EnterpriseLibrary dbCommand.ExecuteScalarAsync错误：操作无效。连接已关闭

Question

我使用EnterpriseLibrary使用异步方法访问数据库。但这是错误的：＆＃34;操作无效。连接已关闭。＆＃34;

我的代码：

        public Database CreateDatabase()
    {
        if (null == this._createDatabase)
        {
            this._createDatabase = new DatabaseProviderFactory().Create("ConnectionString");
        }
        return this._createDatabase;
    }


        public async Task<bool> IsExistCode(string code)
    {
        Database db = base.CreateDatabase();
        using (DbCommand dbCommand = db.GetSqlStringCommand("select top 1 1 from Ads  WITH(NOLOCK)  where Code=@Code"))
        {
            db.AddInParameter(dbCommand, "Code", DbType.AnsiString, code);
            var result = await dbCommand.ExecuteScalarAsync();//error occurrence
            return null != result;
        }
    }

ps：sync方法（dbCommand.ExecuteScalar（））没问题。

注意：我的使用方式有误，请参阅：https://msdn.microsoft.com/zh-cn/library/hh211418.aspx