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Posted 31 Jul 2002

Accessing alternative data-streams of files on an NTFS volume

, 15 Aug 2016
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A pair of classes to encapsulate access to NTFS alternative data streams.


Since NT 3.1, the NTFS file system has supported multiple data-streams for files. There has never been built-in support for viewing or manipulating these additional streams, but the Windows API functions include support for them with a special file syntax: Filename.ext:StreamName. Even Win9x machines can access the alternative data streams of files on any NTFS volume they have access to, e.g., through a mapped drive. Because the Scripting.FileSystemObject and many other libraries call the CreateFile API behind the scenes, even scripts have been able to access alternative streams quite easily (although enumerating the existing streams has always been tricky).

In .NET, however, it seems someone decided to add some checking to the format of filenames. If you attempt to open a FileStream on an alternative stream, you will get a "Path Format not supported" exception. I have been unable to find any class in the CLR that provides support for alternative data streams, so I decided to roll my own.


I originally wrote this code in 2002, targeting v1 of the .NET Framework. Looking at the code now, it seems quite messy, and has several bugs and problems which were mentioned in the comments. I have since completely re-written the code for .NET v3.5, and (hopefully) fixed the bugs.

The new code is not compatible with the original version. However, I have included a sample compatibility wrapper which maps the old API to the new API. You can find these files under the "other/Compatibility wrapper" folder in the download.

Bugs / Issues Fixed

  • The code now uses the FileSystemInfo class rather than the FileInfo class. This allows you to access alternate data streams attached to folders as well as files. (Suggested by Dan Elebash.)
  • The code now uses SafeFileHandle instead of IntPtr for the file handle. (Suggested by Moomansun.)
  • The code is now 64-bit compatible. (Reported by John SMith 5634552745.)
  • The code now correctly calls BackupRead with the bAbort parameter set to true after enumerating the streams. (Reported by scooter_jsm.)
  • The number of global memory allocations required to read the streams from a file has been reduced. (Suggested by scooter_jsm.)
  • v2.1: ValidateStreamName now accepts stream names which contain characters with ASCII codes between 1 and 31. (Reported by Andy Missico.)
  • v2.1: The code will now attempt to map standard IO errors to the equivalent .NET Exception type. (Suggested by Andy Missico.)

Using the Classes

The AlternateDataStreamInfo class represents the details of an individual stream, and provides methods to create, open, or delete the stream.

The static FileSystem class provides methods to retrieve the list of streams for a file, retrieve a specific stream from a file, determine whether a stream exists, and delete a specific stream.

All methods on the FileSystem class offer overloads which accept either a path or a FileSystemInfo object. The overloads which accept a FileSystemInfo object can also be invoked as extension methods.


using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using Trinet.Core.IO.Ntfs;


FileInfo file = new FileInfo(path);

// List the additional streams for a file:
foreach (AlternateDataStreamInfo s in file.ListAlternateDataStreams())
    Console.WriteLine("{0} - {1} bytes", s.Name, s.Size);

// Read the "Zone.Identifier" stream, if it exists:
if (file.AlternateDataStreamExists("Zone.Identifier"))
    Console.WriteLine("Found zone identifier stream:");
    AlternateDataStreamInfo s = 
    using (TextReader reader = s.OpenText())
    // Delete the stream:
    Console.WriteLine("No zone identifier stream found.");

// Alternative method to delete the stream:

Files Included

  • The Trinet.Core.IO.Ntfs folder contains the source code.
  • The doc folder contains the documentation and FxCop project.
  • The bin folder contains the compiled assembly.
  • The other folder contains the compatibility wrapper, and a sample to recursively delete the "Zone.Identifier" stream from all files in a given path.


If you want more information on NTFS programming, or the C++ code I based this on, see Dino Esposito's MSDN article from March 2000: [^].


