EventWaitHandle.OpenExisting Method

Reference

Definition

Namespace:: System.Threading

Assemblies:: mscorlib.dll, System.Threading.dll

Assemblies:: netstandard.dll, System.Threading.dll

Assembly:: System.Threading.dll

Assembly:: mscorlib.dll

Assembly:: netstandard.dll

Important

Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.

Opens a specified named synchronization event, if it already exists.

Overloads

OpenExisting(String)	Opens the specified named synchronization event, if it already exists.
OpenExisting(String, EventWaitHandleRights)	Opens the specified named synchronization event, if it already exists, with the desired security access.

OpenExisting(String)

Source:: EventWaitHandle.cs

Source:: EventWaitHandle.cs

Source:: EventWaitHandle.cs

Opens the specified named synchronization event, if it already exists.

public:
 static System::Threading::EventWaitHandle ^ OpenExisting(System::String ^ name);

[System.Security.SecurityCritical]
public static System.Threading.EventWaitHandle OpenExisting(string name);

[System.Runtime.Versioning.SupportedOSPlatform("windows")]
public static System.Threading.EventWaitHandle OpenExisting(string name);

public static System.Threading.EventWaitHandle OpenExisting(string name);

[<System.Security.SecurityCritical>]
static member OpenExisting : string -> System.Threading.EventWaitHandle

[<System.Runtime.Versioning.SupportedOSPlatform("windows")>]
static member OpenExisting : string -> System.Threading.EventWaitHandle

static member OpenExisting : string -> System.Threading.EventWaitHandle

Public Shared Function OpenExisting (name As String) As EventWaitHandle

Parameters

name: String

The name of the synchronization object to be opened and shared with other processes. The name is case-sensitive. The backslash character (\) is reserved and may only be used to specify a namespace. For more information on namespaces, see the remarks section. There may be further restrictions on the name depending on the operating system. For example, on Unix-based operating systems, the name after excluding the namespace must be a valid file name.

Returns

EventWaitHandle

An object that represents the named system event.

Attributes: SecurityCriticalAttribute SupportedOSPlatformAttribute

Exceptions

WaitHandleCannotBeOpenedException

A synchronization object with the provided name cannot be opened. It may not exist, or a synchronization object of a different type might have the same name. In some cases, this exception may be thrown for invalid names.

ArgumentException

name is an empty string.

-or-

.NET Framework only: name is longer than MAX_PATH (260 characters).

ArgumentNullException

name is null.

IOException

name is invalid. This can be for various reasons, including some restrictions that may be placed by the operating system, such as an unknown prefix or invalid characters. Note that the name and common prefixes "Global\" and "Local\" are case-sensitive.

-or-

There was some other error. The HResult property may provide more information.

DirectoryNotFoundException

Windows only: name specified an unknown namespace. See Object Names for more information.

PathTooLongException

The name is too long. Length restrictions may depend on the operating system or configuration.

UnauthorizedAccessException

The named event exists, but the user does not have the security access required to use it.

Remarks

The name may be prefixed with Global\ or Local\ to specify a namespace. When the Global namespace is specified, the synchronization object may be shared with any processes on the system. When the Local namespace is specified, which is also the default when no namespace is specified, the synchronization object may be shared with processes in the same session. On Windows, a session is a login session, and services typically run in a different non-interactive session. On Unix-like operating systems, each shell has its own session. Session-local synchronization objects may be appropriate for synchronizing between processes with a parent/child relationship where they all run in the same session. For more information about synchronization object names on Windows, see Object Names.

If a synchronization object of the requested type exists in the namespace, the existing synchronization object is opened. If a synchronization object does not exist in the namespace, or a synchronization object of a different type exists in the namespace, a WaitHandleCannotBeOpenedException is thrown.

The OpenExisting method tries to open the specified named system event. To create the system event when it does not already exist, use one of the EventWaitHandle constructors that has a name parameter.

Multiple calls to this method that use the same value for name do not necessarily return the same EventWaitHandle object, even though the objects that are returned represent the same named system event.

