using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using Tool;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.InteropServices;
namespace 计算器
{
  static class Program
  {
    [DllImport("user32.dll")]
    public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
    /// <summary>
    /// 该函数设置由不同线程产生的窗口的显示状态。
    /// </summary>
    /// <param name="hWnd">窗口句柄</param>
    /// <param name="cmdShow">指定窗口如何显示。查看允许值列表，请查阅ShowWlndow函数的说明部分。</param>
    /// <returns>如果函数原来可见，返回值为非零；如果函数原来被隐藏，返回值为零。</returns>
    [DllImport("User32.dll")]
    private static extern bool ShowWindow(IntPtr hWnd, int cmdShow);
    /// <summary>
    /// 该函数将创建指定窗口的线程设置到前台，并且激活该窗口。键盘输入转向该窗口，并为用户改各种可视的记号。系统给创建前台窗口的线程分配的权限稍高于其他线程。
    /// </summary>
    /// <param name="hWnd">将被激活并被调入前台的窗口句柄。</param>
    /// <returns>如果窗口设入了前台，返回值为非零；如果窗口未被设入前台，返回值为零。</returns>
    [DllImport("User32.dll")]
    private static extern bool SetForegroundWindow(IntPtr hWnd);
    private const int SW_SHOWNORMAL = 1;
    /// <summary>
    /// 应用程序的主入口点。
    /// </summary>
    [STAThread]
    static void Main()
    {
      Application.EnableVisualStyles();
      Application.SetCompatibleTextRenderingDefault(false);
      Process processes = RunningInstance();
      if (processes == null)
      {
        Application.Run(new Form1());
      }
      else
      {
        HandleRunningInstance(processes);
      }
    }
    /// <summary>
    /// 获取正在运行的实例，没有运行的实例返回null;
    /// </summary>
    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;
    }
    /// <summary>
    /// 显示已运行的程序。
    /// </summary>
    public static void HandleRunningInstance(Process instance)
    {
      try
      {
        IntPtr formHwnd = FindWindow(null, "计算器");
        ShowWindow(formHwnd, SW_SHOWNORMAL);  //显示
        SetForegroundWindow(formHwnd);     //放到前端
      }
      catch (Exception ex)
      {
        MessageBox.Show(ex.Message);
      }
    }
  }
}

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using Tool;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.InteropServices;
namespace 计算器
{
  static class Program
  {
    [DllImport("user32.dll")]
    public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
    /// <summary>
    /// 该函数设置由不同线程产生的窗口的显示状态。
    /// </summary>
    /// <param name="hWnd">窗口句柄</param>
    /// <param name="cmdShow">指定窗口如何显示。查看允许值列表，请查阅ShowWlndow函数的说明部分。</param>
    /// <returns>如果函数原来可见，返回值为非零；如果函数原来被隐藏，返回值为零。</returns>
    [DllImport("User32.dll")]
    private static extern bool ShowWindow(IntPtr hWnd, int cmdShow);
    /// <summary>
    /// 该函数将创建指定窗口的线程设置到前台，并且激活该窗口。键盘输入转向该窗口，并为用户改各种可视的记号。系统给创建前台窗口的线程分配的权限稍高于其他线程。
    /// </summary>
    /// <param name="hWnd">将被激活并被调入前台的窗口句柄。</param>
    /// <returns>如果窗口设入了前台，返回值为非零；如果窗口未被设入前台，返回值为零。</returns>
    [DllImport("User32.dll")]
    private static extern bool SetForegroundWindow(IntPtr hWnd);
    private const int SW_SHOWNORMAL = 1;
    /// <summary>
    /// 应用程序的主入口点。
    /// </summary>
    [STAThread]
    static void Main()
    {
      Common.AutoRegister();
      Application.EnableVisualStyles();
      Application.SetCompatibleTextRenderingDefault(false);
      bool createNew;
      using (System.Threading.Mutex m = new System.Threading.Mutex(true, Application.ProductName, out createNew))
      {
        if (createNew)
        {
          FileOperator.SetValue("ProcessId", Process.GetCurrentProcess().Id.ToString()); //进程ID写入文件
          Application.Run(new Form1());
        }
        else
        {
          try
          {
            string strProcessId = FileOperator.GetValue("ProcessId"); //从文件中获取进程ID
            int processId = Convert.ToInt32(strProcessId);
            Process process = Process.GetProcessById(processId);
            HandleRunningInstance(process);
          }
          catch
          {
            FileOperator.SetValue("ProcessId", Process.GetCurrentProcess().Id.ToString()); //进程ID写入文件
            Application.Run(new Form1());
          }
        }
      }
    }
    /// <summary>
    /// 显示已运行的程序。
    /// </summary>
    public static void HandleRunningInstance(Process instance)
    {
      try
      {
        IntPtr formHwnd = FindWindow(null, "计算器");
        ShowWindow(formHwnd, SW_SHOWNORMAL);  //显示
        SetForegroundWindow(formHwnd);     //放到前端
      }
      catch (Exception ex)
      {
        MessageBox.Show(ex.Message);
      }
    }
  }
}

private void Form1_Shown(object sender, EventArgs e)
{
  FileOperator.SetValue("hwnd", Process.GetCurrentProcess().MainWindowHandle.ToString());
}

/// <summary>
/// 显示已运行的程序
/// </summary>
public static void HandleRunningInstance(Process instance)
{
  try
  {
    IntPtr hwnd = new IntPtr(Convert.ToInt32(FileOperator.GetValue("hwnd")));
    ShowWindow(hwnd, SW_SHOWNORMAL); //显示
    SetForegroundWindow(hwnd); //放到前端
  }
  catch (Exception ex)
  {
    MessageBox.Show(ex.Message);
  }
}