我在用C＃语言执行3层项目时，尝试关闭阅读器时发生无效尝试调用Read的错误。我想做的是通过将两个表连接在一起来检索地址数据列，并显示在下拉列表中。这是我的数据访问层：

    public List<Distribution> getDistributionAll()
    {
        List<Distribution> distributionAll = new List<Distribution>();
        string address;
        SqlDataReader dr = FoodBankDB.executeReader("SELECT b.addressLineOne FROM dbo.Beneficiaries b INNER JOIN dbo.Distributions d ON d.beneficiary = b.id");

        while (dr.Read())
        {
            address = dr["addressLineOne"].ToString();
            distributionAll.Add(new Distribution(address));
        }

        return distributionAll;
    }

这是我的FoodBankDB类：

   public class FoodBankDB
{
    public static string connectionString = Properties.Settings.Default.connectionString;
    public static SqlDataReader executeReader(string query)
    {
        SqlDataReader result = null;
        System.Diagnostics.Debug.WriteLine("FoodBankDB executeReader: " + query);
        SqlConnection connection = new SqlConnection(connectionString);
        SqlCommand command = new SqlCommand(query, connection);
        connection.Open();
        result = command.ExecuteReader();
        connection.Close();
        return result;
    }

我将它们分为两个类，以便每当更改连接字符串时，都可以通过更改FoodBankDB类轻松修改整个项目。

这是我的业务逻辑层：

public List<Distribution> getAllScheduledDistribution()
    {
        List<Distribution> allDistribution = new List<Distribution>();
        Distribution distributionDAL = new Distribution();
        allDistribution = distributionDAL.getDistributionAll();
        return allDistribution;
    }

最后但并非最不重要的一点是，我的表示层：

List<Distribution> scheduledList = new List<Distribution>();
scheduledList = packBLL.getAllScheduledDistribution();
ddlScheduleList.DataSource = scheduledList;
ddlScheduleList.DataTextField = "address";
ddlScheduleList.DataValueField = "address";
ddlScheduleList.DataBind();

如果我不拆分数据访问层和连接字符串类，则效果很好。有人知道如何解决此错误吗？

提前致谢。

更新部分

        public static string GetConnectionString()
    {
        return connectionString;
    }