using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Data;using System.Data.OleDb;namespace AccessDataBase{    class DBManager    {        const string connStr = @"Provider=Microsoft.Jet.OLEDB.4.0;data source=db\test.mdb";        private static DBManager db;        private DBManager()        {        }        public static DBManager getDBManager()        {            if (db == null)                db = new DBManager();            return db;        }        /// 
        /// 返回指定key的值        /// 
        ///         /// 
    
     对应key的值，如果没有对应的值返回null
            public String QueryByKey(string key)        {            using (OleDbConnection conn = new OleDbConnection(connStr))            {                conn.Open();                OleDbCommand command = new OleDbCommand("select [v] from confirm where [k]=@key", conn);                command.Parameters.Add("@key", key);                OleDbDataReader reader =  command.ExecuteReader();                using (reader)                {                    if (reader.Read())                    {                        return reader.GetString(0);     // 返回key值                    }                    else                    {                        return null;                    }                }            }        }        /// 
        /// 将键值对插入confirm表中，如果表中已经存在key，则抛出KeyHasExistException        /// 
        ///         ///         public void Insert(string key, string value)        {            using (OleDbConnection conn = new OleDbConnection(connStr))            {                conn.Open();                // 检查key是否已经存在                OleDbCommand command = new OleDbCommand("SELECT [k] from confirm where k=@key", conn);                command.Parameters.Add("@key", key);                OleDbDataReader reader = command.ExecuteReader();                // key不存在                if (!reader.Read())                {                    string sql = String.Format("INSERT INTO confirm(k,v) VALUES('{0}','{1}')", key, value);                    command = new OleDbCommand(sql,conn);                    Console.WriteLine(command.CommandText);                    command.ExecuteNonQuery();                }                else                {                    throw new KeyHasExistException("数据库已存在该key");   // key存在抛出异常                }            }        }        /// 
        /// 更新给定的键值对，如果confirm没有记录，则加入一条新的记录        /// 
        ///         ///         public void Upadte(string key, string value)        {            using (OleDbConnection conn = new OleDbConnection(connStr))            {                conn.Open();                // 判断是否存在key                OleDbCommand command = new OleDbCommand("SELECT [k] from confirm where [k]=@key", conn);                command.Parameters.Add("@key",key);                OleDbDataReader reader = command.ExecuteReader();                // key存在                if (reader.Read())                {                    string sql = string.Format("UPDATE confirm SET [k]='{0}', [v]='{1}' WHERE k='{2}'", key, value, key);                    command = new OleDbCommand(sql, conn);                    Console.WriteLine(command.CommandText);                    command.ExecuteNonQuery();                }                else                {                    Insert(key, value);                }            }        }    }    class KeyHasExistException : Exception    {        public KeyHasExistException(String msg)            : base(msg)        {        }    }}