C# Signature:

[DllImport("kernel32.dll")]
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 ////////////////////////////
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)0)
                {
                    // 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

Alternative Managed API:

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