In .NET Framework, this method overload is equivalent to calling the System.Threading.EventWaitHandle.OpenExisting(System.String,System.Security.AccessControl.EventWaitHandleRights) method overload and specifying EventWaitHandleRights.Synchronize and EventWaitHandleRights.Modify rights, combined by using the bitwise OR operation.

Specifying the EventWaitHandleRights.Synchronize flag allows a thread to wait on the named system event, and specifying the EventWaitHandleRights.Modify flag allows a thread to call the Set and Reset methods.

Applies to

.NET 10 and other versions

Product	Versions
.NET	Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9, 10
.NET Framework	2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard	1.0, 1.1, 1.2, 1.3, 1.4, 1.6, 2.0, 2.1
UWP	10.0

OpenExisting(String, EventWaitHandleRights)

Opens the specified named synchronization event, if it already exists, with the desired security access.

public:
 static System::Threading::EventWaitHandle ^ OpenExisting(System::String ^ name, System::Security::AccessControl::EventWaitHandleRights rights);

public static System.Threading.EventWaitHandle OpenExisting(string name, System.Security.AccessControl.EventWaitHandleRights rights);

[System.Security.SecurityCritical]
public static System.Threading.EventWaitHandle OpenExisting(string name, System.Security.AccessControl.EventWaitHandleRights rights);

static member OpenExisting : string * System.Security.AccessControl.EventWaitHandleRights -> System.Threading.EventWaitHandle

[<System.Security.SecurityCritical>]
static member OpenExisting : string * System.Security.AccessControl.EventWaitHandleRights -> System.Threading.EventWaitHandle

Public Shared Function OpenExisting (name As String, rights As EventWaitHandleRights) As EventWaitHandle

Parameters

name: String

rights: EventWaitHandleRights

A bitwise combination of the enumeration values that represent the desired security access.

Returns

EventWaitHandle

An object that represents the named system event.

Attributes: SecurityCriticalAttribute

Exceptions

ArgumentException

name is an empty string.

-or-

.NET Framework only: name is longer than MAX_PATH (260 characters).

ArgumentNullException

name is null.

WaitHandleCannotBeOpenedException

IOException

-or-

There was some other error. The HResult property may provide more information.

DirectoryNotFoundException

Windows only: name specified an unknown namespace. See Object Names for more information.

PathTooLongException

The name is too long. Length restrictions may depend on the operating system or configuration.

UnauthorizedAccessException

The named event exists, but the user does not have the desired security access.

Examples

The following code example demonstrates the cross-process behavior of a named system event with access control security. The example uses the OpenExisting(String) method overload to test for the existence of a named event.

If the event does not exist, it is created with initial ownership and access control security that denies the current user the right to use the event, but grants the right to read and change permissions on the event.

If you run the compiled example from two command windows, the second copy will throw an access violation exception on the call to OpenExisting(String). The exception is caught, and the example uses the OpenExisting(String, EventWaitHandleRights) method overload to wait on the event with the rights needed to read and change the permissions.

After the permissions are changed, the event is opened with the rights required to wait on it and signal it. If you run the compiled example from a third command window, the example runs using the new permissions.

using namespace System;
using namespace System::Threading;
using namespace System::Security::AccessControl;
using namespace System::Security::Permissions;

public ref class Example
{
public:
   [SecurityPermissionAttribute(SecurityAction::Demand,Flags=SecurityPermissionFlag::UnmanagedCode)]
   static void Main()
   {
      String^ ewhName = L"EventWaitHandleExample5";

      EventWaitHandle^ ewh = nullptr;
      bool doesNotExist = false;
      bool unauthorized = false;
      
      // The value of this variable is set by the event
      // constructor. It is true if the named system event was
      // created, and false if the named event already existed.
      //
      bool wasCreated;
      
      // Attempt to open the named event.
      try
      {
         // Open the event with (EventWaitHandleRights.Synchronize
         // | EventWaitHandleRights.Modify), to wait on and
         // signal the named event.
         //
         ewh = EventWaitHandle::OpenExisting( ewhName );
      }
      catch ( WaitHandleCannotBeOpenedException^ ) 
      {
         Console::WriteLine( L"Named event does not exist." );
         doesNotExist = true;
      }
      catch ( UnauthorizedAccessException^ ex ) 
      {
         Console::WriteLine( L"Unauthorized access: {0}", ex->Message );
         unauthorized = true;
      }

      // There are three cases: (1) The event does not exist.
      // (2) The event exists, but the current user doesn't
      // have access. (3) The event exists and the user has
      // access.
      //
      if ( doesNotExist )
      {
         // The event does not exist, so create it.

