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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

General[My vote of 1] Something happening somewhere Pin
Member 31085637-Sep-15 3:47
memberMember 31085637-Sep-15 3:47 
QuestionRe: [My vote of 1] Something happening somewhere Pin
Thomas Freudenberg8-Sep-15 23:44
memberThomas Freudenberg8-Sep-15 23:44 
AnswerRe: [My vote of 1] Something happening somewhere Pin
Ouroborus7775-Oct-15 6:27
memberOuroborus7775-Oct-15 6:27 
GeneralRe: [My vote of 1] Something happening somewhere Pin
Thomas Freudenberg5-Oct-15 10:03
memberThomas Freudenberg5-Oct-15 10:03 
QuestionWarning! Does not work for instances where the monitored key may not always exist. Pin
nmg19617-Aug-15 2:13
membernmg19617-Aug-15 2:13 
AnswerRe: Warning! Does not work for instances where the monitored key may not always exist. Pin
Thomas Freudenberg8-Sep-15 23:41
memberThomas Freudenberg8-Sep-15 23:41 
Questionmuch appriciated, thanks Pin
vdoser16-May-13 8:56
membervdoser16-May-13 8:56 
AnswerRe: much appriciated, thanks Pin
Thomas Freudenberg8-Sep-15 23:45
memberThomas Freudenberg8-Sep-15 23:45 
Questiontnx Pin
Rohit Dubey from Hyderabad15-Mar-12 5:56
memberRohit Dubey from Hyderabad15-Mar-12 5:56 
QuestionGet the process causing the registry key change? Pin
Jon777777718-May-10 8:13
memberJon777777718-May-10 8:13 

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