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Posted 1 Jul 2006


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Fast Excel file reader with basic functionality

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4.88/5 (104 votes)
1 Jul 20063 min read
A set of managed classes for native reading of data from MS Excel files


When I was writing one of the projects (multi-format data parser) I had to deal with Microsoft Excel files.

First of all I tried to read data via COM interop, but this way was very slow (and sometimes input files have up to 100,000 rows and even more) and required Excel to be installed on the target machine.

Then I tried to read data via Excel's OleDb driver. When DataReader ran through data rows without reading them (i.e. while (rd.Read());) it was quite fast, but retrieving data was still very slow.

When I had tried out one of commercial products (TMS FlexCel Studio), it showed rather good results (several seconds for loading 17 MB file, 2 sheets, ~ 80 000 rows), but access to them was not very fast (at least not suitable for displaying data using DataGridView in virtual mode).

So, I decided to write my own reader for Excel files.


First of all I decided that the reader will produce a DataSet object, with sheets represented as DataTables within it. Access to DataTable is rather fast, and its TableName property is suitable for sheet title.

To read an Excel file, we need to get BIFF stream from xls OLE2 container. Working with OLE2 container is implemented in the following classes:

  • XlsHeader (file header representation)
  • XlsFat (support for file system in OLE2 container)
  • XlsRootDirectory (directory tree representation)
  • XlsDirectoryEntry, and
  • XlsStream (file stream)

Note that implementation of OLE2 container object model is limited (e.g. I didn't write any MiniFAT support code) and Excel-specific (XlsHeader supports only MS Excel header field values, any other will produce InvalidHeaderException).

Now we can parse BIFF stream with spreadsheet data. For handy reading data from stream we have a XlsBiffStream class derived from XlsStream. It provides a set of methods for reading XlsBiffRecord common structures. And then, each structure is converted to the appropriate type. Note that only a few record types are supported. All other structures are read as basic XlsBiffRecord and are ignored.

My implementation of reader ignores any text formatting, graphics, embedded objects and charts. Only cell values are read. When a cell contains a formula, the last calculated formula value is used as the cell value. Also, only indexed sheets are read (i.e. sheets with index data) - I didn't write not-indexed sheets support due to my laziness (I didn't come across any file without an index yet).

Using the code

Now, it's very easy to use the parser. All you need is a Stream with an Excel file.

FileStream fs = new FileStream(@"c:\file.xls", 
        FileMode.Open, FileAccess.Read);
ExcelDataReader rd = new ExcelDataReader(fs);

// Now we can access data:
DataSet data = rd.WorkbookData;

Points Of Interest

Latest versions of Excel use SST (Shared String Table) to store string information, and text cells only reference string by index. Also, the size of one BIFF section is limited, and SST can be rather big, so it uses the so-called CONTINUE sections. Every string in SST can be saved as Unicode (2 byte) or as ANSI (single byte per symbol). But when the string is broken with the continue section, it can change its encoding from one to another! This took me a lot of debugging, and I wish nobody has to do the same again...


This implementation of reader is rather nonoptimal (I was pressed for time), but it showed a good speed. It surely beats any interop or OLE method and even some commercial analogs.


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Written By
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Russian Federation Russian Federation
.NET Developer
Russian Federation

Comments and Discussions

GeneralRe: Requirements Pin
Messir11-Jul-06 3:13
MemberMessir11-Jul-06 3:13 
GeneralFound one bug [modified] Pin
Messir9-Jul-06 9:26
MemberMessir9-Jul-06 9:26 
GeneralGreat job Pin
egozi138-Jul-06 10:50
Memberegozi138-Jul-06 10:50 
GeneralNice piece of code but had to make some changes Pin
dsmile7-Jul-06 9:22
Memberdsmile7-Jul-06 9:22 
GeneralRe: Nice piece of code but had to make some changes Pin
pleury12-Nov-06 17:44
Memberpleury12-Nov-06 17:44 
GeneralRe: Nice piece of code but had to make some changes Pin
ecunha19-Sep-14 5:35
Memberecunha19-Sep-14 5:35 
GeneralThanks for the work Pin
Brian Leach5-Jul-06 8:36
MemberBrian Leach5-Jul-06 8:36 
GeneralXML or excel spread sheet? [modified] Pin
Keith Vinson5-Jul-06 5:35
MemberKeith Vinson5-Jul-06 5:35 
Sorry I haven't had a chance to read the code, But if speed is the issue, why not save the file as CSV and then use a CSV reader class to process the file. In my experience I have seen the CSV class be able to process 4-5 MB files in less than a second.


If you can control the format that the file is saved as, and if the format is fixed, and given that the data structure must be preknown in order for you to work with the data then check out using a simpler data format.

However, if the processing is somewhat ad hoc then I like the ability to use XLS files, its much easier that trying to get some user to save the file as CSV. Believe me I have suffered through that very issue (you would be amazed at how many different ways a CSV file can be saved from the same excel workbook!).


-- modified at 11:47 Wednesday 5th July, 2006
GeneralRe: XML or excel spread sheet? Pin
Messir5-Jul-06 5:56
MemberMessir5-Jul-06 5:56 
GeneralRe: XML or excel spread sheet? Pin
Keith Vinson5-Jul-06 6:09
MemberKeith Vinson5-Jul-06 6:09 
GeneralExcel Data Writer Pin
BradleyC4-Jul-06 4:44
MemberBradleyC4-Jul-06 4:44 
GeneralRe: Excel Data Writer Pin
Messir4-Jul-06 6:25
MemberMessir4-Jul-06 6:25 
GeneralHyperlink Pin
kuerbis2-Jul-06 10:41
Memberkuerbis2-Jul-06 10:41 
GeneralRe: Hyperlink Pin
Messir2-Jul-06 18:14
MemberMessir2-Jul-06 18:14 
GeneralNice Article Pin
Paul Conrad1-Jul-06 17:58
professionalPaul Conrad1-Jul-06 17:58 
GeneralRe: Nice Article Pin
Messir1-Jul-06 18:48
MemberMessir1-Jul-06 18:48 

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