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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?


Member 11894340 wrote:
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 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:[^].

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:[^],[^].

Also, you could embed the popular VCL player component:[^],[^].

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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:,

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:

Good luck,
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Prakash1206 22-Sep-15 17:26pm    
Thanks for your reply.
I changed my code..
Added below lines

//i run demo(); after initilize components

public: void demo(){

aTimer = gcnew System::Timers::Timer();
aTimer->Interval = 100;
aTimer->Elapsed += gcnew System::Timers::ElapsedEventHandler(this, &MyForm::OnTimedEvent);
aTimer->AutoReset = true;
aTimer->Enabled = true;

public: delegate void MyCallback(System::String^ str);
VideoCapture cap(0);
Mat image;
IplImage frame, *frame2;

public :System::Void OnTimedEvent(Object^ source, System::Timers::ElapsedEventArgs^ e)
MyForm ^f = gcnew MyForm();
MyCallback^ callback = gcnew MyCallback(f,&MyForm::run);

private: System::Void run(System::String^ str){
cap >> image;
if (image.empty()) MessageBox::Show("empty pic"); else imshow("frame", image);
frame = image;
frame2 = &frame;
pictureBox1->Image = gcnew
System::Drawing::Bitmap(frame2->width, frame2->height, frame2->widthStep,
System::Drawing::Imaging::PixelFormat::Format24bppRgb, (System::IntPtr) frame2->imageData);


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
i did try commenting imshow(), still no change

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