C#合并BitMap图像生成超大bitmap

//设置文件头里面文件大小信息

public void SetBitmapFileSizeInfo(string filePath)
        {
            FileInfo fileInfo = new FileInfo(filePath);
            long le = fileInfo.Length;
            string hexSize = le.ToString("X").PadLeft(8, '0');
            int size1 = Convert.ToInt32(hexSize.Substring(0, 2), 16);
            int size2 = Convert.ToInt32(hexSize.Substring(2, 2), 16);
            int size3 = Convert.ToInt32(hexSize.Substring(4, 2), 16);
            int size4 = Convert.ToInt32(hexSize.Substring(6, 2), 16);
            byte[] sizeBytes = new byte[] { (byte)size4, (byte)size3, (byte)size2, (byte)size1 };
            using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Write))
            {
                using (BinaryWriter r = new BinaryWriter(fs))
                {
                    r.Seek(2, 0);
                    r.Write(sizeBytes, 0, sizeBytes.Length);
                }
            }
        }

 public void SetBitmapSizeInfo(string filePath,int width=0,int height=0)
        {
            if (height != 0)
            {
                string hexHeight = height.ToString("X").PadLeft(8, '0');
                int h1 = Convert.ToInt32(hexHeight.Substring(0, 2), 16);
                int h2 = Convert.ToInt32(hexHeight.Substring(2, 2), 16);
                int h3 = Convert.ToInt32(hexHeight.Substring(4, 2), 16);
                int h4 = Convert.ToInt32(hexHeight.Substring(6, 2), 16);
                byte[] sizeHeight = new byte[] { (byte)h4, (byte)h3, (byte)h2, (byte)h1 };
                using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite))
                {
                    using (BinaryWriter r = new BinaryWriter(fs))
                    {
                        r.Seek(22, 0);//高度保存位置
                        r.Write(sizeHeight, 0, sizeHeight.Length);
                    }
                }
            }
            if (width != 0)
            {
                string hexWidth = height.ToString("X").PadLeft(8, '0');
                int w1 = Convert.ToInt32(hexWidth.Substring(0, 2), 16);
                int w2 = Convert.ToInt32(hexWidth.Substring(2, 2), 16);
                int w3 = Convert.ToInt32(hexWidth.Substring(4, 2), 16);
                int w4 = Convert.ToInt32(hexWidth.Substring(6, 2), 16);
                byte[] sizeWidth = new byte[] { (byte)w4, (byte)w3, (byte)w2, (byte)w1 };
                using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite))
                {
                    using (BinaryWriter r = new BinaryWriter(fs))
                    {
                        r.Seek(18, 0);//高度保存位置
                        r.Write(sizeWidth, 0, sizeWidth.Length);
                    }
                }
            }
        }

private void CreateBitMap(string tempPath,string imagePath)
        {
            string[] files = Directory.GetFiles(tempPath, "*.png");
            Bitmap bmp;
            int height=0;
            for (int i = files.Length-1; i >0; i--)
            {
                string fileName = files[i];
                bmp = new Bitmap(fileName);
                if (i == files.Length - 1)
                {
                    bmp.Save(imagePath, ImageFormat.Bmp);
                    height += bmp.Height;
                    bmp.Dispose();
                    continue;
                }
                else
                {
                    byte[] bytes = GetImageRasterBytes(bmp, PixelFormat.Format32bppRgb);
                    using (FileStream fs = new FileStream(imagePath, FileMode.Open, FileAccess.Write))
                    {
                        fs.Seek(fs.Length, 0);
                        fs.Write(bytes, 0, bytes.Length);
                    }
                    height += bmp.Height;
                    bmp.Dispose();
                }
            }
            SetBitmapFileSizeInfo(imagePath);
            SetBitmapSizeInfo(imagePath, height: height);
            //MessageBox.Show("合并成功");
        }
         private static byte[] GetImageRasterBytes(Bitmap bmp, PixelFormat format)
        {
            Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
            byte[] bits = null;
            try
            {
                // Lock the managed memory
                BitmapData bmpdata = bmp.LockBits(rect, ImageLockMode.ReadWrite, format);
                // Declare an array to hold the bytes of the bitmap.
                bits = new byte[bmpdata.Stride * bmpdata.Height];
                // Copy the values into the array.
                System.Runtime.InteropServices.Marshal.Copy(bmpdata.Scan0, bits, 0, bits.Length);
                // Release managed memory
                bmp.UnlockBits(bmpdata);
            }
            catch
            {
                return null;
            }
            return bits;
        }