Canon SDK - callback function

[Michael]

Hi developers,

I downloaded the Canon SDK Version 7.1 and tried to migrate the
"RELCTRL" sample to C# (with PInvoke). Almost everything is working
except the following callback function that should receive the image
data from the camera. I´m always getting the same error:

An unhandled exception of type 'System.NullRef erenceException '
occurred in Unknown Module.

Additional information: Object reference not set to an instance of an
object.

This error shows up after the callback functions has been called 6
times. Unfortunatley I don´t know why this happens and how I can solve
the problem.

=============== =============== =============== =============== =============== ====

/* The function which receives the picture from a camera */
UInt32 ViewFinderCallB ackFun(UInt32 pBuf, UInt32 Size, UInt32 Format,
UInt32 Context)
{
UInt32 err = cdError.cdOK;

if ( Format == cdType.FILEFORM AT_BMP )
{
IntPtr pBuffer = new IntPtr(pBuf);

// Get the BITMAPHEADER from pBuf
BITMAPHEADER frameHeader = new BITMAPHEADER();
frameHeader = (BITMAPHEADER)M arshal.PtrToStr ucture(pBuffer,
frameHeader.Get Type());

// Create byte array (BITMAPHEADER + image data) and copy
byte[] VideoData = new byte[frameHeader.bmf Header.bfSize];
Marshal.Copy(pB uffer, VideoData, 0,
(int)frameHeade r.bmfHeader.bfS ize);

// Get image size
int width = frameHeader.bmi Header.biWidth;
int height = frameHeader.bmi Header.biHeight ;

// Create new Bitmap
Bitmap frame = new Bitmap(width, height);

BitmapData bmpData = frame.LockBits(
new Rectangle(0, 0, width, height),
ImageLockMode.R eadWrite,
PixelFormat.For mat24bppRgb);

// Copy image data from memory to Scan0 of our Bitmap
// Start at index 54 because of BITMAPHEADER
for (int i = 54; i < VideoData.Lengt h; i++)
{
Marshal.WriteBy te(bmpData.Scan 0, i-54, VideoData);
}

frame.UnlockBit s(bmpData);

// Create delegate
if (this.RecievedF rame != null)
{
this.RecievedFr ame(this, new StreamEventArgs (frame));
}
}
return err;
}

[StructLayout(La youtKind.Sequen tial, Pack=1)]
public struct BITMAPINFOHEADE R
{
public UInt32 biSize;
public Int32 biWidth;
public Int32 biHeight;
public Int16 biPlanes;
public Int16 biBitCount;
public UInt32 biCompression;
public UInt32 biSizeImage;
public Int32 biXPelsPerMeter ;
public Int32 biYPelsPerMeter ;
public UInt32 biClrUsed;
public UInt32 biClrImportant;
}

[StructLayout(La youtKind.Sequen tial, Pack=1)]
public struct BITMAPFILEHEADE R // 14 bytes
{
public Int16 bfType; //"magic cookie" - must be "BM"
public Int32 bfSize;
public Int16 bfReserved1;
public Int16 bfReserved2;
public Int32 bfOffBits;
}

[StructLayout(La youtKind.Sequen tial)]
public struct BITMAPHEADER
{
[MarshalAs(Unman agedType.Struct , SizeConst=14)]
public BITMAPFILEHEADE R bmfHeader;
[MarshalAs(Unman agedType.Struct , SizeConst=40)]
public BITMAPINFOHEADE R bmiHeader;
}

=============== =============== =============== =============== =============== ====

If anybody can help me ... please.

Regards

Michael

[Mattias Sjögren]

Re: Canon SDK - callback function

[color=blue]
>This error shows up after the callback functions has been called 6
>times. Unfortunatley I don´t know why this happens and how I can solve
>the problem.[/color]

This type of error is often caused by garbage collection of the
callback delegate you pass to native code. You must keep a reference
to it as long as its needed to ensure it stays alive.



Mattias

--
Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.

[Michael]

Re: Canon SDK - callback function

Thanks a lot! That did the trick.
[color=blue]
> This type of error is often caused by garbage collection of the
> callback delegate you pass to native code. You must keep a reference
> to it as long as its needed to ensure it stays alive.[/color]

[Bjoern M. Albrecht]

Re: Canon SDK - callback function

wurst.michael@w eb.de (Michael) wrote in message news:<2ddb19ed. 0503030108.437c fc88@posting.go ogle.com>...[color=blue]
> Thanks a lot! That did the trick.
>[color=green]
> > This type of error is often caused by garbage collection of the
> > callback delegate you pass to native code. You must keep a reference
> > to it as long as its needed to ensure it stays alive.[/color][/color]


Hi there,

Can you please explain your solution in some details - I think, i have
similar problems with some other sdk .

Thank you,
Bjoern M. Albrecht