SqlConnection conn=new Sqlconnection();
conn.ConnectionString = ConfigurationSettings.AppSettings.Get("connecionString");
SqlCommand cmd......

------解决方案--------------------
C# code
#region 连接字符串
        const string CONNSTRING = "server=.;database=Library;Trusted_connection=true";
        static SqlConnection conn = new SqlConnection(CONNSTRING);
        public static SqlConnection Conn
        {
          get { return DBsql.conn; }
        }
        #endregion

        #region 打开连接
        public static void Open()
        {
            if (conn.State == ConnectionState.Closed)
            {
                conn.Open();
            }
            else if(conn.State== ConnectionState.Broken)
            {
                conn.Close();
                conn.Open();
            }
        }
        #endregion

        #region 关闭连接
        public static void Close()
        {
            if (conn.State == ConnectionState.Closed || conn.State == ConnectionState.Broken)
            {
                conn.Close();
            }
        }
        #endregion
public static void Ti(string sql,ref SqlDataReader reader)
        {
            SqlCommand comm = new SqlCommand(sql, conn);
            Open();
            reader = comm.ExecuteReader(CommandBehavior.CloseConnection);
        }