我试着用PDF为一个项目做一些事情。我正在尝试与Pdfium进行一些互操作：https://pdfium.googlesource.com/pdfium/。
我想将JPG图像转换为PDF（只是开始）。所以我在网上找到了一些资源，我来到了这个：

///// My main file
// Create the stream
using FileStream image = new FileStream(@"test.jpg", FileMode.Open, FileAccess.Read);
using FileStream pdf = new FileStream(@"test.pdf", FileMode.OpenOrCreate, FileAccess.Write);

// Init the library to use it
FPDF.InitLibrary();

// Create the PDF
IntPtr document = FPDF.CreateNewDocument();
IntPtr page = FPDFPage.New(document, 0, 3000, 4000);
IntPtr pageObject = FPDFPageObj.NewImageObj(document);

// Create a copy of the delegate
FPDF.m_GetBlockDelegate del = Delegates.m_GetBlockDelegate;
FPDF.FILEACCESS fileAccess = new FPDF.FILEACCESS
{
    m_FileLen = (ulong)image.Length,
    m_Param = image.SafeFileHandle.DangerousGetHandle(),
    m_GetBlock = del,
};

// Create the FILEWRITE struct
byte[] buffer = null;
FPDF.FILEWRITE fileWrite = new FPDF.FILEWRITE
{
    WriteBlock = delegate (ref FPDF.FILEWRITE pThis, byte[] pData, ulong size)
    {
        if (buffer == null || (ulong)buffer.Length < size)
            buffer = new byte[size];

        pData.CopyTo(buffer, 0);
        pdf.Write(buffer, 0, (int)size);

        return true;
    }
};

// Load the image
FPDPImageObj.LoadJpegFile(IntPtr.Zero, 0, pageObject, ref fileAccess);
FPDFPage.InsertObject(page, pageObject);

// Save the PDF
FPDF.SaveAsCopy(document, ref fileWrite, 0);

// Destroy the library
FPDF.DestroyLibrary();

这个方法的问题是：结构体有一个委托作为回调函数来做一些事情（它在C中做一种流实现）。
下面是我的委托的代码，struct和委托的实现：

public struct FILEACCESS
{
    public ulong m_FileLen { get; set; }
    public m_GetBlockDelegate m_GetBlock { get; set; }
    public IntPtr m_Param { get; set; }
}
public struct FILEWRITE
{
    // Must be 1
    public int version = 1;
    public WriteBlockDelegate WriteBlock { get; set; }
}

public delegate int m_GetBlockDelegate(IntPtr param, ulong position, [In] byte[] pBuf, ulong size);
public delegate bool WriteBlockDelegate(ref FILEWRITE pThis, [In] byte[] pData, ulong size);

// The implementation of my delegate in another class
public static int m_GetBlockDelegate(IntPtr param, ulong position, [In] byte[] pBuf, ulong size)
{
    // Original source code for the delegate
    //FILE* fileHandle = (FILE*)param;
    //fseek(imageFile, position, SEEK_SET);
    //fread(pBuf, size, 1, fileHandle);

    using FileStream fs = new FileStream(new SafeFileHandle(param, false), FileAccess.Read);
    fs.Seek((long)position, SeekOrigin.Begin);
    fs.Read(pBuf, 0, (int)size);

    return (int)size;
}

目前的问题是pBuf应该是一个缓冲区，用于保存所有要读取的数据。所以它的大小必须与size相同。在我的例子中，pBuf的大小为1。我已经测试了我的代码的C变体，它工作得很好（它读取所有数据）。代码如下：

#include <stdio.h>
#include <fpdfview.h>
#include <fpdf_edit.h>
#include <fpdf_save.h>

FILE* pdfFile = nullptr;

int m_GetBlockImpl(void* param, unsigned long position, unsigned char* pBuf, unsigned long size)
{
    FILE* fileHandle = (FILE*)param;
    fseek(fileHandle, position, SEEK_SET);
    fread(pBuf, size, 1, fileHandle);

    return size;
}

int WriteBlockImpl(FPDF_FILEWRITE* pThis, const void* pData, unsigned long size)
{
    fwrite(pData, size, 1, pdfFile);

    return TRUE;
}

int main()
{
    FILE* jpgFile = fopen("C:\\Users\\tcroi\\Desktop\\test.jpg", "rb");
    pdfFile = fopen("C:\\Users\\tcroi\\Desktop\\test.pdf", "wb");

    if (jpgFile == nullptr || pdfFile == nullptr)
        return 1;

    FPDF_InitLibrary();

    // Create the PDF
    FPDF_DOCUMENT pdf_doc = FPDF_CreateNewDocument();
    FPDF_PAGE pdf_page = FPDFPage_New(pdf_doc, 0, 3000, 4000);

    // Seek the stream and get the size
    fseek(jpgFile, 0, SEEK_END);
    long fileSize = ftell(jpgFile);
    fseek(jpgFile, 0, SEEK_SET);

    // Create the FILEACCESS
    FPDF_FILEACCESS fileReadObj;
    fileReadObj.m_FileLen = fileSize;
    fileReadObj.m_Param = jpgFile;
    fileReadObj.m_GetBlock = m_GetBlockImpl;

    // Load the image to the PDF
    FPDF_PAGEOBJECT page_obj_image = FPDFPageObj_NewImageObj(pdf_doc);
    FPDFImageObj_LoadJpegFile(nullptr, 0, page_obj_image, &fileReadObj);
    FPDFPage_InsertObject(pdf_page, page_obj_image);

    // Create the FILEWRITE
    FPDF_FILEWRITE fileWriteObj;
    fileWriteObj.version = 1;
    fileWriteObj.WriteBlock = WriteBlockImpl;

    FPDF_SaveAsCopy(pdf_doc, &fileWriteObj, 0);

    FPDF_DestroyLibrary();

    fclose(jpgFile);
    fclose(pdfFile);

    return 0;
}

这段代码运行良好，直到我想保存PDF，我只得到一个空白页，也许我错过了一些东西，但根据例子没有。下面是一个例子：https://groups.google.com/g/pdfium/c/P73DgvJtgTs