  • v1 - 1 August 2002 - Initial release, targeting .NET 1.0.
  • v2 - 16 September 2008 - Re-written to target .NET 3.5; major changes include:
    • Replaced FileInfo with FileSystemInfo.
    • Replaced IntPtr with SafeFileHandle.
    • Made the P/Invoke calls 64-bit compatible.
    • Made the code abort the BackupRead operation after enumerating the streams.
    • Reduced the number of global memory allocations required.
  • v2.1 - 28 July 2010 - Minor fixes suggested by Andy Missico:
    • Changed ValidateStreamName to accept stream names containing characters with ASCII codes between 1 and 31 (see
    • Added mapping of standard IO errors to the correct .NET exceptions.
  • v2.2 - 15 August 2016 - Fix for bug reported by Member 12683101, where zero-length alternate streams prevented subsequent streams from being enumerated.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Richard Deeming
Software Developer Nevalee Business Solutions
United Kingdom United Kingdom
No Biography provided

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Comments and Discussions

GeneralProblem accessing alternate data stream of a network shared drive Pin
m.vinod859-Nov-09 11:40
memberm.vinod859-Nov-09 11:40 
GeneralRe: Problem accessing alternate data stream of a network shared drive Pin
Richard Deeming10-Nov-09 3:02
memberRichard Deeming10-Nov-09 3:02 
GeneralRe: Problem accessing alternate data stream of a network shared drive Pin
m.vinod8510-Nov-09 6:44
memberm.vinod8510-Nov-09 6:44 
GeneralRe: Problem accessing alternate data stream of a network shared drive Pin
Richard Deeming10-Nov-09 7:54
memberRichard Deeming10-Nov-09 7:54 
Questionhow to compile these sourecs Pin
hefeilixin29-Sep-09 18:36
memberhefeilixin29-Sep-09 18:36 
AnswerRe: how to compile these sourecs Pin
Richard Deeming30-Sep-09 2:31
memberRichard Deeming30-Sep-09 2:31 
Newsanother question Pin
hefeilixin9-Oct-09 22:44
memberhefeilixin9-Oct-09 22:44 
GeneralRe: another question Pin
Richard Deeming12-Oct-09 2:08
memberRichard Deeming12-Oct-09 2:08 
The CHM file is probably blocked because you downloaded it from the Internet. To unblock it, right-click on the file and choose "Properties". At the bottom of the General tab, you will see "Security: This file came from another computer...", with an "Unblock" button next to it. Click the unblock button, and you should be able to open the file.

"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer

GeneralRe: another question Pin
John Simmons / outlaw programmer11-Feb-11 2:51
mvpJohn Simmons / outlaw programmer11-Feb-11 2:51 
GeneralFileInfo question Pin
Brendan Chong16-Sep-09 8:33
memberBrendan Chong16-Sep-09 8:33 
GeneralRe: FileInfo question Pin
Richard Deeming16-Sep-09 8:49
memberRichard Deeming16-Sep-09 8:49 
GeneralEdit File summary Pin
=>Joe<=30-Jun-09 0:55
member=>Joe<=30-Jun-09 0:55 
GeneralRe: Edit File summary Pin
Richard Deeming30-Jun-09 2:42
memberRichard Deeming30-Jun-09 2:42 
GeneralRe: Edit File summary Pin
=>Joe<=1-Jul-09 2:33
member=>Joe<=1-Jul-09 2:33 
GeneralPlz simple sample code [modified] Pin
moris200069-Jun-09 19:55
membermoris200069-Jun-09 19:55 
GeneralRe: Plz simple sample code Pin
Richard Deeming10-Jun-09 2:20
memberRichard Deeming10-Jun-09 2:20 
GeneralRe: Plz simple sample code Pin
moris2000610-Jun-09 4:28
membermoris2000610-Jun-09 4:28 
GeneralBUG: Does not work in Vista SP1 Pin
remit.ukraine@gmail.com28-Apr-09 21:48
memberremit.ukraine@gmail.com28-Apr-09 21:48 
GeneralRe: BUG: Does not work in Vista SP1 Pin
Richard Deeming29-Apr-09 2:21
memberRichard Deeming29-Apr-09 2:21 
GeneralRe: BUG: Does not work in Vista SP1 Pin
ini18329-Apr-09 3:19
memberini18329-Apr-09 3:19 
GeneralVery nice Pin
GSerjo25-Sep-08 13:51
memberGSerjo25-Sep-08 13:51 
GeneralBug: BackupRead() documentation points to bug in this code. Pin
scooter_jsm16-Sep-08 8:40
memberscooter_jsm16-Sep-08 8:40 
GeneralRe: Bug: BackupRead() documentation points to bug in this code. Pin
Richard Deeming19-Sep-08 9:33
memberRichard Deeming19-Sep-08 9:33 
GeneralNot for 64-bit Pin
John SMith 563455274511-Apr-07 18:56
memberJohn SMith 563455274511-Apr-07 18:56 
NewsRe: Not for 64-bit Pin
unRheal21-May-09 10:19
memberunRheal21-May-09 10:19 

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