using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

using Microsoft.VisualBasic.ApplicationServices;

namespace VisualBasic
{
    static class Program
    {

        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            string[] args = Environment.GetCommandLineArgs();
            SingleInstanceController controller = new SingleInstanceController();
            controller.Run(args);

        }
    }

    public class SingleInstanceController : WindowsFormsApplicationBase
    {
        public SingleInstanceController()
        {
            IsSingleInstance = true;

            StartupNextInstance += this_StartupNextInstance;
        }

        void this_StartupNextInstance(object sender, StartupNextInstanceEventArgs e)
        {
            Form1 form = MainForm as Form1; //My derived form type
            form.LoadFile(e.CommandLine[1]);
        }

        protected override void OnCreateMainForm()
        {
            MainForm = new Form1();
        }
    }

}


using System;
using System.IO;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace VisualBasic
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            string[] args = Environment.GetCommandLineArgs();
            LoadFile(args[1]);
        }

        public void LoadFile(string file)
        {
        textBox1.Text += file;
        }

    }
}