using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.Diagnostics;
using System.Reflection;
using System.Threading;

namespace TEST
{
    static class Program
    {
        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new FrmMain());
            Process instance = RunningInstance();
            if (instance == null)
            {
                Application.Run(new FrmMain());
            }
            else
            {
                HandleRunningInstance(instance);
            } 
        }


        #region 进程名
        public static Process RunningInstance()
        {
            Process current = Process.GetCurrentProcess();
            Process[] processes = Process.GetProcessesByName(current.ProcessName);
            foreach (Process process in processes)
            {
                if (process.Id != current.Id)
                {
                    if (Assembly.GetExecutingAssembly().Location.Replace("/ ", "\\ ") == current.MainModule.FileName)
                    {
                        return process;
                    }
                }
            }
            return null;
        }
        public static void HandleRunningInstance(Process instance)
        {
            ShowWindowAsync(instance.MainWindowHandle, WS_SHOWNORMAL);
            SetForegroundWindow(instance.MainWindowHandle);
        }
        [DllImport("User32.dll ")]
        private static extern bool ShowWindowAsync(IntPtr hWnd, int cmdShow);
        [DllImport("User32.dll ")]
        private static extern bool
            SetForegroundWindow(IntPtr hWnd);
        private const int WS_SHOWNORMAL = 1;
        #endregion
    }
}

------解决方案--------------------
C# code

   private void button1_Click(object sender, EventArgs e)
        {
            Form2 frm = Form2.CreateInstance();
            if (frm.WindowState == FormWindowState.Minimized)
            {
                frm.WindowState = FormWindowState.Normal;
            }
            else
            {
                frm.Show();
            }
        }

 public partial class Form2 : Form
    {
        private static readonly object m_SyncObject = new object();

        private static Form2 frm;

        private Form2()
        {
            InitializeComponent();
        }

        public static Form2 CreateInstance()
        {
            if (frm == null)
            {
                lock (m_SyncObject)
                {
                    if (frm == null)
                    {
                        frm = new Form2();
                    }
                }
            }
            return frm;
        }

    }