SqlDataReader hangs on Dispose()

问题

I use the following approach to execute queries over database and read data:

using(SqlConnection connection = new SqlConnection("Connection string"))
{
    connection.Open();

    using(SqlCommand command = new SqlCommand("SELECT * FROM TableName", connection))
    {
        using (SqlDataReader reader = command.ExecuteReader())
        {
              // read and process data somehow (possible source of exceptions)
        } // <- reader hangs here if exception occurs
    } 
}

While reading and processing data some exceptions can occur. The problem is when exception is thrown DataReader hangs on Close() call. Do you have any ideas why??? And how to solve this issue in a proper way? The problem has gone when I wrote try..catch..finally block instead of using and called command.Cancel() before disposing the reader in finally.

Working version:

    using(SqlConnection connection = new SqlConnection("Connection string"))
    {
        connection.Open();

        using(SqlCommand command = new SqlCommand("SELECT * FROM TableName", connection))
        {
            SqlDataReader reader = command.ExecuteReader();
            try
            {
                // read and process data somehow (possible source of exceptions)
            }
            catch(Exception ex)
            {
                // handle exception somehow
            }
            finally
            {
               command.Cancel(); // !!!
               reader.Dispose();
            }
        } 
    }

回答1:

When an exception occurs you stop processing data before all data is received. You can reproduce this issue even without exceptions if you abort processing after a few rows.

When the command or reader is disposed, the query is still running on the server. ADO.NET just reads all remaining rows and result sets like mad and throws them away. It does that because the server is sending them and the protocol requires receiving them.

Calling SqlCommand.Cancel sends an "attention" to SQL Server causing the query to truly abort. It is the same thing as pressing the cancel button in SSMS.

To summarize, this issue occurs whenever you stop processing rows although many more rows are inbound. Your workaround (calling SqlCommand.Cancel) is the correct solution.

回答2:

About the Dispose method of the SqlDataReader, MSDN (link) has this to say:

Releases the resources used by the DbDataReader and calls Close.

Emphasis added by me. And if you then go look at the Close method (link), it states this:

The Close method fills in the values for output parameters, return values and RecordsAffected, increasing the time that it takes to close a SqlDataReader that was used to process a large or complex query. When the return values and the number of records affected by a query are not significant, the time that it takes to close the SqlDataReader can be reduced by calling the Cancel method of the associated SqlCommand object before calling the Close method.

So if you need to stop iterating through the reader, it's best to cancel the command first just like your working version is doing.

回答3:

I would not format it that way.
Open(); is not in a try block and it can throw an exception
ExecuteReader(); is not in a try block and it can throw an exception
I like reader.Close - cause that is what I see in MSDN samples
And I catch SQLexception as they have numbers (like for timeout)

SqlConnection connection = new SqlConnection();
SqlDataReader reader = null;
try
{
    connection.Open();  // you are missing this as a possible source of exceptions
    SqlCommand command = new SqlCommand("SELECT * FROM TableName", connection);
    reader = command.ExecuteReader();  // you are missing this as a possible source of exceptions
    // read and process data somehow (possible source of exceptions)
}
catch (SqlException ex)
{
}
catch (Exception ex)
{
    // handle exception somehow
}
finally
{
    if (reader != null) reader.Close();
    connection.Close();
}

来源：https://stackoverflow.com/questions/19686488/sqldatareader-hangs-on-dispose

标签

.net

sqldatareader