I have some code which has the error “AccessViolationException was unhandled by user code: Attempted to read or write protected memory…”
A trimmed down version of the offending function is as follows:
protected override void OnPaint(PaintEventArgs pe)
{
if ((updatingFastBackground) || (Calculating)) return; //ADDED FOR DEBUGGING, SEE BELOW
BitmapData canvasData = Canvas.LockBits(new Rectangle(Point.Empty, Canvas.Size), ImageLockMode.WriteOnly, FastPixelFormat);
BitmapData fbgData = fastBackground.LockBits(new Rectangle(Point.Empty, fastBackground.Size), ImageLockMode.ReadOnly, FastPixelFormat);
try
{
unsafe
{
byte* canvasDataScan0 = (byte*)canvasData.Scan0.ToPointer();
byte* fbgDataScan0 = (byte*)fbgData.Scan0.ToPointer();
Rectangle spriteBounds = new Rectangle(Point.Empty, ButtonImages.ImageSize);
for (int i = 0; i < ButtonImages.Images.Count; i++)
{
// Button offset location
Point l = new Point(
(int)((i % columnCount) * hStep + myIVM.Location.X),
(int)((i / columnCount) * vStep + myIVM.Location.Y));
// Paint at current location?
if (buttonPaintBounds.Contains(l))
{
BitmapData spriteData = buttonBitmaps[i].LockBits(spriteBounds, ImageLockMode.ReadOnly, FastPixelFormat);
try
{
int spriteLeft = Math.Max(l.X, 0);
int spriteRight = Math.Min(l.X + ButtonImages.ImageSize.Width, canvasData.Width);
int spriteTop = Math.Max(l.Y, 0);
int spriteBottom = Math.Min(l.Y + ButtonImages.ImageSize.Height, canvasData.Height);
int spriteWidth = spriteRight - spriteLeft;
int spriteHeight = spriteBottom - spriteTop;
byte* canvasRowLeft = canvasDataScan0 + (spriteTop * canvasData.Stride) + spriteLeft * 4;
byte* spriteRowLeft =
(byte*)spriteData.Scan0.ToPointer() +
Math.Max((spriteTop - l.Y), 0) * spriteData.Stride +
Math.Max((spriteLeft - l.X), 0) * 4;
for (int y = 0; y < spriteHeight; y++)
{
canvasRowLeft += canvasData.Stride;
spriteRowLeft += spriteData.Stride;
Byte* canvasWalk = (Byte*)canvasRowLeft;
Byte* spriteWalk = (Byte*)spriteRowLeft;
for (int x = 0; x < spriteWidth; x++)
{
if (spriteWalk[3] != 255)
{
canvasWalk[0] = (byte)(canvasWalk[0] * spriteWalk[3] / 255 + spriteWalk[0]);
canvasWalk[1] = (byte)(canvasWalk[1] * spriteWalk[3] / 255 + spriteWalk[1]);
canvasWalk[2] = (byte)(canvasWalk[2] * spriteWalk[3] / 255 + spriteWalk[2]);
}
canvasWalk += 4;
spriteWalk += 4;
}
}
thesePoints.Add(l);
}
finally
{
buttonBitmaps[i].UnlockBits(spriteData);
}
}
The error occurs on the line:
canvasWalk[0] = (byte)(canvasWalk[0] * spriteWalk[3] / 255 + spriteWalk[0]);
and even when replaced with:
canvasWalk[0] = 0;
The iteration variables y and x have different values each time it crashes so this leads me to believe an external function is modifying the Canvas bitmap.
IF this is in fact my problem, is there a way to prevent fastBackground and Canvas from being externally modified? I thought LockBits was supposed to do that that…
If that’s not enough to answer, here’s some more I’ve tried:
I added the line
if ((updatingFastBackground) || (Calculating)) return;
to exit OnPaint if fastBackground Canvas or dimensions are being modified by other functions.
I could use a mutex to prevent the functions that modify the bitmaps fastBackground and Canvas from being run at the same time as paint (as I think they must be) but I’d rather block them another way as Canvas is public and I don’t want to require passing a mutex out of the class.
per @usr ‘s suggestion, this further trimmed down version doesn’t fail… Must have been a PTD error. (programmer too dumb) i.e. arithmetic error
protected override void OnPaint(PaintEventArgs pe)
{
if ((updatingFastBackground) || (Calculating)) return; //ADDED FOR DEBUGGING, SEE BELOW
BitmapData canvasData = Canvas.LockBits(new Rectangle(Point.Empty, Canvas.Size), ImageLockMode.WriteOnly, FastPixelFormat);
BitmapData fbgData = fastBackground.LockBits(new Rectangle(Point.Empty, fastBackground.Size), ImageLockMode.ReadOnly, FastPixelFormat);
try
{
unsafe
{
byte* canvasDataScan0 = (byte*)canvasData.Scan0.ToPointer();
byte* fbgDataScan0 = (byte*)fbgData.Scan0.ToPointer();
Rectangle spriteBounds = new Rectangle(Point.Empty, ButtonImages.ImageSize);
for (int i = 0; i < ButtonImages.Images.Count; i++)
{
// Button offset location
Point l = new Point(
(int)((i % columnCount) * hStep + myIVM.Location.X),
(int)((i / columnCount) * vStep + myIVM.Location.Y));
// Paint at current location?
if (buttonPaintBounds.Contains(l))
{
BitmapData spriteData = buttonBitmaps[i].LockBits(spriteBounds, ImageLockMode.ReadOnly, FastPixelFormat);
try
{
byte* canvasRowLeft = canvasDataScan0;
byte* spriteRowLeft = (byte*)spriteData.Scan0.ToPointer();
for (int y = 0; y < 145; y++)
{
canvasRowLeft += canvasData.Stride;
spriteRowLeft += spriteData.Stride;
Byte* canvasWalk = (Byte*)canvasRowLeft;
Byte* spriteWalk = (Byte*)spriteRowLeft;
for (int x = 0; x < 145; x++)
{
if (spriteWalk[3] != 255)
{
canvasWalk[0] = 0;
canvasWalk[1] = 0;
canvasWalk[2] = 0;
}
canvasWalk += 4;
spriteWalk += 4;
}
}
thesePoints.Add(l);
}
finally
{
buttonBitmaps[i].UnlockBits(spriteData);
}
}
Moving my comment into an answer because it helped solve the problem:
Fixed is not required for the buffer returned by LockBits because it is unmanaged memory. Your pointer arithmetic is wrong. Find the bug. Create a simple repro to help find the bug.