SignIncremental.cs
// 完毕:
using System;
using System.IO;
using System.Drawing;
using GrapeCity.Documents.Pdf;
using GrapeCity.Documents.Pdf.AcroForms;
using GrapeCity.Documents.Text;
using System.Security.Cryptography.X509Certificates;

namespace DsPdfWeb.Demos
{
    // 此示例​​生成并签署 PDF(使用与 SignDoc 示例类似的代码),
    // 然后使用第二个签名对生成的 PDF 进行签名,而不会使原始文件失效
    // 签名,通过使用增量更新(使用 Sign() 方法时的默认设置)。
    public class SignIncremental
    {
        public int CreatePDF(Stream stream)
        {
            var doc = new GcPdfDocument();
            // 加载签名文档(我们使用类似于SignDoc示例的代码):
            doc.Load(CreateAndSignPdf());
            // 初始化第二个证书:
            var pfxPath = Path.Combine("Resources", "Misc", "JohnDoe.pfx");
            var cert = new X509Certificate2(File.ReadAllBytes(pfxPath), "secret",
                X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable);
            var sp2 = new SignatureProperties()
            {
                SignatureBuilder = new Pkcs7SignatureBuilder()
                {
                    CertificateChain = new X509Certificate2[] { cert }
                },
                Location = "DsPdfWeb Demo Browser",
                SignerName = "DsPdfWeb",
                SigningDateTime = Common.Util.TimeNow(),
            };
            // 找到第二个(尚未填写)签名字段:
            var sfld2 = doc.AcroForm.Fields["SecondSignature"] as SignatureField;
            // 连接签名字段和签名属性:
            sp2.SignatureField = sfld2 ?? throw new Exception("Unexpected: could not find 'SecondSignature' field");
            // 签署并保存文档:
            doc.Sign(sp2, stream);

            // 倒回流以读取刚刚创建的文档
            // 进入另一个 GcPdfDocument 并验证所有签名:
            stream.Seek(0, SeekOrigin.Begin);
            var doc2 = new GcPdfDocument();
            doc2.Load(stream);
            foreach (var fld in doc2.AcroForm.Fields)
                if (fld is SignatureField sfld)
                    if (!sfld.Value.VerifySignatureValue())
                        throw new Exception($"Failed to verify signature for field {sfld.Name}");

            // 完成(生成并签名的文档已保存到“流”)。
            return doc.Pages.Count;
        }

        // 这个方法与SignDoc示例几乎完全相同,
        // 但添加了第二个签名字段(但不签名):
        private Stream CreateAndSignPdf()
        {
            var doc = new GcPdfDocument();
            var page = doc.NewPage();
            var tf = new TextFormat() { Font = StandardFonts.Times, FontSize = 14 };
            page.Graphics.DrawString("你好世界!" +
                "",
                tf, new PointF(72, 72));

            // 初始化测试证书:
            var pfxPath = Path.Combine("Resources", "Misc", "DsPdfTest.pfx");
            var cert = new X509Certificate2(File.ReadAllBytes(pfxPath), "qq",
                X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable);
            var sp = new SignatureProperties()
            {
                SignatureBuilder = new Pkcs7SignatureBuilder()
                {
                    CertificateChain = new X509Certificate2[] { cert }
                },
                Location = "DsPdfWeb Demo Browser",
                SignerName = "DsPdfWeb",
                SigningDateTime = Common.Util.TimeNow(),
            };

            // 初始化一个签名字段来保存签名:
            var sf = new SignatureField();
            sf.Widget.Rect = new RectangleF(72, 72 * 2, 72 * 4, 36);
            sf.Widget.Page = page;
            sf.Widget.BackColor = Color.LightSeaGreen;
            // 将签名字段添加到文档中:
            doc.AcroForm.Fields.Add(sf);
            // 连接签名字段和签名属性:
            sp.SignatureField = sf;

            // 添加第二个签名字段:
            var sf2 = new SignatureField() { Name = "SecondSignature" };
            sf2.Widget.Rect = new RectangleF(72, 72 * 3, 72 * 4, 36);
            sf2.Widget.Page = page;
            sf2.Widget.BackColor = Color.LightYellow;
            // 将签名字段添加到文档中:
            doc.AcroForm.Fields.Add(sf2);

            var ms = new MemoryStream();
            doc.Sign(sp, ms);
            return ms;
        }
    }
}