"There is already an open DataReader associated with this Command which must be closed first."

Question

我正在开发需要连接到另一个数据库以获取一些数据的应用程序，为此，我决定使用 SqlConnection、reader 等。

而且我需要执行一些查询，例如首先我需要为某个用户获取 CARD ID，然后我需要通过该 CARD ID 获取一些数据..

这是我的代码：

#region Connection to another Database

SqlConnection sqlConnection1 = new SqlConnection("Data Source=ComputerOne; Initial Catalog=TestDatabase;Integrated Security=False; User ID=test; Password=test123;");
SqlCommand cmd = new SqlCommand();
SqlDataReader reader;

cmd.CommandText = "Select * From Users Where CardID=" + "'" + user.CardID + "'";
cmd.CommandType = CommandType.Text;
cmd.Connection = sqlConnection1;

sqlConnection1.Open();

reader = cmd.ExecuteReader();

string cardID = "";
string quantity="";

while (reader.Read())
{
    cardID = reader["CardID"].ToString();
}
//HOW COULD I WRITE ANOTHER QUERY NOW, FOR EXAMPLE, OK I GOT CARDID NOW GIVE ME SOME OTHER THINGS FROM THAT DATABASE BY THAT cardID
//here I tried to change CommandText and to keep working with reader.. but its not working like this because its throwing me exception mention in question title.

cmd.CommandText = "Select T1.CardID, T2.Title, Sum(T1.Quantity) as Quantity From CardTransactions as T1 JOIN Adds as T2 ON T1.AddsID = T2.AddsID Where T1.CardID =" + cardID + "AND T1.Type = 1 Group By T1.CardID, T2.Title";

reader = cmd.ExecuteReader();

while (reader.Read())
{
    quantity = reader["Quantity"].ToString();
}

// Data is accessible through the DataReader object here.

sqlConnection1.Close();

#endregion

伙计们，我如何使用这个例子执行一些查询语句。

非常感谢！干杯

Answer 1

您打开的reader仍然有效并打开。您一次只能有一个活动 reader。您应该将所有 Sql... 个实例包装在 using 中以确保它们正确关闭。

using (SqlConnection connection = new SqlConnection(...))
{
    using (SqlDataReader reader = cmd.ExecuteReader())
    {
        // the code using reader
    }
}

Answer 2

嗯...您收到错误消息是因为第一个调用所用的 reader 未关闭。当您使用完 DataReader 对象时，您应该始终调用 Close 方法，以确保 reader 使用的连接返回到连接池（该连接仅由该 DataReader 使用）。部分代码：

reader = cmd.ExecuteReader();
try
{
  while(myReader.Read()) 
  {
    while (reader.Read())
    {
        cardID = reader["CardID"].ToString();
    }
}
finally
{
  myReader.Close();
}
...
reader = cmd.ExecuteReader();
try
{
  while(myReader.Read()) 
  {
        reader = cmd.ExecuteReader();

        while (reader.Read())
        {
            quantity = reader["Quantity"].ToString();
        }
  }
}
finally
{
  myReader.Close();
  myConnection.Close();
}

此外...作为一个干净的代码规则，将您的调用分开在不同的方法中（SOLID 原则）

Answer 3

你的问题是你没有处理你正在使用的对象。为此，最好始终使用 using 结构，因为它可以保证您将处理所有内容。试试下面的代码：

SqlConnection sqlConnection1 = new SqlConnection("Data Source=ComputerOne; Initial Catalog=TestDatabase;Integrated Security=False; User ID=test; Password=test123;");
SqlCommand cmd = new SqlCommand();
SqlDataReader reader;
string cardID = "";
string quantity="";

using(sqlConnection1 = new SqlConnection("Data Source=ComputerOne; Initial Catalog=TestDatabase;Integrated Security=False; User ID=test; Password=test123;"))
{
    sqlConnection1.Open();

    using(cmd = new SqlCommand())
    {
        cmd.CommandText = "Select * From Users Where CardID=" + "'" + user.CardID + "'";
        cmd.CommandType = CommandType.Text;
        cmd.Connection = sqlConnection1;

        using(reader = cmd.ExecuteReader())
        {


            while (reader.Read())
            {
                cardID = reader["CardID"].ToString();
            }
        } //reader gets disposed right here
    } //cmd gets disposed right here

    using(cmd = new SqlCommand())
    {
        cmd.CommandText = "Select T1.CardID, T2.Title, Sum(T1.Quantity) as Quantity From CardTransactions as T1 JOIN Adds as T2 ON T1.AddsID = T2.AddsID Where T1.CardID =" + cardID + "AND T1.Type = 1 Group By T1.CardID, T2.Title";
        cmd.CommandType = CommandType.Text;
        cmd.Connection = sqlConnection1;

        using(reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                quantity = reader["Quantity"].ToString();
            }
        } //reader gets disposed right here
    } //cmd gets disposed right here
    sqlConnection1.Close();
} //sqlConnection1 gets disposed right here

"There is already an open DataReader associated with this Command which must be closed first."

"There is already an open DataReader associated with this Command which must be closed first."

c#

sql

database

connection

reader