@msdn=http://search.microsoft.com/search/results.aspx?qu=$$$ @pinvoke=http://pinvoke.net/$$$.htm Summary: The CreateMutex API !!!!C# Signature: [DllImport("kernel32.dll", SetLastError = true)] static extern IntPtr CreateMutex(IntPtr lpMutexAttributes, bool bInitialOwner, string lpName); !!!!VB.Net Signature: <DllImport("Kernel32.dll", CharSet:=CharSet.Auto, setlasterror:=True)> _ Public Function CreateMutex(ByVal lpMutexAttributes As IntPtr, _ ByVal bInitialOwner As Boolean, _ ByVal lpName As String) As IntPtr End Function !!!!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 //////////////////////////// namespace nsWin32Calls { /// <summary> /// Win32 calls encapsulation object. /// </summary> public class Win32Calls { ... [DllImport("kernel32.dll", SetLastError = true)] public static extern IntPtr CreateMutex( IntPtr lpMutexAttributes, bool bInitialOwner, string lpName ); [DllImport("kernel32.dll")] public static extern bool ReleaseMutex( IntPtr hMutex ); ... /// <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; ... } } //// frmMain.cs /////////////////////////////// ... 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 = new IntPtr( 0 ); IntPtr ipHMutex = new IntPtr( 0 ); try { // Create the mutex and verify its status BEFORE construction // of the main form. ipHMutex = Win32Calls.CreateMutex( ipMutexAttr, true, "CompanyName_AppName_MUTEX" ); if (ipHMutex != IntPtr.Zero) { // 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 != (IntPtr)0) Win32Calls.ReleaseMutex( ipHMutex ); // cleanup the main form object instance. if (form != null) { form.Dispose(); } } } ... } } I tried to format as well as possible, but this thing is a bit weird. If I leave one blank line, this darn thing leaves two. If I leave none, it leaves none. :s !!!!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: See: System.Threading.Mutex in .NET Framework 1.1 Documentation: CreateMutex@msdn on MSDN
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