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SemaphoreSecurity Class

Note: This class is new in the .NET Framework version 2.0.

Represents the Windows access control security for a named semaphore. This class cannot be inherited.

Namespace: System.Security.AccessControl
Assembly: System (in system.dll)

public sealed class SemaphoreSecurity : NativeObjectSecurity
/** @attribute ComVisibleAttribute(false) */ 
public final class SemaphoreSecurity extends NativeObjectSecurity
public final class SemaphoreSecurity extends NativeObjectSecurity

A SemaphoreSecurity object specifies access rights for a named system semaphore, and also specifies how access attempts are audited. Access rights to the semaphore are expressed as rules, with each access rule represented by a SemaphoreAccessRule object. Each auditing rule is represented by a SemaphoreAuditRule object.

This mirrors the underlying Windows security system, in which each securable object has at most one discretionary access control list (DACL) that controls access to the secured object, and at most one system access control list (SACL) that specifies which access attempts are audited. The DACL and SACL are ordered lists of access control entries (ACE) that specify access and auditing for users and groups. A SemaphoreAccessRule or SemaphoreAuditRule object might represent more than one ACE.


A Semaphore object can represent a local semaphore or a named system semaphore. Windows access control security is meaningful only for named system semaphores.

The SemaphoreSecurity, SemaphoreAccessRule, and SemaphoreAuditRule classes hide the implementation details of ACLs and ACEs. They allow you to ignore the seventeen different ACE types and the complexity of correctly maintaining inheritance and propagation of access rights. These objects are also designed to prevent the following common access control errors:

  • Creating a security descriptor with a null DACL. A null reference to a DACL allows any user to add access rules to an object, potentially creating a denial-of-service attack. A new SemaphoreSecurity object always starts with an empty DACL, which denies all access for all users.

  • Violating the canonical ordering of ACEs. If the ACE list in the DACL is not kept in the canonical order, users might inadvertently be given access to the secured object. For example, denied access rights must always appear before allowed access rights. SemaphoreSecurity objects maintain the correct order internally.

  • Manipulating security descriptor flags, which should be under resource manager control only.

  • Creating invalid combinations of ACE flags.

  • Manipulating inherited ACEs. Inheritance and propagation are handled by the resource manager, in response to changes you make to access and audit rules.

  • Inserting meaningless ACEs into ACLs.

The only capabilities not supported by the .NET security objects are dangerous activities that should be avoided by the majority of application developers, such as the following:

  • Low-level tasks that are normally performed by the resource manager.

  • Adding or removing access control entries in ways that do not maintain the canonical ordering.

To modify Windows access control security for a named semaphore, use the Semaphore.GetAccessControl method to get the SemaphoreSecurity object. Modify the security object by adding and removing rules, and then use the Semaphore.SetAccessControl method to reattach it.


Changes you make to a SemaphoreSecurity object do not affect the access levels of the named semaphore until you call the Semaphore.SetAccessControl method to assign the altered security object to the named semaphore.

To copy access control security from one semaphore to another, use the Semaphore.GetAccessControl method to get a SemaphoreSecurity object representing the access and audit rules for the first semaphore, then use the Semaphore.SetAccessControl method, or a constructor that accepts a SemaphoreSecurity object, to assign those rules to the second semaphore.

Users with an investment in the security descriptor definition language (SDDL) can use the SetSecurityDescriptorSddlForm method to set access rules for a named semaphore, and the GetSecurityDescriptorSddlForm method to obtain a string that represents the access rules in SDDL format. This is not recommended for new development.

The following code example demonstrates the separation between Allow rules and Deny rules, and shows the combination of rights in compatible rules. The example creates a SemaphoreSecurity object, adds rules that allow and deny various rights for the current user, and displays the resulting pair of rules. The example then allows new rights for the current user and displays the result, showing that the new rights are merged with the existing Allow rule.


This example does not attach the security object to a Semaphore object. Examples that attach security objects can be found in Semaphore.GetAccessControl and Semaphore.SetAccessControl.

using System;
using System.Threading;
using System.Security.AccessControl;
using System.Security.Principal;

public class Example
    public static void Main()
        // Create a string representing the current user.
        string user = Environment.UserDomainName + "\\" + 

        // Create a security object that grants no access.
        SemaphoreSecurity mSec = new SemaphoreSecurity();

        // Add a rule that grants the current user the 
        // right to enter or release the semaphore.
        SemaphoreAccessRule rule = new SemaphoreAccessRule(user, 
            SemaphoreRights.Synchronize | SemaphoreRights.Modify, 

        // Add a rule that denies the current user the 
        // right to change permissions on the semaphore.
        rule = new SemaphoreAccessRule(user, 

        // Display the rules in the security object.

        // Add a rule that allows the current user the 
        // right to read permissions on the semaphore. This rule
        // is merged with the existing Allow rule.
        rule = new SemaphoreAccessRule(user, 


    private static void ShowSecurity(SemaphoreSecurity security)
        Console.WriteLine("\r\nCurrent access rules:\r\n");

        foreach(SemaphoreAccessRule ar in 
            security.GetAccessRules(true, true, typeof(NTAccount)))
            Console.WriteLine("        User: {0}", ar.IdentityReference);
            Console.WriteLine("        Type: {0}", ar.AccessControlType);
            Console.WriteLine("      Rights: {0}", ar.SemaphoreRights);

/*This code example produces output similar to following:

Current access rules:

        User: TestDomain\TestUser
        Type: Deny
      Rights: ChangePermissions

        User: TestDomain\TestUser
        Type: Allow
      Rights: Modify, Synchronize

Current access rules:

        User: TestDomain\TestUser
        Type: Deny
      Rights: ChangePermissions

        User: TestDomain\TestUser
        Type: Allow
      Rights: Modify, ReadPermissions, Synchronize

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 98, Windows 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.

.NET Framework

Supported in: 2.0

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