         // Create an access control list (ACL) that denies the
         // current user the right to wait on or signal the
         // event, but allows the right to read and change
         // security information for the event.
         //
         String^ user = String::Concat( Environment::UserDomainName, L"\\",
            Environment::UserName );
         EventWaitHandleSecurity^ ewhSec = gcnew EventWaitHandleSecurity;
         //following constructor fails
         EventWaitHandleAccessRule^ rule = gcnew EventWaitHandleAccessRule(
            user,
            static_cast<EventWaitHandleRights>(
               EventWaitHandleRights::Synchronize | 
               EventWaitHandleRights::Modify),
            AccessControlType::Deny );
         ewhSec->AddAccessRule( rule );

         rule = gcnew EventWaitHandleAccessRule( user,
            static_cast<EventWaitHandleRights>(
               EventWaitHandleRights::ReadPermissions | 
               EventWaitHandleRights::ChangePermissions),
            AccessControlType::Allow );
         ewhSec->AddAccessRule( rule );
         
         // Create an EventWaitHandle object that represents
         // the system event named by the constant 'ewhName',
         // initially signaled, with automatic reset, and with
         // the specified security access. The Boolean value that
         // indicates creation of the underlying system object
         // is placed in wasCreated.
         //
         ewh = gcnew EventWaitHandle( true,
            EventResetMode::AutoReset,
            ewhName,
            wasCreated,
            ewhSec );
         
         // If the named system event was created, it can be
         // used by the current instance of this program, even
         // though the current user is denied access. The current
         // program owns the event. Otherwise, exit the program.
         //
         if ( wasCreated )
         {
            Console::WriteLine( L"Created the named event." );
         }
         else
         {
            Console::WriteLine( L"Unable to create the event." );
            return;
         }
      }
      else if ( unauthorized )
      {
         // Open the event to read and change the access control
         // security. The access control security defined above
         // allows the current user to do this.
         //
         try
         {
            ewh = EventWaitHandle::OpenExisting( ewhName, 
               static_cast<EventWaitHandleRights>(
                  EventWaitHandleRights::ReadPermissions |
                  EventWaitHandleRights::ChangePermissions) );
            
            // Get the current ACL. This requires
            // EventWaitHandleRights.ReadPermissions.
            EventWaitHandleSecurity^ ewhSec = ewh->GetAccessControl();
            String^ user = String::Concat( Environment::UserDomainName, L"\\",
               Environment::UserName );
            
            // First, the rule that denied the current user
            // the right to enter and release the event must
            // be removed.
            EventWaitHandleAccessRule^ rule = gcnew EventWaitHandleAccessRule(
               user,
               static_cast<EventWaitHandleRights>(
                  EventWaitHandleRights::Synchronize |
                  EventWaitHandleRights::Modify),
               AccessControlType::Deny );
            ewhSec->RemoveAccessRule( rule );
            
            // Now grant the user the correct rights.
            //
            rule = gcnew EventWaitHandleAccessRule( user,
               static_cast<EventWaitHandleRights>(
                  EventWaitHandleRights::Synchronize |
                  EventWaitHandleRights::Modify),
               AccessControlType::Allow );
            ewhSec->AddAccessRule( rule );
            
            // Update the ACL. This requires
            // EventWaitHandleRights.ChangePermissions.
            ewh->SetAccessControl( ewhSec );
            Console::WriteLine( L"Updated event security." );
            
