方法一:

/// <summary>
/// 从这里开始运行
/// </summary>
[STAThread]
static void Main()
{
    Process instance = RunningInstance();
    if (instance == null)
    {
        //没有实例在运行
        WeatherApp appInstance = new WeatherApp();
        appInstance.StartMainGui();
    }
    else
    {
        //已经有一个实例在运行
        HandleRunningInstance(instance);
    }
}
#region 确保只有一个实例
public static Process RunningInstance()
{
    Process current = Process.GetCurrentProcess();
    Process[] processes = Process.GetProcessesByName(current.ProcessName);
    //遍历与当前进程名称相同的进程列表
    foreach (Process process in processes)
    {
        //Ignore the current process
        if (process.Id != current.Id)
        {
            //Make sure that the process is running from the exe file.
            if (Assembly.GetExecutingAssembly().Location.Replace("/", "\\") == current.MainModule.FileName)
            {
                //Return the other process instance.
                return process;
            }
        }
    }
    return null;
}
private static void HandleRunningInstance(Process instance)
{
    MessageBox.Show("该应用系统已经在运行！", "提示信息", MessageBoxButtons.OK, MessageBoxIcon.Information);
    ShowWindowAsync(instance.MainWindowHandle, 1); //调用api函数，正常显示窗口
    SetForegroundWindow(instance.MainWindowHandle); //将窗口放置最前端。
}
[DllImport("User32.dll")]
private static extern bool ShowWindowAsync(System.IntPtr hWnd, int cmdShow);
[DllImport("User32.dll")]
private static extern bool SetForegroundWindow(System.IntPtr hWnd);
#endregion