FreeCoTaskMem (Memory Access Exception) in Vista, fine in XP .NET 2

GavH

Hi Guys,

I have a problem migrating an application to Vista. The following method which runs fine on XP, will randomly throw a memory access exception (violation) on Vista.

/// <summary>
/// Initiates a frame capture through VMR9/7 GetCurrentImage()
///
/// Returns the frame as bmp through the CaptureComplete event, along with
/// the published frame number (ie where the frame will appear in the avi) which is
/// at best an approximate
/// </summary>
[STAThread]
public void CaptureFrame()
{
if((! captureLock) && previewing)
{
captureLock = true; //we need to avoid concurrent calls, as they use the same objects

int hr = 0;
IntPtr bmpPtr = IntPtr.Zero;
IntPtr pDib = IntPtr.Zero;
Bitmap bmp = null;
BitmapInfoHeader bmi = new BitmapInfoHeader();
byte[] buffer;

try
{
//get a pointer to the image
if(! vmr7Mode) hr = videoWindow.GetCurrentImage(out pDib);
else hr = videoWindow7.GetCurrentImage(out pDib);
DsError.ThrowExceptionForHR(hr);

bmi = (BitmapInfoHeader) Marshal.PtrToStructure(pDib, typeof(BitmapInfoHeader));

if (IntPtr.Size == 32)
bmpPtr = new IntPtr(pDib.ToInt32() + bmi.Size);
else
bmpPtr = new IntPtr(pDib.ToInt64() + bmi.Size);

bmp = new Bitmap(bmi.Width, bmi.Height, System.Drawing.Imaging.PixelFormat.Format32bppRgb);
System.Drawing.Imaging.BitmapData bmpData = bmp.LockBits(new Rectangle(0, 0, bmi.Width, bmi.Height),
System.Drawing.Imaging.ImageLockMode.WriteOnly,
System.Drawing.Imaging.PixelFormat.Format32bppRgb);

buffer = new byte[bmi.ImageSize];
Marshal.Copy(bmpPtr, buffer, 0, bmi.ImageSize);

Marshal.Copy(buffer, 0,bmpData.Scan0, bmi.ImageSize);

bmp.UnlockBits(bmpData);
<<EXCEPTION>> Marshal.FreeCoTaskMem(pDib); //exception here


bmp.RotateFlip(RotateFlipType.Rotate180FlipX);

//fire the event with the bmp
CaptureComplete(new Bitmap(bmp),CapturedVideoFrames,CurrentFrame);
bmp.Dispose();
}
catch
{

}
finally
{
//clean up anything we have not already dealt with
if(bmpPtr != IntPtr.Zero) Marshal.FreeCoTaskMem(bmpPtr); bmpPtr = IntPtr.Zero;
if(pDib != IntPtr.Zero) Marshal.FreeCoTaskMem(pDib); pDib = IntPtr.Zero;
GC.Collect();
captureLock = false; //re-enable capture
}
}
}

The exception is thrown by Marshal.FreeCoTaskMem() The code works a seemingly random number of times and then explodes.

Any pointers (chessey pun intended) greatfully received.

Thanks

Citizen on the earth

Mattias Sjogren

GavH

Thanks guys,

Both very useful, I will bare in mind with regard to Vista Compatibility.

"The FreeCoTaskMem(pDib) call inside the try block is unecessary since you're calling it again in the finally block"

Sorry that was simply a remnant of moving the dealocation around, the point was that the call to FreeCoTaskMem in the try block throws an exception, I simply moved it forward from the finally block to test that and left it in my pasted code.

"Good thing Vista errors since it exposes bugs in your code" you are totally correct, the intptr.size is sloppy, guess I must have been thinking 32, 64 (perhaps I was drinking and coding that night!) the worst thing is that has been running seemingly well, and most certainly un detected on XP systems for a long, long while now, and I guess that within that light the tighter sensitivity and control on Vista can only be a good thing.

Thankyou all.