SingleInstanceHelper

using System;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;


namespace Core {

    /// <summary>
    /// Ensures that only process is executed at a time
    /// </summary>
    internal class SingleInstanceHelper:IDisposable {

        [DllImport("user32.dll",EntryPoint = "SetForegroundWindow")]
        static extern bool SetForegroundWindow(IntPtr hWnd);

        readonly bool _onlyInstance;
        readonly Process _firstProcess;

        public Process FirstProcess {
            get { return _firstProcess; }
        }

        public bool OnlyInstance {
            get {
                return _onlyInstance;
            }
        }

        public SingleInstanceHelper() {
            _firstProcess = PriorProcess();
            _onlyInstance = _firstProcess == null;
        }

        public bool ShowFirstInstance() {

            if(_firstProcess == null)
                return false;

            if(_firstProcess.MainWindowHandle == IntPtr.Zero)
                return false;

            SetForegroundWindow(_firstProcess.MainWindowHandle);

            return true;
        }


        /// <summary>
        /// Returns a System.Diagnostics.Process pointing to
        /// a pre-existing process with the same name as the
        /// current one, if any; or null if the current process is unique.
        /// </summary>
        /// <returns>Existing Process if any, otherwise null if no previous processes are found</returns>
        Process PriorProcess() {

            using(Process curr = Process.GetCurrentProcess()) {

                Process[] procs = Process.GetProcessesByName(curr.ProcessName);
                Process[] exceptPrior = null;

                try {
                    foreach(Process p in procs) {

                        if((p.Id != curr.Id) &&
                            (p.MainModule.FileName == curr.MainModule.FileName)) {
                            exceptPrior = procs.Except(new Process[] { p }).ToArray();
                            return p;
                        }
                    }
                    exceptPrior = procs;
                }
                finally {
                    foreach(var item in exceptPrior)
                        item.Dispose();
                }
                return null;
            }
        }


        public void Dispose() {
            if(_firstProcess != null) {
                _firstProcess.Dispose();
            }
        }
    }

}