C#

using System;
using System.Windows.Forms;
using Microsoft.VisualBasic.ApplicationServices;

    class Program
    {
        [STAThread]
        static void Main(string[] args) { new App().Run(args); }

        class App : WindowsFormsApplicationBase
        {
            public App()
            {
                IsSingleInstance = EnableVisualStyles = true;
                ShutdownStyle = Microsoft.VisualBasic.ApplicationServices.ShutdownMode.AfterMainFormCloses;
                StartupNextInstance += new StartupNextInstanceEventHandler(NextInstance);
            }

            protected override void OnCreateMainForm()
            {
                MainForm = new Form1();
                ((Form1)MainForm).Args = new string[CommandLineArgs.Count];
                CommandLineArgs.CopyTo(((Form1)MainForm).Args, 0);
            }

            protected void NextInstance(object sender, StartupNextInstanceEventArgs eventArgs)
            {
                string[] args = new string[eventArgs.CommandLine.Count];
                eventArgs.CommandLine.CopyTo(args, 0);
                MainForm.Invoke(new Form1.InstanceDelegate(((Form1)MainForm).processParameters), new object[] { MainForm, args });
            }
        }
    }

// In Form1 class:

public void processParameters(object sender, string[] args) 
{ 
    if (WindowState == FormWindowState.Minimized) 
        WindowState = FormWindowState.Normal; 
    if (args != null && args.Length != 0) 
        openFiles(args); // process args
}