msdn.microsoft.com/en-us/library/bb383953 ％28v = vs.90％29.aspx [ ^ ]。



此外，你可以嵌入流行的VCL播放器组件：

https://wiki.videolan.org/.Net_Interface_to_VLC [ ^ ]，

HTTPS：// WW w.youtube.com/watch?v=aAdKP79jT6o [ ^ ]。

-SA

PictureBox is really inefficient control for showing video. Why would you need a picture box (control) showing Image, redundant middleman between video data and the control, if you need to render video itself? You should better use your own custom control which renders video graphics directly onto the Graphics pixels. Please see: https://msdn.microsoft.com/en-us/library/system.drawing.bitmap.lockbits%28v=vs.110%29.aspx[^].

See also my past answers:
Append a picture within picturebox[^],
draw a rectangle in C#[^],
How do I clear a panel from old drawing[^].

But it would be much better to embed existing media player in your form. Please see:
https://msdn.microsoft.com/en-us/library/windows/desktop/dd562851%28v=vs.85%29.aspx[^],
https://msdn.microsoft.com/en-us/library/bb383953%28v=vs.90%29.aspx[^].

Also, you could embed the popular VCL player component:
https://wiki.videolan.org/.Net_Interface_to_VLC[^],
https://www.youtube.com/watch?v=aAdKP79jT6o[^].

—SA

首先，你无法操作除相应的UI线程（被称为 Application.Run PictureBox 或其他任何内容） $ C>）。但间接地，如果使用UI线程调用机制将方法委托给此UI线程，则可以触发任何更改。我在过去的答案中对此进行了解释：

控制。 Invoke（）vs. Control.BeginInvoke（） [ ^ ]，

Treeview扫描仪和MD5的问题 [ ^ ]，

主线程上的.NET事件 [ ^ ]。



至于 NullReferenceException ，你没有用mes显示异常的位置sage对象引用未设置为对象的实例被抛出。



不用担心。这是检测和修复的最简单的案例之一。它只是意味着某些引用类型的某个成员/变量通过使用和它的实例（非静态）成员解除引用，这要求此成员/变量为非null，但实际上它似乎为null。只需在调试器下执行它，它将停止抛出异常的执行。在该行上设置一个断点，重新启动应用程序并再次到达这一点。评估下一行中涉及的所有引用，并查看哪一个为null，而不需要为null。解决这个问题之后，修复代码：要么确保将成员/变量正确初始化为非空引用，要么将其检查为null，如果为null，则执行其他操作。



另请参阅：想要在按钮点击时显示下一条记录。但是如果下一个记录功能的条件对象引用未设置为对象的实例则会出错。



有时候，你不能这样做在调试器下，由一个或另一个原因。一个非常讨厌的情况是，只有在调试信息不​​可用时构建软件时才会出现问题。在这种情况下，你必须使用更难的方式。首先，你需要确保你永远不会通过静默处理异常来阻止异常的传播（这是开发人员对自己的犯罪，但很常见）。您需要在每个线程的最顶层堆栈帧上捕获绝对所有异常。如果处理类型 System.Exception 的异常，则可以执行此操作。在处理程序中，您需要记录所有异常信息，尤其是 System.Exception.StackTrace ：

http://msdn.microsoft.com/en-us/library/system.exception.aspx ，

http://msdn.microsoft.com/en-us/library/ system.exception.stacktrace.aspx 。



堆栈跟踪只是一个字符串，显示从throw语句到处理程序的异常传播的完整路径。通过阅读，您总能找到目的。对于日志记录，使用类 System.Diagnostics.EventLog ：

http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx 。



祝你好运，

-SA

First of all, you cannot operate with any elements (PictureBox or anything else) included in presently running UI in any thread except the corresponding UI thread (the called Application.Run). But indirectly, you can trigger any changes if you delegate the method to this UI thread using UI thread invocation mechaism. This is explained in my past answers:
Control.Invoke() vs. Control.BeginInvoke()[^],
Problem with Treeview Scanner And MD5[^],
.NET event on main thread[^].

As to the NullReferenceException, you did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object".

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx,
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx.

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx.

Good luck,

—SA

这篇关于在单独的线程c ++ / cli上运行Picturebox的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！