iText7（.net）SignExternalSignatureContainer NullReferenceException

我正在尝试创建一个C#可以对PDF文档进行数字签名的程序。为了将签名元素包含到PDF中，我正在使用iText7。现在，如果我在没有调试器的情况下运行该程序，System.NullReferenceException将会抛出一个错误，并且程序将失败。但是如果我使用调试器运行程序，也会发生异常，并且代码继续并正确签名PDF ...

我不确定这是一个问题iText7还是在创建签名字段时犯了一个错误或忘记了一些重要的事情。

任何想法如何解决这个问题？

例外

System.NullReferenceException：未在对象实例上设置对象引用。适用于C中的iText.Signatures.PdfSignatureAppearance.GetAppearance（）：\ Development \ Others \ itext7 \ itext \ itext.sign \ itext \ signatures \ PdfSignatureAppearance.cs：第584行。 ）在C：\ Development \ Others中的C：\ Development \ Others \ itext7 \ itext \ itext.sign \ itext \ signatures \ PdfSigner.cs：第808行。 \ itext7 \ itext \ itext.sign \ itext \签名\ PdfSigner.cs：582行，位于C：\ Development \ Signature \ SignService.Engine \ Core \ PdfEngine中的SignService.Engine.Core.PdfEngine.AddSignature（SignTask任务）处。 cs：第122行。

我的密码

/// <summary>
/// Add SignatureField to Pdf
/// and digitally sign it 
/// </summary>
public SignatureResult AddSignature(SignTask task)
{
    _logger.Info("Start Signing PDF");

    var prop = task.SignatureProperties;

    try
    {
        var reader = new PdfReader(new MemoryStream(prop.Document));
        var stream = new ByteArrayOutputStream();
        var signer = new PdfSigner(reader, stream, new StampingProperties().UseAppendMode());

        // set appearance
        var appearance = signer.GetSignatureAppearance();
        appearance.SetReason(prop.SignReason)
                  .SetLocation(prop.SignLocation)
                  .SetContact(prop.SignContact);

        // set rendering mode
        switch (_settings.Pdf.RenderingMode)
        {
            case SignatureRenderingMode.Description:
                appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.DESCRIPTION);
                break;
            case SignatureRenderingMode.Graphic:
                appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.GRAPHIC);
                break;
            case SignatureRenderingMode.GraphicAndDescription:
                appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.GRAPHIC_AND_DESCRIPTION);
                break;
            case SignatureRenderingMode.NameAndDescription:
                appearance.SetRenderingMode(PdfSignatureAppearance.RenderingMode.NAME_AND_DESCRIPTION);
                break;
        }

        // set image
        if (!string.IsNullOrEmpty(_settings.Pdf.SignatureGraphicPath))
        {
            if (File.Exists(_settings.Pdf.SignatureGraphicPath))
            {
                var imageData = ImageDataFactory.Create(_settings.Pdf.SignatureGraphicPath);
                appearance.SetSignatureGraphic(imageData);
            }
            else if (_settings.Pdf.RenderingMode == SignatureRenderingMode.GraphicAndDescription || _settings.Pdf.RenderingMode == SignatureRenderingMode.GraphicAndDescription)
            {
                throw new Exception("Failed to create Signature Field.\nIf rendering mode is graphic or graphic and description, a signature image must be provided");
            }
        }

        // set visibility
        if (prop.Visible)
        {
            var rect = new Rectangle(prop.X, prop.Y, prop.Width, prop.Height);
            appearance.SetPageRect(rect);
            appearance.SetPageNumber(1); // todo create setting for this
        }

        signer.SetFieldName(_settings.Pdf.SignatureFieldName);

        // sign field
        var signatureContainer = new ExternalSignatureContainer(task, _settings.Ais);
        // **Exeption thrown here**
        signer.SignExternalContainer(signatureContainer, GetEstimatedSize(task.TimestampOnly));

        var tempResult = stream.ToArray();

        // set revocation info if active
        if (tempResult.Length > 0 && _settings.Ais.AddRevocationInfo)
        {
            tempResult = AddRevocationInfo(signatureContainer.Crl, signatureContainer.Ocsp, tempResult);
        }

        reader.Close();
        stream.Close();

        _logger.Info("Finished signing");

        // return sign result
        return tempResult.Length > 0 
          ? new SignatureResult
          {
               Message = "",
               Status = RequestStatus.Success,
               Document = stream.ToArray(),
               Id = prop.Id
          }
          : new SignatureResult
          {
               Message = "Failed to sign the Document",
               Status = RequestStatus.Failed,
               Document = null,
               Id = prop.Id
          };
    }
    catch (AisServiceException aisServiceException)
    {
        _logger.Error($"While requesting Signature an error occured: {aisServiceException.Message}", aisServiceException);
        throw;
    }
    catch (Exception exception)
    {
        _logger.Error($"While creating signed pdf an error occured: {exception.Message}", exception);
        throw new PdfException($"While creating Signed Pdf an error Occured: {exception.Message}");
    }
}

设定：

{
  "Pdf": {
    "Visible": true,
    "Position": {
      "X": 50,
      "Y": 50,
      "Height": 100,
      "Width": 200
    },
    "SignatureFieldName": "SignatureField",
    "SignatureGraphicPath": "",
    "RenderingMode": 1
  }
}

任务：

  "SignatureProperties": {
    "Id": "1",
    "Document": [DocumentAsByteArray],
    "Visible": true,
    "SignReason": "Test",
    "SignLocation": "Test",
    "SignContact": "Test",
    "Height": 100,
    "Width": 200,
    "X": 50,
    "Y": 50
  }