C# Signature:
using Microsoft.Win32.SafeHandles;
[DllImport("kernel32.dll")]
public static extern SafeWaitHandle CreateMutex(IntPtr lpMutexAttributes, bool bInitialOwner, string lpName);
Alternative declaration:
[DllImport("kernel32.dll")]
public static extern IntPtr CreateMutex(IntPtr lpMutexAttributes, bool bInitialOwner, string lpName);
User-Defined Types:
None.
Notes:
None.
Tips & Tricks:
Please add some!!!
Sample Code:
Here's how I use CreateMutex() to prevent more than one instance of my application from running at any given time.
//// Win32Calls.cs ////////////////////////////
using Microsoft.Win32.SafeHandles;
namespace nsWin32Calls
{
/// <summary>
/// Win32 calls encapsulation object.
/// </summary>
public class Win32Calls
{
...
[DllImport("kernel32.dll", SetLastError = true)]
public static extern SafeWaitHandle CreateMutex( IntPtr lpMutexAttributes,
bool bInitialOwner, string lpName );
[DllImport("kernel32.dll")]
public static extern bool ReleaseMutex( SafeWaitHandle hMutex );
...
/// <summary>
/// If CreateMutex() returns zero, GetLastError() can return this error.
/// </summary>
public const int ERROR_ALREADY_EXISTS = 183;
...
}
}
//// frmMain.cs ///////////////////////////////
...
using Microsoft.Win32.SafeHandles;
using nsWin32Calls; // encapsulates Win32 calls
...
namespace dsh
{
public class frmMain : System.Windows.Forms.Form
{
...
// helper objects
private static frmMain form = null;
...
/// <summary>
/// Main entry point for the application.
/// </summary>
[STAThread]
public static void Main()
{
// create IntPtrs for use with CreateMutex()
IntPtr ipMutexAttr = IntPtr.Zero;
SafeWaitHandle ipHMutex = new SafeWaitHandle();
try
{
// Create the mutex and verify its status BEFORE construction
// of the main form.
ipHMutex = Win32Calls.CreateMutex( ipMutexAttr,
true, "CompanyName_AppName_MUTEX" );
if (!ipHMutex.IsInvalid)
{
// check GetLastError value (MUST use this call. See MSDN)
int iGLE = Marshal.GetLastWin32Error();
// if we get the ERROR_ALREADY_EXISTS value, there is
// already another instance of this application running.
if (iGLE == Win32Calls.ERROR_ALREADY_EXISTS)
// So, don't allow this instance to run.
return;
}
else
{ // CreateMutex() failed.
// once the app is up and running, I log the failure from
// within the frmMain constructor.
m_bMutexFailed = true;
}
// construct the main form object and
form = new frmMain();
// run the app.
Application.Run( form );
}
catch( Exception oEx )
{
...handle it...
}
finally
{
// release the mutex
if (!ipHMutex.IsInvalid)
Win32Calls.ReleaseMutex( ipHMutex );
// cleanup the main form object instance.
if (form != null) {
form.Dispose();
}
}
}
...
}
}
Another Sample implementation
This implements a Mutex using a Null DACL (with all the security problems with that). This allows the Mutex to be sharable cross-process. In my implementation I access this Mutex from both ASP.Net and a Windows service running under different ids.
I put this together from links on this site as well as searching other sites. Credit to all those who posted to help me implement this.
/// <summary>
/// Summary description for Native.
/// </summary>
internal sealed class Native
{
/// <summary>
/// This value can be returned by CreateMutex() and is found in
/// C++ in the error.h header file.
