Class1.cs

private static Form form;
private static Thread thread;
private static event Action showRequest;

public static void OpenForm()
{
	if (thread == null)
	{
		thread = new Thread(() =>
		{
			form = new Form();
			form.Show();

			showRequest += delegate
			{
				form.Show();
			};

			form.FormClosing += delegate(object sender, FormClosingEventArgs args)
			{
				if (args.CloseReason == CloseReason.UserClosing)
				{
					args.Cancel = true;
					form.Hide();
				}
			};

			Application.EnableVisualStyles();
			Application.Run();
		});
	}
	if (thread.ThreadState != ThreadState.Running) thread.Start();
	else
	{
		if (showRequest != null) showRequest.BeginInvoke(null, null);
	}
}