C# – Why does the cell style index returning wrong CellFormat value when parsing excel cells using OpenXml SDK

The s attribute references a style xf entry in styles.xml. The style xf in turn references a number format mask. To identify a cell that contains a date, you need to perform the style xf -> numberformat lookup, then identify whether that numberformat mask is a date/time numberformat mask (rather than, for example, a percentage or an accounting numberformat mask).

The style.xml file has elements like:

<xf numFmtId="14" ... applyNumberFormat="1" />
<xf numFmtId="1" ... applyNumberFormat="1" />

These are the xf entries, which in turn give you a numFmtId that references the number format mask.

You should find the numFmts section somewhere near the top of style.xml, as part of the styleSheet element

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?> 
    <styleSheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
        <numFmts count="3">
            <numFmt numFmtId="164" formatCode="[$-414]mmmm\ yyyy;@" /> 
            <numFmt numFmtId="165" formatCode="0.000" /> 
            <numFmt numFmtId="166" formatCode="#,##0.000" /> 
        </numFmts>

The number format id may be here, or it may be one of the built-in formats. Number format codes (numFmtId) less than 164 are "built-in".

The list that I have is incomplete:

0 = 'General';
1 = '0';
2 = '0.00';
3 = '#,##0';
4 = '#,##0.00';

9 = '0%';
10 = '0.00%';
11 = '0.00E+00';
12 = '# ?/?';
13 = '# ??/??';
14 = 'mm-dd-yy';
15 = 'd-mmm-yy';
16 = 'd-mmm';
17 = 'mmm-yy';
18 = 'h:mm AM/PM';
19 = 'h:mm:ss AM/PM';
20 = 'h:mm';
21 = 'h:mm:ss';
22 = 'm/d/yy h:mm';

37 = '#,##0 ;(#,##0)';
38 = '#,##0 ;[Red](#,##0)';
39 = '#,##0.00;(#,##0.00)';
40 = '#,##0.00;[Red](#,##0.00)';

44 = '_("$"* #,##0.00_);_("$"* \(#,##0.00\);_("$"* "-"??_);_(@_)';
45 = 'mm:ss';
46 = '[h]:mm:ss';
47 = 'mmss.0';
48 = '##0.0E+0';
49 = '@';

27 = '[$-404]e/m/d';
30 = 'm/d/yy';
36 = '[$-404]e/m/d';
50 = '[$-404]e/m/d';
57 = '[$-404]e/m/d';

59 = 't0';
60 = 't0.00';
61 = 't#,##0';
62 = 't#,##0.00';
67 = 't0%';
68 = 't0.00%';
69 = 't# ?/?';
70 = 't# ??/??';

The missing values are mainly related to east asian variant formats.

All strings in an Excel worksheet are stored in a array like structure called the SharedStringTable. The goal of this table is to centralize all strings in an index based array and then if that string is used multiple times in the document to just reference the index in this array. That being said, the 0 you received when you got the text value of the A1 cell is the index into the SharedStringTable. To get the real value you can use this helper function:

public static SharedStringItem GetSharedStringItemById(WorkbookPart workbookPart, int id)
{
    return workbookPart.SharedStringTablePart.SharedStringTable.Elements<SharedStringItem>().ElementAt(id);
}

Then in your code call it like this to get the real value:

Cell cell = GetCell(worksheet, "A", 1);

string cellValue = string.Empty;

if (cell.DataType != null)
{
    if (cell.DataType == CellValues.SharedString)
    {
       int id = -1;

       if (Int32.TryParse(cell.InnerText, out id))
       {
           SharedStringItem item = GetSharedStringItemById(workbookPart, id);

           if (item.Text != null)
           {
               cellValue = item.Text.Text;
           }
           else if (item.InnerText != null)
           {
               cellValue = item.InnerText;
           }
           else if (item.InnerXml != null)
           {
               cellValue = item.InnerXml;
           }
       }
    }
}