ExcelToHtmlConverter.ProcessWorkbook throws Illegal IndexedColor index: 0 error on .NET Core 3.1

[Evanyi0016]

以下代码在.net framework中可顺利执行，但在core 3.1下2.5.4版本报错：Illegal IndexedColor index: 0；

` ///

/// （NPOI）Excel转HTML
///

/// HTML文件的的名称（不带后缀）
/// 需要转换Excel的绝对路径
///
public string ExcelToHtml(string Filename)
{
IWorkbook workbook;
//获取后缀名称
string fileExt = Path.GetExtension(Filename).ToLower();
//判断是否可以打开该文件，如果出错则文件已经打开或者有异常
try
{
System.IO.FileStream stream = System.IO.File.OpenWrite(Filename);
stream.Close();
}
catch
{
return null;
}
using (FileStream fs = new FileStream(Filename, FileMode.Open, FileAccess.Read))
{
//如果是XLSX格式选择XSSFWorkbook ，如果是XLS格式选择HSSFWorkbook
if (fileExt == ".xlsx") { workbook = new XSSFWorkbook(fs); } else if (fileExt == ".xls") { workbook = new HSSFWorkbook(fs); } else { workbook = null; }
if (workbook == null) { return null; }
}

        ExcelToHtmlConverter eth = new ExcelToHtmlConverter();

        // 设置输出参数
        eth.OutputColumnHeaders = true; //是否输出ColumnHeader
        eth.OutputHiddenColumns = false;//是否在表头插入一行序列号
        eth.OutputHiddenRows = false;//是否输出HiddenRow
        eth.OutputLeadingSpacesAsNonBreaking = true; //不清楚
        eth.OutputRowNumbers = false;//是否在列头插入一行序列号
        eth.UseDivsToSpan = false; //不清楚


        // 加载Excel文件
        eth.ProcessWorkbook(workbook); //***此处报错：Illegal IndexedColor index: 0****

        // 保存文件值当前路径
        return eth.Document.InnerXml;

      //  string htmlFile = Application.StartupPath + "\\" + HtmlName + ".html";
      //  eth.Document.Save(htmlFile);
      //   return htmlFile;

    }`

以下是Excel样例。
simple.xlsx

[tonyqus]

Can you provide the Excel file?

[Evanyi0016]

Can you provide the Excel file?

The sample has been uploaded.

[tonyqus]

The exception thrown by IndexedColor.ValueOf() method. In some cases, the color value=0. But there is no pre-defined color whose index equals 0.

[isaacfi]

Hi,

Is there any way to avoid this exception?

[PhenX]

Here is a simple workaround :

(typeof(IndexedColors).GetField("mappingIndex", BindingFlags.Static | BindingFlags.NonPublic)
   .GetValue(null) as Dictionary<int, IndexedColors>)[0] = IndexedColors.Black;

But a real fix is still very welcome

[quiian]

Thanks for the "fix", @PhenX. In my case I needed white instead of black, so it seems the color is not fixed, but you lead me on the right track. :)