未将对象的引用设置到对象的实例，怎么解决啊

[qijun1994]

直接上代码，从网上下载的一个将AVI视频拆分为BMP图像文件的程序。
自己单步调试了一下，发现 private void cmdGetBitmaps_Click(object sender, EventArgs e)函数中，执行到 bih = (BITMAPINFOHEADER)Marshal.PtrToStructure(new IntPtr(pDib), bih.GetType());
语句之后，便没有执行后面的处理得到BMP文件的代码，直接跳到了报错，未将对象的引用设置到对象的实例。
前面已经定义了一下BITMAPINFOHEADER bih = new BITMAPINFOHEADER();为什么还会报错呢？求大神指点

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.Globalization;
using System.IO;

namespace GetAVIFrames
{
    public partial class Form1 : Form
    {
        public const int BMP_MAGIC_COOKIE = 19778;
        [DllImport("avifil32.dll")] public static extern void AVIFileInit();
        [DllImport("avifil32.dll")] public static extern int AVIFileOpen(ref int ppFile, String szFile, int Mode, int pclsidHandler);
        [DllImport("avifil32.dll")] public static extern int AVIFileGetStream(int pFile, out IntPtr ppAvi, int fccType, int lParam);

        [DllImport("avifil32.dll")] public static extern int AVIStreamStart(int pavi);
        [DllImport("avifil32.dll")] public static extern int AVIStreamLength(int pavi);
        [DllImport("avifil32.dll")] public static extern int AVIStreamInfo(int pAVIStream, ref AVI_STREAM_INFO pStrInfo, int Size);

        [DllImport("avifil32.dll")] static extern int AVIFileInfo(int pFile, ref AVI_FILE_INFO aviInfo, int Size);
        [DllImport("avifil32.dll")] static extern int AVIStreamGetFrameOpen(IntPtr pAvi, ref BITMAPINFOHEADER bih);
        [DllImport("avifil32.dll")] public static extern int AVIStreamGetFrame(int pGetFrame, int Pos);
        [DllImport("avifil32.dll")] public static extern int AVIStreamGetFrameClose(int pGetFrame);

        public struct AVI_RECT
        {
            public UInt32 left;
            public UInt32 top;
            public UInt32 right;
            public UInt32 bottom;
        };

        public struct AVI_STREAM_INFO
        {
            public UInt32 fccType;
            public UInt32 fccHandler;
            public UInt32 dwFlags;
            public UInt32 dwCaps;
            public UInt16 wPriority;
            public UInt16 wLanguage;
            public UInt32 dwScale;
            public UInt32 dwRate;
            public UInt32 dwStart;
            public UInt32 dwLength;
            public UInt32 dwInitialFrames;
            public UInt32 dwSuggestedBufferSize;
            public UInt32 dwQuality;
            public UInt32 dwSampleSize;
            public AVI_RECT rcFrame;
            public UInt32 dwEditCount;
            public UInt32 dwFormatChangeCount;
            public string szName;
        } ;


        public struct AVI_FILE_INFO
        {
            long dwMaxBytesPerSecond;
            long dwFlags;
            long dwCaps;
            long dwStreams;
            long dwSuggestedBufferSize;
            long dwWidth;
            long dwHeight;
            long dwScale;
            long dwRate;
            long dwLength;
            long dwEditCount;
            long szFileType;
        }

        [StructLayout(LayoutKind.Sequential, Pack=1)]
        public struct BITMAPINFOHEADER
        {
            public UInt32 biSize;
            public Int32 biWidth;
            public Int32 biHeight;
            public Int16 biPlanes;
            public Int16 biBitCount;
            public UInt32 biCompression;
            public UInt32 biSizeImage;
            public Int32 biXPelsPerMeter;
            public Int32 biYPelsPerMeter;
            public UInt32 biClrUsed;
            public UInt32 biClrImportant;
        }

        [StructLayout(LayoutKind.Sequential, Pack = 1)]
        public struct BITMAPFILEHEADER
        {
            public Int16 bfType;
            public Int32 bfSize;
            public Int16 bfReserved1;
            public Int16 bfReserved2;
            public Int32 bfOffBits;
        }

        int writeDenied = 0x20;
        BITMAPINFOHEADER bih = new BITMAPINFOHEADER();

        int streamtypeVIDEO = 1935960438; // 'equivalent to: mmioStringToFOURCC("vids", 0&)

        int AVIFile = 0;
        IntPtr aviStream;
      

        public Form1()
        {
            InitializeComponent();
        }

        private IntPtr IntPtr(int pDib)
        {
            throw new Exception("The method or operation is not implemented.");
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            AVIFileInit();
        }

        private void cmdGetBitmaps_Click(object sender, EventArgs e)
        {
            int result;
            String fileName;
            int pointerAVI = 0;
            int pointerAVIStream = 0;
            int numberFrames = 0;
            int firstFrame = 0;

