Error with OracleDataReader. Error: Invalid operation. The connection is closed

问题

When I try to assign the reader C# throws an exception:

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.

          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1:

When you use `OracleCommand', you have to associate a valid OracleConnection object to it.

  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.

Thanks.

回答2:

You should open the connection and you should also use sql-parameters. Hopefully this is the correct oracle syntax because i cannot test it:

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).

Edit: Since you are selecting just a single value i suggest to use 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();
    }
}

来源：https://stackoverflow.com/questions/17062954/error-with-oracledatareader-error-invalid-operation-the-connection-is-closed

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winforms

Oracle