Question

I’m exporting a grid with NPOI v1.2.3, and am having trouble getting cell formatting to work.

I have a class that exports a list of objects to an XLS file. A row is created for each object, and a cell is added for each configured property. The cell data format can be set on a per-property level.

I’ve read that you shouldn’t create a new style for each cell. I can’t hard-code my styles, since my exporter needs to support any class. Instead, I wrote a little cache system that only creates a new CellStyle if one hasn’t already been created for the current cell’s format.

Unfortunately, this still hasn’t solved the problem. The formatting is not correctly applied in the final XLS file. In my test case, most cells in the XLS are using the “Date” format, even though only a few columns are dates. The first column correctly uses a custom format, however. No cells are set to text, even though that should be most of them.

What am I doing wrong?

Code

The “AddRecords” method below is used to add the data rows (header and footer rows are added separately). The last bit of code is the method that lazy-loads CellStyles.

private void AddRecords( Sheet sheet, IList<T> records )
{
    foreach( var record in records )
    {
        // append row
        var row = sheet.CreateRow ( sheet.LastRowNum + 1 );

        // iterate through all configured columns
        foreach ( var column in GetColumns() )
        {
            // append cell
            Cell cell = row.CreateCell ( row.LastCellNum == -1 ? 0 : row.LastCellNum );

            // get the property value of the column from the record
            object value = GetCellValue ( column, record );

            // extension method that takes an object value and calls the appropriate type-specific SetCellValue overload
            cell.SetCellValue ( value );

            // get format from the column definition ("m/d", "##.###", etc.), or use the default
            string dataFormat = column.DataFormat ?? GetDefaultDataFormat ( value );

            // find/create cell style
            cell.CellStyle = GetCellStyleForFormat( sheet.Workbook, dataFormat );
        }
    }
}

/// <summary>
/// Returns a default format string based on the object type of value.
///
/// http://poi.apache.org/apidocs/org/apache/poi/ss/usermodel/BuiltinFormats.html
/// </summary>
/// <param name="value"></param>
/// <returns></returns>
private string GetDefaultDataFormat( object value )
{
    if( value == null )
    {
        return "General";
    }

    if( value is DateTime )
    {
        return "m/d";
    }

    if( value is bool )
    {
        return "[=0]\"Yes\";[=1]\"No\"";
    }

    if( value is byte || value is ushort || value is short ||
         value is uint || value is int || value is ulong || value is long )
    {
        return "0";
    }

    if( value is float || value is double )
    {
        return "0.00";
    }

    // strings and anything else should be text
    return "text";
}

private readonly Dictionary<string, CellStyle> _cellStyleCache = new Dictionary < string, CellStyle > ();

private CellStyle GetCellStyleForFormat( Workbook workbook, string dataFormat )
{
    if( !_cellStyleCache.ContainsKey ( dataFormat ) )
    {
        var newDataFormat = workbook.CreateDataFormat ();
        var style = workbook.CreateCellStyle ();
        style.DataFormat = newDataFormat.GetFormat ( dataFormat );

        _cellStyleCache[dataFormat] = style;
    }

    return _cellStyleCache[dataFormat];
}

Answer 1

It looks like the problem has something to do with creating new formats that match built-in formats. I changed my lazy-loading method to use the built-in format if available, and the cell formats in my final XLS are all correct now.

if( !_cellStyleCache.ContainsKey ( dataFormat ) )
{
    var style = workbook.CreateCellStyle ();

    // check if this is a built-in format
    var builtinFormatId = HSSFDataFormat.GetBuiltinFormat ( dataFormat );

    if( builtinFormatId != - 1)
    {
        style.DataFormat = builtinFormatId;
    }
    else
    {
        // not a built-in format, so create a new one
        var newDataFormat = workbook.CreateDataFormat ();
        style.DataFormat = newDataFormat.GetFormat ( dataFormat );
    }

    _cellStyleCache[dataFormat] = style;
}