C#将PDF文件转成图片文件

新建一个WinForms的应用。

安装Nuget包：

Install-Package PDFsharp

官方网站：https://docs.pdfsharp.net/

看了一下官方的介绍，参考代码推测，PDFsharp实现的功能是把PDF文件里面用到的图片文件给提取出来，而并不是把PDF页面转换成图片。
所以想把PDF用类似截图的方式弄成图片的，下面的代码应该是行不通的。

下面是从网站弄过来的示例代码：

using PdfSharp.Pdf;
using PdfSharp.Pdf.Advanced;
using PdfSharp.Pdf.IO;
using System;
using System.Collections.Generic;
using System.IO;
using System.Windows.Forms;

namespace PdfDemo
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            var fileDialog = new OpenFileDialog();
            fileDialog.Filter = "PDF文件(*.pdf)|*.pdf|所有文件(*.*)|*.*";
            if (fileDialog.ShowDialog() == DialogResult.OK)
            {
                var fileName = fileDialog.FileName;

                PdfDocument document = PdfReader.Open(fileName);

                int imageCount = 0;
                // Iterate pages
                foreach (PdfPage page in document.Pages)
                {
                    // Get resources dictionary
                    PdfDictionary resources = page.Elements.GetDictionary("/Resources");
                    if (resources != null)
                    {
                        // Get external objects dictionary
                        PdfDictionary xObjects = resources.Elements.GetDictionary("/XObject");
                        if (xObjects != null)
                        {
                            ICollection<PdfItem> items = xObjects.Elements.Values;
                            // Iterate references to external objects
                            foreach (PdfItem item in items)
                            {
                                PdfReference reference = item as PdfReference;
                                if (reference != null)
                                {
                                    PdfDictionary xObject = reference.Value as PdfDictionary;
                                    // Is external object an image?
                                    if (xObject != null && xObject.Elements.GetString("/Subtype") == "/Image")
                                    {
                                        ExportImage(xObject, ref imageCount);
                                    }
                                }
                            }
                        }
                    }
                }

                MessageBox.Show("操作成功");
            }
        }

        private static void ExportImage(PdfDictionary image, ref int count)
        {
            string filter = image.Elements.GetName("/Filter");
            switch (filter)
            {
                case "/DCTDecode":
                    ExportJpegImage(image, ref count);
                    break;

                case "/FlateDecode":
                    ExportAsPngImage(image, ref count);
                    break;
            }
        }

        private static void ExportJpegImage(PdfDictionary image, ref int count)
        {
            // Fortunately JPEG has native support in PDF and exporting an image is just writing the stream to a file.
            byte[] stream = image.Stream.Value;
            FileStream fs = new FileStream(String.Format("Image{0}.jpeg", count++), FileMode.Create, FileAccess.Write);
            BinaryWriter bw = new BinaryWriter(fs);
            bw.Write(stream);
            bw.Close();
        }

        private static void ExportAsPngImage(PdfDictionary image, ref int count)
        {
            int width = image.Elements.GetInteger(PdfImage.Keys.Width);
            int height = image.Elements.GetInteger(PdfImage.Keys.Height);
            int bitsPerComponent = image.Elements.GetInteger(PdfImage.Keys.BitsPerComponent);

            // TODO: You can put the code here that converts vom PDF internal image format to a Windows bitmap
            // and use GDI+ to save it in PNG format.
            // It is the work of a day or two for the most important formats. Take a look at the file
            // PdfSharp.Pdf.Advanced/PdfImage.cs to see how we create the PDF image formats.
            // We don't need that feature at the moment and therefore will not implement it.
            // If you write the code for exporting images I would be pleased to publish it in a future release
            // of PDFsharp.
        }
    }
}