This is sometimes confusing because you just don’t think about it — you have to access a file that you rarely access when making Windows forms applications, “Program.cs”

     static class Program
    {
        ///
        /// The main entry point for the application.
        ///
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
    }

There are so many ways to skin this cat, so you get to see how I skinned my last cat.

    static class Program
    {
        ///
        /// The main entry point for the application.
        ///
        [STAThread]
        static void Main(string[] args)
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            Form1 mainf = new Form1();

            if (args.Length > 0)
            {

                try
                {

                    if (System.IO.File.Exists(args[0]))
                    {
                        mainf.LoadFile= args[0];

                    }
                }
                catch
                {
                    MessageBox.Show("Could not open file.", "Could not open file.", MessageBoxButtons.OK, MessageBoxIcon.Information);

                }

            }

            Application.Run(mainf);

        }
    }

You will notice the mainf.LoadFile property. In the main form of my program I have a property for public string LoadFile ... and the field private string loadFile = String.Empty; in the forms load event I check the value of this field.

    private void Form1_Load(object sender, EventArgs e)
    {
       if(loadFile != String.Empty){
             //  The only way this field is NOT String.empty is if we set it in
             //   static void Main() of program.cs
             //   LOAD it however it is needed OpenFile, SetDatabase, whatever you use.

       }

    }