“已经有一个打开的 DataReader 与此命令关联，必须先关闭它。”

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、阅读器等。

我需要执行一些查询，例如首先我需要获取某个用户的 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 

那么伙计们，我怎样才能使用这个示例执行一些查询语句。

非常感谢！ 干杯

Answer 1

您打开的阅读器仍然处于活动状态并处于打开状态。而且您一次只能有一个活跃的读者。您应该将所有 Sql... 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...您收到错误是因为第一次调用使用的阅读器未关闭。使用完 DataReader 对象后，您应该始终调用 Close 方法，以确保读取器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();
}

另外...作为一个干净的代码规则，用不同的方法分隔你的调用（SOLID 原则）

Answer 3

你的问题是你没有处理你正在使用的对象。为此目的，最好始终使用using 结构，因为它可以保证你所有的东西都会被处理掉。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