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Posted 7 Jul 2003

RegistryMonitor - a .NET wrapper class for RegNotifyChangeKeyValue

, 16 Jan 2006 CPOL
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The Windows API provides a function RegNotifyChangeKeyValue, which is not covered by the Microsoft.Win32.RegistryKey class. This solution imports that function and encapsulates it in a convenient manner.

Sample Image - RegistryMonitor.png


The Windows API provides a function RegNotifyChangeKeyValue, which notifies the caller about changes to attributes or the content of a specified registry key. Unfortunately, this function is not provided by the Microsoft.Win32.RegistryKey class. Because I needed that functionality, I've written a simple wrapper class.



RegistryMonitor has three constructors, the parameter lists should be self-explanatory:

RegistryMonitor(RegistryKey registryKey)
RegistryMonitor(string name)
RegistryMonitor(RegistryHive registryHive, string subKey)


RegistryMonitor supports two events:

public event EventHandler RegChanged;
public event ErrorEventHandler Error;

The RegChanged event is raised when the registry key specified during construction has changed. Error is raised when an exception occurs.

The latter event is necessary because the monitoring is hosted in a different thread.


RegistryMonitor has only one property:

public RegChangeNotifyFilter RegChangeNotifyFilter { get; set; }

RegChangeNotifyFilter is an enum. Since I don't want to repeat its implementation here in the article, I just want to say that it controls which kinds of registry changes will be detected, e.g. only key or value changes.


RegistryMonitor has two public methods which are declared as follows:

public void Start();
public void Stop();

I don't think that these methods require much explanation. The former creates a separate thread, which will monitor the registry, and the latter will stop that thread.


Because a simple example will say more than a thousand words, here's a console sample monitoring HKCU\Environment (that's where the current user's environment variables are stored):

public class MonitorSample
    static void Main() 
        RegistryMonitor monitor = new 
          RegistryMonitor(RegistryHive.CurrentUser, "Environment");
        monitor.RegChanged += new EventHandler(OnRegChanged);
    private void OnRegChanged(object sender, EventArgs e)
        Console.WriteLine("registry key has changed");

Provided with this article is another demo, which is a WinForms application (however, a VS.NET 2003 solution).

Points of interest

The first version of RegistryMonitor used reflection to retrieve the private hkey field of the Microsoft.Win32.RegistryKey class. However, Microsoft changed the internal implementation, so this hack didn't work anymore (see the comments below). Therefore I changed my implementation, so that RegistryMonitor now uses P/Invoke to open and close the registry key explicitly.


  • 08-Jul-2003 - Initial release.
  • 15-Jan-2005 - Updated to work for both .NET 1.1 and 2.0.


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


About the Author

Thomas Freudenberg
Technical Lead Cubeware GmbH
Germany Germany
No Biography provided

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Comments and Discussions

AnswerRe: Win32Exception: The system cannot find the file specified Pin
Member 432876816-Dec-14 19:01
memberMember 432876816-Dec-14 19:01 
GeneralRe: Win32Exception: The system cannot find the file specified Pin
Opata Chibueze3-Sep-15 21:25
memberOpata Chibueze3-Sep-15 21:25 
GeneralThanks so much for this! Pin
Teufel121216-Mar-09 14:02
memberTeufel121216-Mar-09 14:02 
QuestionHOW TO: get changed key and value Pin
Jared James Sullivan13-Feb-09 21:17
memberJared James Sullivan13-Feb-09 21:17 
GeneralOnRegChanged - Object Ref not set to instance Pin
Bergeir1-Jul-08 8:24
memberBergeir1-Jul-08 8:24 
QuestionMissing files? Pin
tatowaki12-Feb-08 9:54
membertatowaki12-Feb-08 9:54 
AnswerRe: Missing files? Pin
Thomas Freudenberg12-Feb-08 10:54
memberThomas Freudenberg12-Feb-08 10:54 
Generalthread issues [modified] Pin
TheShaver20-Aug-07 1:10
memberTheShaver20-Aug-07 1:10 
First off - nice article. Very helpful Smile | :)
It looks like there's a thread problem here, though. If I call Start(), then immediately modify the registry programmatically, then the ThreadLoop() call may not have reached RegNotifyChangeKeyValue(), so the change will go unnoticed.
This can be partially fixed by at least waiting in Start() until that call is reached (although that's not a perfect solution).
Also, if a registry change is picked up, then it's possible that another change could occur between OnRegChanged() and the next RegNotifyChangeKeyValue() call, which would also fall through the cracks.
Finally, if the terminate event is signalled, then OnRegChanged() will get called - the code doesn't differentiate between termination or notification.

Is there no alternative method of monitoring the registry, that could be done with, say, callbacks, rather than signalling?

Edit: Ha. My bad - I just checked: RegNotifyChangeKeyValue() will pick up on changes between calls, so my second point is nonsense.

-- modified at 6:52 Monday 20th August, 2007

Questionused in .net 2005? Pin
Paul Pang20-Jul-07 7:59
memberPaul Pang20-Jul-07 7:59 
AnswerRe: used in .net 2005? Pin
eddewind9-Oct-07 19:15
membereddewind9-Oct-07 19:15 
Generalmonitor more precisely Pin
Alexandre GRANVAUD14-May-07 22:54
memberAlexandre GRANVAUD14-May-07 22:54 
AnswerRe: monitor more precisely Pin
Thomas Freudenberg15-May-07 5:55
memberThomas Freudenberg15-May-07 5:55 
GeneralRe: monitor more precisely Pin
jgehman13-Jun-07 4:57
memberjgehman13-Jun-07 4:57 
AnswerRe: monitor more precisely Pin
Thomas Freudenberg13-Jun-07 5:04
memberThomas Freudenberg13-Jun-07 5:04 
GeneralRe: monitor more precisely Pin
N_Nguyen26-Nov-08 11:58
memberN_Nguyen26-Nov-08 11:58 
GeneralAny way of knowing which process initiated the change Pin
drinkwater11-Apr-07 18:44
memberdrinkwater11-Apr-07 18:44 
GeneralRe: Any way of knowing which process initiated the change Pin
Thomas Freudenberg15-Apr-07 22:43
memberThomas Freudenberg15-Apr-07 22:43 
AnswerRe: Any way of knowing which process initiated the change Pin
Vivek_India7-Aug-09 0:54
memberVivek_India7-Aug-09 0:54 
GeneralRe: Any way of knowing which process initiated the change [modified] Pin
Thomas Freudenberg7-Aug-09 1:15
memberThomas Freudenberg7-Aug-09 1:15 
AnswerRe: Any way of knowing which process initiated the change Pin
Vivek_India9-Aug-09 23:39
memberVivek_India9-Aug-09 23:39 
Question~(*|*)~How can I monitor Registry for a special process? Pin
behnam_iut14-Dec-06 11:28
memberbehnam_iut14-Dec-06 11:28 
AnswerRe: ~(*|*)~How can I monitor Registry for a special process? Pin
Thomas Freudenberg14-Dec-06 11:31
memberThomas Freudenberg14-Dec-06 11:31 
GeneralProposal for Enhancements Pin
LJEstrada25-Sep-06 6:19
memberLJEstrada25-Sep-06 6:19 
GeneralRe: Proposal for Enhancements Pin
Thomas Freudenberg27-Sep-06 12:40
memberThomas Freudenberg27-Sep-06 12:40 
GeneralNot able to run multiple instances Pin
sanzay mishra22-Sep-06 8:24
membersanzay mishra22-Sep-06 8:24 

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