            AVI_FILE_INFO fileInfo = new AVI_FILE_INFO();
            AVI_STREAM_INFO streamInfo = new AVI_STREAM_INFO();

            fileName = "D:\\highway.avi";

            try
            {
                result = AVIFileOpen(ref pointerAVI, fileName, writeDenied, 0);
                result = AVIFileGetStream(pointerAVI, out aviStream, streamtypeVIDEO, 0);
                firstFrame = AVIStreamStart(aviStream.ToInt32());
                numberFrames = AVIStreamLength(aviStream.ToInt32());
             
                result = AVIFileInfo(pointerAVI, ref fileInfo, System.Runtime.InteropServices.Marshal.SizeOf(fileInfo));
                result = AVIStreamInfo(aviStream.ToInt32(), ref streamInfo, Marshal.SizeOf(streamInfo));

                bih.biBitCount = 24;
                bih.biClrImportant = 0;
                bih.biClrUsed = 0;
                bih.biCompression = 0;
                bih.biHeight = (Int32)(streamInfo.rcFrame.bottom);
                bih.biWidth = (Int32)streamInfo.rcFrame.right;
                bih.biPlanes = 1;
                bih.biSize = (UInt32)Marshal.SizeOf(bih);
                bih.biXPelsPerMeter = 0;
                bih.biYPelsPerMeter = 0;

                int getFrameObject = 0;

                String dstFileName = "D:\\bitmap";

                getFrameObject = AVIStreamGetFrameOpen(aviStream, ref bih);



                for (int i = firstFrame; i < numberFrames - 1; i++)
                {
                    int pDib = AVIStreamGetFrame(getFrameObject, firstFrame + i);
                  
                 
                    bih = (BITMAPINFOHEADER)Marshal.PtrToStructure(new IntPtr(pDib), bih.GetType());

                    //Copy the image
                    byte[] bitmapData = new byte[bih.biSizeImage];
                    int address = pDib + Marshal.SizeOf(bih);
                    for (int offset = 0; offset < bitmapData.Length; offset++)
                    {
                        bitmapData[offset] = Marshal.ReadByte(new IntPtr(address));
                        address++;
                    }


                    //Copy bitmap info
                    byte[] bitmapInfo = new byte[Marshal.SizeOf(bih)];
                    IntPtr ptr;
                    ptr = Marshal.AllocHGlobal(bitmapInfo.Length);
                    Marshal.StructureToPtr(bih, ptr, false);
                    address = ptr.ToInt32();
                    for (int offset = 0; offset < bitmapInfo.Length; offset++)
                    {
                        bitmapInfo[offset] = Marshal.ReadByte(new IntPtr(address));
                        address++;
                    }

                    //Create file header
                    BITMAPFILEHEADER bfh = new BITMAPFILEHEADER();
                    bfh.bfType = BMP_MAGIC_COOKIE;
                    bfh.bfSize = (Int32)(55 + bih.biSizeImage); //size of file as written to disk
                    bfh.bfReserved1 = 0;
                    bfh.bfReserved2 = 0;
                    bfh.bfOffBits = Marshal.SizeOf(bih) + Marshal.SizeOf(bfh);



                    //Create or overwrite the destination file
                    FileStream fs = new FileStream(dstFileName + i.ToString() + ".bmp", System.IO.FileMode.Create);
                    BinaryWriter bw = new BinaryWriter(fs);

                    //Write header
                    bw.Write(bfh.bfType);
                    bw.Write(bfh.bfSize);
                    bw.Write(bfh.bfReserved1);
                    bw.Write(bfh.bfReserved2);
                    bw.Write(bfh.bfOffBits);
                    //Write bitmap info
                    bw.Write(bitmapInfo);
                    //Write bitmap data
                    bw.Write(bitmapData);
                    bw.Close();
                    fs.Close();
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

    }
}

[qijun1994]

引用 1 楼 sp1234 的回复:

你在异常时，从vs调试器上，确认了 bih 是null了吗？ 没有调试，你怎么认定一定是这个变量值的问题？

十分感谢，跟踪了一下，发现是getFrameObject = AVIStreamGetFrameOpen(aviStream, ref bih);这一句有问题，getFrameObject得值始终为0，导致后面的 int pDib = AVIStreamGetFrame(getFrameObject, firstFrame + i); pDib的值也为0,而aviStream与 bih都不为Null。

[qijun1994]

引用 2 楼 tcmakebest 的回复:

那就可能是pDib是0啊

是的，pDib的值为0，bih没有问题。 再调调看吧，十分感谢

[qijun1994]

引用 1 楼 sp1234 的回复:

你在异常时，从vs调试器上，确认了 bih 是null了吗？ 没有调试，你怎么认定一定是这个变量值的问题？

查看了MSDN中对于AVIStreamGetFrameOpen function的解释，返回值的阐述如下： Return value Returns a GetFrame object that can be used with the AVIStreamGetFrame function. If the system cannot find a decompressor that can decompress the stream to the given format, or to any RGB format, the function returns NULL. 其中的If the system cannot find a decompressor（如果系统无法找到解压器），怎么理解啊，还望大神指点，十分感谢

[threenewbee]

http://hi.baidu.com/dongyuejiang/item/1badc7eadd263c2f5b7cfbe9

[以专业开发人员为伍]

你在异常时，从vs调试器上，确认了 bih 是null了吗？ 没有调试，你怎么认定一定是这个变量值的问题？

[tcmakebest]

那就可能是pDib是0啊

[linghujun2000]

我也遇到这个问题，怎么办？坐等答案