            // Open the event with (EventWaitHandleRights.Synchronize
            // | EventWaitHandleRights.Modify), the rights required
            // to wait on and signal the event.
            //
            ewh = EventWaitHandle::OpenExisting( ewhName );
         }
         catch ( UnauthorizedAccessException^ ex ) 
         {
            Console::WriteLine( L"Unable to change permissions: {0}",
               ex->Message );
            return;
         }

      }
      
      // Wait on the event, and hold it until the program
      // exits.
      //
      try
      {
         Console::WriteLine( L"Wait on the event." );
         ewh->WaitOne();
         Console::WriteLine( L"Event was signaled." );
         Console::WriteLine( L"Press the Enter key to signal the event and exit." );
         Console::ReadLine();
      }
      catch ( UnauthorizedAccessException^ ex ) 
      {
         Console::WriteLine( L"Unauthorized access: {0}", ex->Message );
      }
      finally
      {
         ewh->Set();
      }
   }
};

int main()
{
   Example::Main();
}

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

internal class Example
{
    internal static void Main()
    {
        const string ewhName = "EventWaitHandleExample5";

        EventWaitHandle ewh = null;
        bool doesNotExist = false;
        bool unauthorized = false;

        // The value of this variable is set by the event
        // constructor. It is true if the named system event was
        // created, and false if the named event already existed.
        //
        bool wasCreated;

        // Attempt to open the named event.
        try
        {
            // Open the event with (EventWaitHandleRights.Synchronize
            // | EventWaitHandleRights.Modify), to wait on and 
            // signal the named event.
            //
            ewh = EventWaitHandle.OpenExisting(ewhName);
        }
        catch (WaitHandleCannotBeOpenedException)
        {
            Console.WriteLine("Named event does not exist.");
            doesNotExist = true;
        }
        catch (UnauthorizedAccessException ex)
        {
            Console.WriteLine("Unauthorized access: {0}", ex.Message);
            unauthorized = true;
        }

        // There are three cases: (1) The event does not exist.
        // (2) The event exists, but the current user doesn't 
        // have access. (3) The event exists and the user has
        // access.
        //
        if (doesNotExist)
        {
            // The event does not exist, so create it.

            // Create an access control list (ACL) that denies the
            // current user the right to wait on or signal the 
            // event, but allows the right to read and change
            // security information for the event.
            //
            string user = Environment.UserDomainName + "\\"
                + Environment.UserName;
            EventWaitHandleSecurity ewhSec = 
                new EventWaitHandleSecurity();

            EventWaitHandleAccessRule rule = 
                new EventWaitHandleAccessRule(user, 
                    EventWaitHandleRights.Synchronize | 
                    EventWaitHandleRights.Modify, 
                    AccessControlType.Deny);
            ewhSec.AddAccessRule(rule);

            rule = new EventWaitHandleAccessRule(user, 
                EventWaitHandleRights.ReadPermissions | 
                EventWaitHandleRights.ChangePermissions, 
                AccessControlType.Allow);
            ewhSec.AddAccessRule(rule);

            // Create an EventWaitHandle object that represents
            // the system event named by the constant 'ewhName', 
            // initially signaled, with automatic reset, and with
            // the specified security access. The Boolean value that 
            // indicates creation of the underlying system object
            // is placed in wasCreated.
            //
            ewh = new EventWaitHandle(true, 
                EventResetMode.AutoReset, 
                ewhName, 
                out wasCreated, 
                ewhSec);

            // If the named system event was created, it can be
            // used by the current instance of this program, even 
            // though the current user is denied access. The current
            // program owns the event. Otherwise, exit the program.
            // 
            if (wasCreated)
            {
                Console.WriteLine("Created the named event.");
            }
            else
            {
                Console.WriteLine("Unable to create the event.");
                return;
            }
        }
        else if (unauthorized)
        {
            // Open the event to read and change the access control
            // security. The access control security defined above
            // allows the current user to do this.
            //
            try
            {
                ewh = EventWaitHandle.OpenExisting(ewhName, 
                    EventWaitHandleRights.ReadPermissions | 
                    EventWaitHandleRights.ChangePermissions);

