好奇怪的问题 “ 未能找到类型或命名空间名称“PdfSharp”(是否缺少 using 指令或程序集引用?)”

这是代码：

//.cs  文件类型，便于外部编辑时使用
// 引用必要的命名空间
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Linq;
using PdfSharp.Drawing;
using PdfSharp.Pdf;
using System.Threading.Tasks;

public static void Exec(Quicker.Public.IStepContext context)
{
    string folderPath = (string)context.GetVarValue("每个文件夹路径"); // 获取图片文件夹路径
    string destination = (string)context.GetVarValue("转换后的PDF路径");

    if (!string.IsNullOrEmpty(folderPath) && Directory.Exists(folderPath))
    {
        var imageExtensions = new[] { ".jpg", ".jpeg", ".png", ".gif", ".bmp" }; // 图像文件扩展名列表
        var imageFiles = Directory.GetFiles(folderPath)
                                  .Where(f => imageExtensions.Contains(Path.GetExtension(f).ToLower()))
                                  .OrderBy(f => f)
                                  .ToList();

        if (imageFiles.Count > 0)
        {
            PdfDocument doc = new PdfDocument();

            // 使用并行处理以加快处理速度
            Parallel.ForEach(imageFiles, imagePath =>
            {
                using (Bitmap bitmap = new Bitmap(imagePath))
                {
                    int width = bitmap.Width;
                    int height = bitmap.Height;
                    float dpiX = bitmap.HorizontalResolution;
                    float dpiY = bitmap.VerticalResolution;

                    PdfPage pdfPage = doc.AddPage();
                    pdfPage.Width = XUnit.FromInch(width / dpiX);
                    pdfPage.Height = XUnit.FromInch(height / dpiY);

                    using (XGraphics xgr = XGraphics.FromPdfPage(pdfPage))
                    {
                        using (MemoryStream stream = new MemoryStream())
                        {
                            bitmap.Save(stream, ImageFormat.Png);
                            stream.Seek(0, SeekOrigin.Begin);

                            XImage img = XImage.FromStream(stream);
                            xgr.DrawImage(img, 0, 0, pdfPage.Width.Point, pdfPage.Height.Point);
                        }
                    }
                }
            });

            // 保存PDF文件
            doc.Save(destination);
            doc.Close();
        }
        else
        {
            context.SetVarValue("error", "文件夹中没有找到有效的图片文件");
        }
    }
    else
    {
        context.SetVarValue("error", "未提供有效的图片文件夹路径或文件夹不存在");
    }
}

这是报错：