/// </summary>
public const int ERROR_ALREADY_EXISTS = 183;
public const uint SYNCHRONIZE = 0x00100000;
[DllImport("kernel32.dll")]
public static extern IntPtr CreateMutex(IntPtr lpMutexAttributes, bool bInitialOwner, string lpName);
[DllImport("kernel32.dll")]
public static extern bool ReleaseMutex( IntPtr hMutex );
[DllImport("kernel32.dll")]
public static extern IntPtr OpenMutex(uint dwDesiredAccess, bool bInheritHandle, string lpName);
[DllImport("advapi32.dll", SetLastError=true)]
public static extern bool SetSecurityDescriptorDacl(ref SECURITY_DESCRIPTOR securityDescriptor, bool daclPresent, IntPtr dacl, bool daclDefaulted);
[DllImport("advapi32.dll", SetLastError=true)]
public static extern bool InitializeSecurityDescriptor(out SECURITY_DESCRIPTOR securityDescriptor, uint dwRevision);
[StructLayoutAttribute(LayoutKind.Sequential)]
public struct SECURITY_DESCRIPTOR
{
public byte revision;
public byte size;
public short control;
public IntPtr owner;
public IntPtr group;
public IntPtr sacl;
public IntPtr dacl;
}
[StructLayout(LayoutKind.Sequential)]
public struct SECURITY_ATTRIBUTES
{
public int nLength;
public IntPtr lpSecurityDescriptor;
public int bInheritHandle;
}
/// <summary>
/// Security enumeration from:
/// http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/synchronization_object_security_and_access_rights.asp
/// </summary>
[Flags]
public enum SyncObjectAccess : uint
{
DELETE = 0x00010000,
READ_CONTROL = 0x00020000,
WRITE_DAC = 0x00040000,
WRITE_OWNER = 0x00080000,
SYNCHRONIZE = 0x00100000,
EVENT_ALL_ACCESS = 0x001F0003,
EVENT_MODIFY_STATE = 0x00000002,
MUTEX_ALL_ACCESS = 0x001F0001,
MUTEX_MODIFY_STATE = 0x00000001,
SEMAPHORE_ALL_ACCESS = 0x001F0003,
SEMAPHORE_MODIFY_STATE = 0x00000002,
TIMER_ALL_ACCESS = 0x001F0003,
TIMER_MODIFY_STATE = 0x00000002,
TIMER_QUERY_STATE = 0x00000001
}
}
public class SecurityNeutralMutex
{
[ Serializable ]
public class MutexCreationException : ApplicationException
{
public MutexCreationException( string msg ) : base( msg )
{
}
public MutexCreationException( string msg, Exception ex ) : base( msg, ex )
{
}
public MutexCreationException( SerializationInfo info, StreamingContext context ) : base( info, context )
{
}
}
public SecurityNeutralMutex()
{
}
public static Mutex CeateMutex( bool initiallyOwned, string name, out bool createdNew )
{
Mutex mutex = new Mutex( false ) ;
mutex.Close();
mutex.Handle = InternalCreateMutex( initiallyOwned, name, out createdNew );
return mutex;
}
private static IntPtr InternalCreateMutex( bool initiallyOwned, string name, out bool createdNew )
{
IntPtr mutexAttributesPtr = new IntPtr( 0 );
IntPtr securityDescPtr = new IntPtr( 0 );
bool worked;
const uint SdRevisionLevel = 1;
try
{
int lastError = 0;
// It's faster to first try OpenMutex
IntPtr hMutex = Native.OpenMutex( ( uint ) ( Native.SyncObjectAccess.MUTEX_MODIFY_STATE ), false, name );
createdNew = false;
if ( hMutex == IntPtr.Zero )
{
// Initialize the security descriptor structure
Native.SECURITY_DESCRIPTOR securityDesc;
worked = Native.InitializeSecurityDescriptor( out securityDesc, SdRevisionLevel );
if( ! worked )
{
lastError = Marshal.GetLastWin32Error();
throw new MutexCreationException( string.Format( "Failed to initialize security descriptor. Win32 error num: '{0}'", lastError ) );
}
worked = Native.SetSecurityDescriptorDacl(ref securityDesc, true, IntPtr.Zero, false);
if( ! worked )
{
lastError = Marshal.GetLastWin32Error();
throw new MutexCreationException( string.Format( "Failed to set security descriptor DACL. Win32 error num: '{0}'", lastError ) );
}
Native.SECURITY_ATTRIBUTES secAttribs = new Native.SECURITY_ATTRIBUTES();
secAttribs.nLength = Marshal.SizeOf( secAttribs );
secAttribs.bInheritHandle = 1;
securityDescPtr = Marshal.AllocCoTaskMem( Marshal.SizeOf( securityDesc ) );
Marshal.StructureToPtr( securityDesc, securityDescPtr, false );
secAttribs.lpSecurityDescriptor = securityDescPtr;
mutexAttributesPtr = Marshal.AllocCoTaskMem( Marshal.SizeOf( secAttribs ) );
Marshal.StructureToPtr( secAttribs, mutexAttributesPtr, false );
hMutex = Native.CreateMutex( mutexAttributesPtr, initiallyOwned ? true : false, name );
lastError = Marshal.GetLastWin32Error();
createdNew = ( lastError != Native.ERROR_ALREADY_EXISTS );
if( hMutex == IntPtr.Zero )
{
// If we get here, some sort of unrecoverable error has presumably occurred.
// However, we will try one last time, in case it was merely some sort of race condition.
// Note that I do not believe that there is any opening for a race condition in the above code.
// Nevertheless, we have nothing to lose by giving it one last try.
hMutex = Native.OpenMutex( ( uint ) ( Native.SyncObjectAccess.MUTEX_MODIFY_STATE ), false, name );
createdNew = false;
if( hMutex == IntPtr.Zero )
{
lastError = Marshal.GetLastWin32Error();
throw new MutexCreationException( string.Format( "Unable to create or open mutex. Win32 error num: '{0}'", lastError ) );
}
}
}
return hMutex;
}
finally
{
if( mutexAttributesPtr != IntPtr.Zero )
{
Marshal.FreeCoTaskMem( mutexAttributesPtr );
}
if( securityDescPtr != IntPtr.Zero )
{
Marshal.FreeCoTaskMem( securityDescPtr );
}
}
}
}
Alternative Managed API:
