wince的UnauthorizedAccessException异常如何解决?

代码如下，我简单模拟连续截图，程序刚开始运行好好的，但是在随机的第N幅的时候会弹出UnauthorizedAccessException异常，字面意思是无存取权限，但目录是有存储权限的，前面N幅都能生成，也没有超过大小限制，我猜想是内存没释放的问题，但是我都加了Dispose()后还是出现同样问题，郁闷
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;namespace DeviceApplication1
{
    public partial class Form1 : Form
    {
       
        public Form1()
        {
            InitializeComponent();
        }        private void button1_Click(object sender, EventArgs e)
        {
            for (int i = 0; i <= 1000; i++)
            {
                string fileName = string.Format("SDMMC\\IMAGE\\{0}.jpg",i);
                GetImage(fileName);
            }
            MessageBox.Show("Hello");
        }        public void GetImage(string fileName)
        {
            int iHeight = Screen.PrimaryScreen.Bounds.Height;
            int iWidth = Screen.PrimaryScreen.Bounds.Width;
            IntPtr dc1 = WINAPI.CreateDC(null, null, null, (IntPtr)null);
            Graphics gScreen = Graphics.FromHdc(dc1);
            Bitmap iGetPhoto = new Bitmap(
                iWidth
                , iHeight);
            Graphics gPhoto = Graphics.FromImage(iGetPhoto);
            IntPtr iHandle = gScreen.GetHdc();
            IntPtr iPhoto = gPhoto.GetHdc();            WINAPI.BitBlt(iPhoto, 0, 0, iWidth, iHeight,
                iHandle, 0, 0, WINAPI.SRCCOPY);
            iGetPhoto.Save(fileName, ImageFormat.Jpeg);
            gScreen.ReleaseHdc(iHandle);
            gPhoto.ReleaseHdc(iPhoto);
            gScreen.Dispose();
            gPhoto.Dispose();            iGetPhoto.Dispose();
            WINAPI.DeleteDC(dc1);
        }
    }    public class WINAPI
    {
        public const int SRCCOPY = 0x00CC0020; // BitBlt dwRop parameter        [System.Runtime.InteropServices.DllImportAttribute("coredll.dll")]
        public static extern IntPtr CreateDC(
        string lpszDriver, // 驱动名称  
        string lpszDevice, // 设备名称  
        string lpszOutput, // 无用，可以设定位"NULL"  
        IntPtr lpInitData // 任意的打印机数据  
        );        [DllImport("coredll.dll")]
        public static extern bool BitBlt(IntPtr hObject, int nXDest, int nYDest,
            int nWidth, int nHeight, IntPtr hObjectSource,
            int nXSrc, int nYSrc, int dwRop);
        [DllImport("coredll.dll")]
        public static extern IntPtr CreateCompatibleBitmap(IntPtr hDC, int nWidth,
            int nHeight);
        [DllImport("coredll.dll")]
        public static extern IntPtr CreateCompatibleDC(IntPtr hDC);
        [DllImport("coredll.dll")]
        public static extern bool DeleteDC(IntPtr hDC);
        [DllImport("coredll.dll")]
        public static extern bool DeleteObject(IntPtr hObject);
        [DllImport("coredll.dll")]
        public static extern IntPtr SelectObject(IntPtr hDC, IntPtr hObject);
    }
}