CGBitmapContext in MonoTouch throws invalid handle exception

Question

I'm new to MonoTouch and I've recently been converting code from standard C# to MonoTouch compliant code so it can be used within an iPhone application. I've managed to convert the code, however I've run into a problem with this method which I use to return a bitmap context:

private CGBitmapContext ExtractWriteableBitmap( RGBPaletteRecord rgbPalette, double dpi, ChartIndexFile indexFile, RasterChartFile chartFile )
        {
            CGBitmapContext bitmapImage = null;
            TileRecord tile;

            // calc the number of tiles in each plane
            int tileCountX = indexFile.TileIndexRecords.Max(ti => ti.X);
            int tileCountY = indexFile.TileIndexRecords.Max(ti => ti.Y);

            // create the big picture
            int pixelWidth = (tileCountX + 1) * TileRecord.PixelWidth;
            int pixelHeight = (tileCountY + 1) * TileRecord.PixelHight;

            int intDPI = Convert.ToInt32(dpi);

            int bytesPerRow = (int)TileRecord.PixelWidth * 4; // note that bytes per row should 
    //be based on width, not height.

            CGColorSpace colorSpace = CGColorSpace.CreateDeviceRGB();
            CGImageAlphaInfo info = CGImageAlphaInfo.NoneSkipFirst;

            // create the big picture 
            bitmapImage = new CGBitmapContext(System.IntPtr.Zero, pixelWidth, pixelHeight, intDPI, intDPI, colorSpace, info);

            byte[] sourceArray;
            RectangleF sourceRect;

            //copy the tiles into the big picture
            int index = 0;
            foreach (TileIndexRecord tileIndexRecord in indexFile.TileIndexRecords)
            {
                // get the tile record
                tile = chartFile.TileRecords[index];

                // extract the byte array for the given palette
                sourceArray = tile.GetBytes(rgbPalette);

                GCHandle pinnedArray = GCHandle.Alloc(sourceArray, GCHandleType.Pinned);
                IntPtr pointer = pinnedArray.AddrOfPinnedObject();
                //do your stuff
                //pinnedArray.Free();

                CGImage image = new CGImage(pointer);
                sourceRect = new RectangleF(0, 0, TileRecord.PixelWidth, TileRecord.PixelHight); 

                bitmapImage.DrawImage(new RectangleF(0,0,128,128),image);

                // copy the tile image into the big picture
                //bitmapImage.WritePixels(sourceRect, sourceArray, TileRecord.Stride, (tileIndexRecord.X * TileRecord.PixelWidth), (tileIndexRecord.Y * TileRecord.PixelHight));

                // increment the index
                index++;
            }
            return bitmapImage;
        }

Here is the stack trace:

System.Exception: Invalid handle
  at MonoTouch.CoreGraphics.CGContext..ctor (IntPtr handle, Boolean owns) [0x00022] in /Developer/MonoTouch/Source/monotouch/src/shared/CoreGraphics/CGContext.cs:136
  at MonoTouch.CoreGraphics.CGBitmapContext..ctor (IntPtr data, Int32 width, Int32 height, Int32 bitsPerComponent, Int32 bytesPerRow, MonoTouch.CoreGraphics.CGColorSpace colorSpace, CGImageAlphaInfo bitmapInfo) [0x00000] in <filename unknown>:0
  at Jargoon.Data.Arcs.Loader.ExtractWriteableBitmap (Jargoon.Data.Arcs.Records.RGBPaletteRecord rgbPalette, Double dpi, Jargoon.Data.Arcs.Raschts.ChartIndexFile indexFile, Jargoon.Data.Arcs.Raschts.RasterChartFile chartFile) [0x00079] in /Users/jacknutkins/Desktop/Jargoon/ARCSViewer/ARCSViewer/Loader.cs:569
  at Jargoon.Data.Arcs.Loader.GetHiResImage (Jargoon.Data.Arcs.Records.RGBPaletteRecord rgbPalette) [0x00000] in /Users/jacknutkins/Desktop/Jargoon/ARCSViewer/ARCSViewer/Loader.cs:361
  at ARCSViewer.ARCSViewerViewController.ViewDidLoad () [0x0001c] in /Users/jacknutkins/Desktop/Jargoon/ARCSViewer/ARCSViewer/ARCSViewerViewController.cs:37
  at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
  at MonoTouch.UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00042] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:29
  at ARCSViewer.Application.Main (System.String[] args) [0x00000] in /Users/jacknutkins/Desktop/Jargoon/ARCSViewer/ARCSViewer/Main.cs:17

The error occurs at the line:

// create the big picture 
            bitmapImage = new CGBitmapContext(System.IntPtr.Zero, pixelWidth, pixelHeight, intDPI, intDPI, colorSpace, info);

I'm very new to MonoTouch so try and bear with me if its glaringly obvious but can anyone explain whats going on?

Answer 1

This usually happens when you're running out of memory. The (device) console will usually print more information though.

My guess is that you're not freeing the CGBitmapContext you're creating.