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public class SingleInstance : IDisposable
{
    [DllImport( "Coredll.dll", SetLastError = true )]
    static extern IntPtr CreateEvent( IntPtr ptrAlwaysZero, bool bManualReset, bool bInitialState, string lpName );
 
    [DllImport( "Coredll.dll", SetLastError = true )]
    static extern int CloseHandle( IntPtr handle );
 
    private IntPtr Handle = IntPtr.Zero;
 
    public bool IsSingleInstance { get { return this.Handle != IntPtr.Zero; } }
 
    /// <summary>A running program is identified by an already existing event that is named the same as the executable path.</summary>
    public SingleInstance() : this( Assembly.GetExecutingAssembly().GetModules()[ 0 ].FullyQualifiedName ) { }
 
    /// <summary>Accepts a single string as a parameter.</summary>
    public SingleInstance( string sEventName )
    {
        this.Handle = CreateEvent( IntPtr.Zero, true, false, sEventName );
        if ( Marshal.GetLastWin32Error() != 0 )
            Dispose();
    }
 
    public void Dispose()
    {
        if ( this.Handle != IntPtr.Zero )
            CloseHandle( this.Handle );
        this.Handle = IntPtr.Zero;
    }
}

static class Program
{
    [MTAThread]
    static void Main()
    {
        using ( SingleInstance si = new SingleInstance() )
            if ( si.IsSingleInstance )
                Application.Run();
    }
}