读取器关闭时无效尝试调用Read？

Question

我正在运行DbDataReader查询以从下拉列表中删除项目,如果它们已经附加到特定提交,并且我一直收到错误,告诉我读者已关闭.无法理解为什么我的读者被视为关闭在这里.我错过了什么？

 protected void Page_Load(object sender, EventArgs e)
{

    string x = Request.QueryString["SubId"];
    string connectionString = System.Configuration.ConfigurationManager.
        ConnectionStrings["MyConnectionString"].ConnectionString;
    string displayQuery = "SELECT CustName, CustAdd, CustCity, CustState, " +
        "CustZip FROM Customer WHERE SubId =" + x;
    string broQuery = "SELECT EntityType FROM Broker WHERE SubId =" + x;
    string ddlQuery = "SELECT ProductId FROM SubmissionProducts " +
        "WHERE SubmissionId =" + x;
    using (SqlConnection displayConn = new SqlConnection(connectionString))
    {
        displayConn.Open();
        SqlCommand DlistCmd = new SqlCommand(ddlQuery, displayConn);

        using (SqlDataReader Ddldr = DlistCmd.ExecuteReader())
        {
            while (Ddldr.Read())
            {

                switch (Ddldr.GetInt32(0))
                {
                    case 1:
                        DdlProductList.Items.RemoveAt(1);
                        break;
                    case 2:
                        DdlProductList.Items.RemoveAt(2);
                        break;
                    case 3:
                        DdlProductList.Items.RemoveAt(3);
                        break;
                    case 4:
                        DdlProductList.Items.RemoveAt(4);
                        break;
                    case 5:
                        DdlProductList.Items.RemoveAt(5);
                        break;
                    case 6:
                        DdlProductList.Items.RemoveAt(6);
                        break;
                    case 7:
                        DdlProductList.Items.RemoveAt(7);
                        break;
                    default:
                        break;
                }
                Ddldr.Close();
            }

        }

Answer 1

不要打电话Ddldr.Close();,特别是在里面while.这样你就可以进行第一次迭代,关闭阅读器,第二次迭代当然会在阅读器关闭时进行.该using声明将照顾它.只需从代码中删除此行即可.

所以:

using (SqlDataReader Ddldr = DlistCmd.ExecuteReader())
{
    while (Ddldr.Read())
    {
        switch (Ddldr.GetInt32(0))
        {
            ... your cases here
            default:
                break;
        }
    }
}

还有以下几行:

string x = Request.QueryString["SubId"];
string displayQuery = "SELECT CustName, CustAdd, CustCity, CustState, CustZip FROM Customer WHERE SubId =" + x;
string broQuery = "SELECT EntityType FROM Broker WHERE SubId =" + x;
string ddlQuery = "SELECT ProductId FROM SubmissionProducts WHERE SubmissionId =" + x;

像一堆s..t.您应该使用参数化查询,绝对不会编写任何类似的代码,否则您的应用程序将容易受到SQL注入攻击.每次在编写SQL查询时都使用字符串连接,警报应该响铃告诉您错误.

所以这是正确的方法:

protected void Page_Load(object sender, EventArgs e)
{
    string x = Request.QueryString["SubId"];
    string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;
    using (var conn = new SqlConnection(connectionString))
    using (var cmd = conn.CreateCommand())
    {
        conn.Open();
        cmd.CommandText = "SELECT ProductId FROM SubmissionProducts WHERE SubmissionId = @SubmissionId";
        cmd.Parameters.AddWithValue("@SubmissionId", x)

        using (var reader = cmd.ExecuteReader())
        {
            while (Ddldr.Read())
            {
                switch (reader.GetInt32(reader.GetOrdinal("ProductId")))
                {
                    ... your cases here
                    default:
                        break;
                }
            }

        }
    }
}