                // Get the current ACL. This requires 
                // EventWaitHandleRights.ReadPermissions.
                EventWaitHandleSecurity ewhSec = ewh.GetAccessControl();
                
                string user = Environment.UserDomainName + "\\"
                    + Environment.UserName;

                // First, the rule that denied the current user 
                // the right to enter and release the event must
                // be removed.
                EventWaitHandleAccessRule rule = 
                    new EventWaitHandleAccessRule(user, 
                        EventWaitHandleRights.Synchronize | 
                        EventWaitHandleRights.Modify, 
                        AccessControlType.Deny);
                ewhSec.RemoveAccessRule(rule);

                // Now grant the user the correct rights.
                // 
                rule = new EventWaitHandleAccessRule(user, 
                    EventWaitHandleRights.Synchronize | 
                    EventWaitHandleRights.Modify, 
                    AccessControlType.Allow);
                ewhSec.AddAccessRule(rule);

                // Update the ACL. This requires
                // EventWaitHandleRights.ChangePermissions.
                ewh.SetAccessControl(ewhSec);

                Console.WriteLine("Updated event security.");

                // Open the event with (EventWaitHandleRights.Synchronize 
                // | EventWaitHandleRights.Modify), the rights required
                // to wait on and signal the event.
                //
                ewh = EventWaitHandle.OpenExisting(ewhName);
            }
            catch (UnauthorizedAccessException ex)
            {
                Console.WriteLine("Unable to change permissions: {0}",
                    ex.Message);
                return;
            }
        }

        // Wait on the event, and hold it until the program
        // exits.
        //
        try
        {
            Console.WriteLine("Wait on the event.");
            ewh.WaitOne();
            Console.WriteLine("Event was signaled.");
            Console.WriteLine("Press the Enter key to signal the event and exit.");
            Console.ReadLine();
        }
        catch (UnauthorizedAccessException ex)
        {
            Console.WriteLine("Unauthorized access: {0}", ex.Message);
        }
        finally
        {
            ewh.Set();
        }
    }
}

Imports System.Threading
Imports System.Security.AccessControl

Friend Class Example

    <MTAThread> _
    Friend Shared Sub Main()
        Const ewhName As String = "EventWaitHandleExample5"

        Dim ewh As EventWaitHandle = Nothing
        Dim doesNotExist as Boolean = False
        Dim unauthorized As Boolean = False

        ' The value of this variable is set by the event
        ' constructor. It is True if the named system event was
        ' created, and False if the named event already existed.
        '
        Dim wasCreated As Boolean

        ' Attempt to open the named event.
        Try
            ' Open the event with (EventWaitHandleRights.Synchronize
            ' Or EventWaitHandleRights.Modify), to wait on and 
            ' signal the named event.
            '
            ewh = EventWaitHandle.OpenExisting(ewhName)
        Catch ex As WaitHandleCannotBeOpenedException
            Console.WriteLine("Named event does not exist.")
            doesNotExist = True
        Catch ex As UnauthorizedAccessException
            Console.WriteLine("Unauthorized access: {0}", ex.Message)
            unauthorized = True
        End Try

        ' There are three cases: (1) The event does not exist.
        ' (2) The event exists, but the current user doesn't 
        ' have access. (3) The event exists and the user has
        ' access.
        '
        If doesNotExist Then
            ' The event does not exist, so create it.

            ' Create an access control list (ACL) that denies the
            ' current user the right to wait on or signal the 
            ' event, but allows the right to read and change
            ' security information for the event.
            '
            Dim user As String = Environment.UserDomainName _ 
                & "\" & Environment.UserName
            Dim ewhSec As New EventWaitHandleSecurity()

            Dim rule As New EventWaitHandleAccessRule(user, _
                EventWaitHandleRights.Synchronize Or _
                EventWaitHandleRights.Modify, _
                AccessControlType.Deny)
            ewhSec.AddAccessRule(rule)

            rule = New EventWaitHandleAccessRule(user, _
                EventWaitHandleRights.ReadPermissions Or _
                EventWaitHandleRights.ChangePermissions, _
                AccessControlType.Allow)
            ewhSec.AddAccessRule(rule)

