ConnectionString 属性尚未初始化。 找错解决方法

ConnectionString 属性尚未初始化。 找错

C# code

  OleDbConnection connection = new OleDbConnection(ConfigurationManager.AppSettings["sql"]);        string sql = "select * from [SalesInfo]";        OleDbDataAdapter da = new OleDbDataAdapter(sql, connection);        DataSet ds = new DataSet();        da.Fill(ds);

XML code

<appSettings>        <add key="sql " value="Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\SalesProjects\App_Data\1.accdb;Persist Security Info=True "/>    </appSettings>