程序代码：

/// <summary>
    /// 尝试单例模式
    /// </summary>
    public class Db
    {
        /// <summary>
        /// 需要的数据库
        /// </summary>
        private  static SPMEntities db = new SPMEntities();
        
        private static Db instance;

        /// <summary>
        /// 私有的构造方法
        /// </summary>
        private Db()
        {
            
        }
        public static void GetInstance()
        {
            if(instance==null)
            {
                instance = new Db();
            } 
        }
        public static SPMEntities GetDb()
        {
            GetInstance();
            return db ;
        }