            ' Create an EventWaitHandle object that represents
            ' the system event named by the constant 'ewhName', 
            ' initially signaled, with automatic reset, and with
            ' the specified security access. The Boolean value that 
            ' indicates creation of the underlying system object
            ' is placed in wasCreated.
            '
            ewh = New EventWaitHandle(True, _
                EventResetMode.AutoReset, ewhName, _
                wasCreated, ewhSec)

            ' If the named system event was created, it can be
            ' used by the current instance of this program, even 
            ' though the current user is denied access. The current
            ' program owns the event. Otherwise, exit the program.
            ' 
            If wasCreated Then
                Console.WriteLine("Created the named event.")
            Else
                Console.WriteLine("Unable to create the event.")
                Return
            End If

        ElseIf unauthorized Then

            ' Open the event to read and change the access control
            ' security. The access control security defined above
            ' allows the current user to do this.
            '
            Try
                ewh = EventWaitHandle.OpenExisting(ewhName, _
                    EventWaitHandleRights.ReadPermissions Or _
                    EventWaitHandleRights.ChangePermissions)

                ' Get the current ACL. This requires 
                ' EventWaitHandleRights.ReadPermissions.
                Dim ewhSec As EventWaitHandleSecurity = _
                    ewh.GetAccessControl()
                
                Dim user As String = Environment.UserDomainName _ 
                    & "\" & Environment.UserName

                ' First, the rule that denied the current user 
                ' the right to enter and release the event must
                ' be removed.
                Dim rule As New EventWaitHandleAccessRule(user, _
                    EventWaitHandleRights.Synchronize Or _
                    EventWaitHandleRights.Modify, _
                    AccessControlType.Deny)
                ewhSec.RemoveAccessRule(rule)

                ' Now grant the user the correct rights.
                ' 
                rule = New EventWaitHandleAccessRule(user, _
                    EventWaitHandleRights.Synchronize Or _
                    EventWaitHandleRights.Modify, _
                    AccessControlType.Allow)
                ewhSec.AddAccessRule(rule)

                ' Update the ACL. This requires
                ' EventWaitHandleRights.ChangePermissions.
                ewh.SetAccessControl(ewhSec)

                Console.WriteLine("Updated event security.")

                ' Open the event with (EventWaitHandleRights.Synchronize 
                ' Or EventWaitHandleRights.Modify), the rights required
                ' to wait on and signal the event.
                '
                ewh = EventWaitHandle.OpenExisting(ewhName)

            Catch ex As UnauthorizedAccessException
                Console.WriteLine("Unable to change permissions: {0}", _
                    ex.Message)
                Return
            End Try

        End If

        ' Wait on the event, and hold it until the program
        ' exits.
        '
        Try
            Console.WriteLine("Wait on the event.")
            ewh.WaitOne()
            Console.WriteLine("Event was signaled.")
            Console.WriteLine("Press the Enter key to signal the event and exit.")
            Console.ReadLine()
        Catch ex As UnauthorizedAccessException
            Console.WriteLine("Unauthorized access: {0}", _
                ex.Message)
        Finally
            ewh.Set()
        End Try
    End Sub 
End Class

Remarks

The rights parameter must include the EventWaitHandleRights.Synchronize flag to allow threads to wait on the event, and the EventWaitHandleRights.Modify flag to allow threads to call the Set and Reset methods.

The OpenExisting method tries to open an existing named system event. To create the system event when it does not already exist, use one of the EventWaitHandle constructors that has a name parameter.

Applies to

.NET Framework 4.8.1 and other versions

Product	Versions
.NET Framework	2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1

Share via

EventWaitHandle.OpenExisting Method

Definition

Overloads

OpenExisting(String)

Parameters

Returns

Exceptions

Remarks

See also

Applies to

OpenExisting(String, EventWaitHandleRights)

Parameters

Returns

Exceptions

Examples

Remarks

See also

Applies to

Additional resources