当所有列表项的总和时，字符总数超过 256

在这种情况下的方法将与前面的示例不同，因为 Excel 文件支持对于总字符数小于 256 的项目列表的数据验证，如果所有项目都直接绑定，如上例所示。但在许多情况下，列表项可能超过 256 个字符，在这种情况下，直接绑定将不起作用。

但是，如果从同一个 excel 文件的不同列引用，则 excel 文件支持超过 256 个字符的列表项。因此，一旦解决方法可以从数据库或设置文件中读取所有值并将其从当前视图隐藏到远程列之一并且数据验证可以通过公式读取此隐藏列以创建列表项。下面的代码将通过设置文件读取数据值来显示此方法。

// Read all list items from config file
string[] countryDV = ConfigurationManager.AppSettings["countryDV"].Split(',').Select(s => s.Trim().ToUpper()).ToArray();
// Get the column name where you want to hide the list items, assume distant  column is "ZZ"
string countryDVDataCellColumn = ConfigurationManager.AppSettings["countryDVDataCellColumn"].Trim().ToString(); 

  
int DVRowLimit = (Int16.MaxValue); 
// Creating and Assigning Settings for Data Validation
CreateDropDownUsingCellReference(workbook, countryDV, "CountryConstraint", countryDVDataCellColumn);
CellRangeAddressList countryDVAddList = new CellRangeAddressList(1, DVRowLimit, targetFirstCol, targetLastCol);
dvConstraint = (XSSFDataValidationConstraint)validationHelper.CreateFormulaListConstraint("=CountryConstraint");
dvConstraint.Validate();
dataValidation = (XSSFDataValidation)validationHelper.CreateValidation(dvConstraint, countryDVAddList);
dataValidation.ShowErrorBox = true;
dataValidation.SuppressDropDownArrow = true;
dataValidation.ErrorStyle = 0;
dataValidation.CreateErrorBox("InvalidValue", "Select Valid country.");
dataValidation.ShowErrorBox = true;
dataValidation.CreatePromptBox("country Data Validation", "Enter country.");
dataValidation.ShowPromptBox = true;
sheet.AddValidationData(dataValidation);

private void CreateDropDownUsingCellReference(XSSFWorkbook wb, string[] csvListOfValues, string listName, string headerName)
{
    int columnIndex = CellReference.ConvertColStringToIndex(headerName);
    try
    {
    XSSFName namedCell = (XSSFName)wb.CreateName();
    namedCell.NameName = listName;
    //Creating Cell and Assigning Values from CSVListOfValues;
    for (int i = 0; i < csvListOfValues.Length; i++)
    {
            var namedRow = wb.GetSheetAt(0).CreateRow(i + 1);
            namedRow.CreateCell(columnIndex).SetCellValue(csvListOfValues[i]);
    }

    //Assigning the Reference for sheet 0 With Cell Range, where list items iscopied 
    String reference = wb.GetSheetAt(0).SheetName + "!$" + headerName + "$2:$" + headerName + "$" + (csvListOfValues.Length + 1).ToString();
    namedCell.RefersToFormula = reference;

    //Hiding the Column  now;
    wb.GetSheetAt(0).SetColumnHidden(columnIndex, true);
    }
    catch (Exception Ex)
    { }
}