我正在尝试为Spread.NET创建自定义单元格类型。我得到的错误是：
无法调用抽象基成员：'FarPoint.Web.Spread.BaseCellType.PaintCell（string，System.Web.UI.WebControls.TableCell，FarPoint. Web.Spread.Appearance，FarPoint.Web.Spread.Inset，object，bool）'
代码如下：

[Serializable()]
public class BarcodeCellType : FarPoint.Web.Spread.BaseCellType
{
    public override Control PaintCell(string id, TableCell parent, Appearance style, Inset margin, object value, bool upperLevel)
    {
        parent.Attributes.Add("FpCellType", "BarcodeCellType");

        if (value != null)
        {
            try
            {
                MemoryStream ms = GenerateBarCode(value.ToString());
                var img = Bitmap.FromStream(ms);
                value = img;
            }
            catch (Exception ex)
            {
                value = ex.ToString();
            }
        }

        return base.PaintCell(id, parent, style, margin, value, upperLevel); //ERROR HERE
    }

    private MemoryStream GenerateBarCode(string codeInfo)
    {
        using (MemoryStream ms = new MemoryStream())
        {
            BarCodeBuilder bb = new BarCodeBuilder();
            bb.CodeText = codeInfo;
            bb.SymbologyType = Symbology.Code128;
            bb.BarCodeImage.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
            return ms;
        }
    }
}