oracle OracleDataReader 出错。错误：无效的操作。连接已关闭
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Error with OracleDataReader. Error: Invalid operation. The connection is closed
提问by Alek
When I try to assign the reader C# throws an exception:
当我尝试分配读取器时，C# 抛出异常：
Invalid operation. The connection is closed
Invalid operation. The connection is closed
I try to get a result from a query that returns a single cell with an average value inside. cmd is an oraclecomand that i use to insert a row into a table and so far so good. I see the message box next and after that the exception appears.
我尝试从返回单个单元格的查询中获取结果，该单元格内部具有平均值。cmd 是一个 oraclecomand，我用来在表中插入一行，到目前为止一切顺利。我接下来看到消息框，然后出现异常。
try
{
cmd.ExecuteNonQuery();
MessageBox.Show("Recipe Rated");
OracleCommand cm = new OracleCommand("select round(avg(rating),1) from rates where id_rec = "+id);
OracleDataReader reader = cm.ExecuteReader();
reader.Read();
textBox5.Text =""+reader.GetInt16(0);
}
回答by Tim Schmelter
You should open the connection and you should also use sql-parameters. Hopefully this is the correct oracle syntax because i cannot test it:
您应该打开连接，并且还应该使用 sql-parameters。希望这是正确的 oracle 语法，因为我无法测试它：
using(var con = new OracleConnection("ConnectionString Here"))
using(var cmd = new OracleCommand("ADD YOUR INSERT/UPDATE/DELETE", con))
{
con.Open();
cmd.ExecuteNonQuery();
using (var cm = new OracleCommand("select round(avg(rating),1)As AvgRating from rates where id_rec = @id", con))
{
cm.Parameters.AddWithValue("@id", id);
using (var reader = cm.ExecuteReader())
{
if (reader.Read())
{
textBox5.Text = reader.GetInt16(0).ToString();
}
}
}
}
Note that i have used the using-statement to ensure that all unmanaged resources are disposed as soon as possible. It also closes connections (even on error).
请注意，我使用了 -using语句来确保尽快处理所有非托管资源。它还关闭连接（即使出错）。
Edit: Since you are selecting just a single value i suggest to use ExecuteScalar:
编辑：由于您只选择一个值，我建议使用ExecuteScalar：
using (var cm = new OracleCommand("select round(avg(rating),1)As AvgRating from rates where id_rec = @id", con))
{
cm.Parameters.AddWithValue("@id", id);
object avgRating = cm.ExecuteScalar();
if (!(avgRating is DBNull))
{
textBox5.Text = avgRating.ToString();
}
}
回答by FIre Panda
When you use `OracleCommand', you have to associate a valid OracleConnectionobject to it.
当您使用`OracleCommand' 时，您必须将一个有效的OracleConnection对象与其关联。
using (OracleConnection connection = new OracleConnection(connectionString))
{
MessageBox.Show("Recipe Rated");
OracleCommand cm = new OracleCommand("select round(avg(rating),1) from rates where id_rec = "+id);
try
{
cm.Connection = connection;
connection.Open(); //oracle connection object
OracleDataReader reader = cm.ExecuteReader();
reader.Read();
textBox5.Text =""+reader.GetInt16(0);
}
}
Hope this help.
希望这有帮助。
Thanks.
谢谢。
