How can I make a .NET Windows Forms application that only runs in the System Tray?

As mat1t says - you need to add a NotifyIcon to your application and then use something like the following code to set the tooltip and context menu:

this.notifyIcon.Text = "This is the tooltip";
this.notifyIcon.ContextMenu = new ContextMenu();
this.notifyIcon.ContextMenu.MenuItems.Add(new MenuItem("Option 1", new EventHandler(handler_method)));

This code shows the icon in the system tray only:

this.notifyIcon.Visible = true;  // Shows the notify icon in the system tray

The following will be needed if you have a form (for whatever reason):

this.ShowInTaskbar = false;  // Removes the application from the taskbar
Hide();

The right click to get the context menu is handled automatically, but if you want to do some action on a left click you'll need to add a Click handler:

    private void notifyIcon_Click(object sender, EventArgs e)
    {
        var eventArgs = e as MouseEventArgs;
        switch (eventArgs.Button)
        {
            // Left click to reactivate
            case MouseButtons.Left:
                // Do your stuff
                break;
        }
    }

I've wrote a traybar app with .NET 1.1 and I didn't need a form.
First of all, set the startup object of the project as a Sub Main, defined in a module.
Then create programmatically the components: the NotifyIcon and ContextMenu.
Be sure to include a MenuItem "Quit" or similar.
Bind the ContextMenu to the NotifyIcon.
Invoke Application.Run().
In the event handler for the Quit MenuItem be sure to call set NotifyIcon.Visible = False, then Application.Exit(). Add what you need to the ContextMenu and handle properly :)

1. Create a new Windows Application with the wizard.
2. Delete Form1 from the code.
3. Remove the code in Program.cs starting up the Form1.
4. Use the NotifyIcon class to create your system tray icon (assign an icon to it).
5. Add a contextmenu to it.
6. Or react to NotifyIcon's mouseclick and differenciate between Right and Left click, setting your contextmenu and showing it for which ever button (right/left) was pressed.
7. Application.Run() to keep the app running with Application.Exit() to quit. Or a bool bRunning = true; while(bRunning){Application.DoEvents(); Thread.Sleep(10);}. Then set bRunning = false; to exit the app.

"System tray" application is just a regular win forms application, only difference is that it creates a icon in windows system tray area. In order to create sys.tray icon use NotifyIcon component , you can find it in Toolbox(Common controls), and modify it's properties: Icon, tool tip. Also it enables you to handle mouse click and double click messages.

And One more thing , in order to achieve look and feels or standard tray app. add followinf lines on your main form show event:

private void MainForm_Shown(object sender, EventArgs e)
{
    WindowState = FormWindowState.Minimized;
    Hide();
} 

As far as I'm aware you have to still write the application using a form, but have no controls on the form and never set it visible. Use the NotifyIcon (an MSDN sample of which can be found here) to write your application.

I adapted the accepted answer to .NET Core, using the recommended replacements for deprecated classes:

• ContextMenu -> ContextMenuStrip
• MenuItem -> ToolStripMenuItem

Program.cs

namespace TrayOnlyWinFormsDemo
{
    internal static class Program
    {
        [STAThread]
        static void Main()
        {
            ApplicationConfiguration.Initialize();
            Application.Run(new MyCustomApplicationContext());
        }
    }
}

MyCustomApplicationContext.cs

using TrayOnlyWinFormsDemo.Properties;  // Needed for Resources.AppIcon

namespace TrayOnlyWinFormsDemo
{
    public class MyCustomApplicationContext : ApplicationContext
    {
        private NotifyIcon trayIcon;

        public MyCustomApplicationContext()
        {
            trayIcon = new NotifyIcon()
            {
                Icon = Resources.AppIcon,
                ContextMenuStrip = new ContextMenuStrip()
                {
                    Items = { new ToolStripMenuItem("Exit", null, Exit) }
                },
                Visible = true
            };
        }

        void Exit(object? sender, EventArgs e)
        {
            trayIcon.Visible = false;
            Application.Exit();
        }
    }
}

notifyIcon1->ContextMenu = gcnew

System::Windows::Forms::ContextMenu();
System::Windows::Forms::MenuItem^ nIItem = gcnew
System::Windows::Forms::MenuItem("Open");

nIItem->Click += gcnew System::EventHandler(this, &your_class::Open_NotifyIcon);

notifyIcon1->ContextMenu->MenuItems->Add(nIItem);

You can create the form, modify it then pass it to the Application.Run as a parameter. :

    internal static class Program
    {
        /// <summary>
        ///  The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            ApplicationConfiguration.Initialize();
            var form = new Form1();
            form.Hide();
            form.Opacity = 0;
            form.ShowInTaskbar = false;
            Application.Run(form);
        }
    }

Add your NotifyIcon and ContextMenu (if needed) to your form at design time as a regular app. Make sure your Notifyicon is Visible and has an icon associated. This will also let you work with a form that you may need later for any reason