如果没有剩余要读取的行，请关闭SqlDataReader-C＃

Question

This is causing me a headache. I know this question (or atleast variants of it) has been asked many times but before one flags it as a duplicate please consider the following code:

    string myConnectionString =  myConnectionString = ConfigurationManager.ConnectionStrings["DBCS"].ToString();
     SqlConnection mySQLConnection;
     SqlCommand mySQLCommand;
     SqlDataReader mySQLDataReader;

    using (mySQLConnection = new SqlConnection(myConnectionString))
    {
       mySQLCommand = new SqlCommand("SELECT TOP 1 * FROM Table ORDER BY Id DESC", mySQLConnection);
       mySQLCommand.Connection = mySQLConnection;
       mySQLCommand.Connection.Open();
       using(mySQLDataReader = mySQLCommand.ExecuteReader())
       {
         if (mySQLDataReader.HasRows)
         {
             if (mySQLConnection.State == ConnectionState.Open)
             {
                while (mySQLDataReader.Read())
                {
                  //Perform Logic : If the last record being returned meets some condition then call the below method
                   MethodCalled();
                }            
            }
         }
      }
         MessageBox.Show("Connection state: " + mySQLConnection.State);
   }

I would like to find a way to either:

1. Close the reader after it has finished reading
2. Break out of the while-loop when it has finished reading and there are no more rows left

But I just keep on getting a SqlException stating the following:

invalid attempt to call read when reader is closed

Just from broad observation, I can trace that error is due to me returning data that contains one row only. The problem is that after it has read that row, the compiler goes back to While(mySQLDataReader.Read()){} and attempts to read through a table that does not contain any rows.

I attempted the following:

1. Wrapping the ExecuteReader() from the command object in a using block so that it automatically closes the reader and the connection respectively once it has done reading like so:

    using(mySQLDataReader = mySQLCommand.ExecuteReader()) { //Logic performed } 

2. Just before the closing brace of the while-loop , I tried checking if there are any more rows left/returned from the sql command and breaking out the loop once that condition is satisfied:

    if(mySQLDataReader.HasRows == false) //No more rows left to read { break; //break out of loop } 

Both attempts were unsuccessful. How can I get around this?

Answer 1

It must be one of the following 3 things:

1. You're using Read() OUTSIDE the using block. Remember that using block will implicitly call Close and Dispose on your reader. Thus any Read() calls must be placed inside the using block.
2. The body of your using block is explicitly closing the reader. This seems improbable.
3. Apparently you have declared your mySQLDataReader at a higher level. It could be that some other (async) code is closing the reader. This also is unlikely. You shouldn't, in most cases, define a DataReader at global level.

Edit

Reading the full code block that you have posted now, I'd suggest a few changes. Can you run the following and tell us if it runs:

using (var mySQLConnection = new SqlConnection(myConnectionString))
{
   mySQLCommand = new SqlCommand("SELECT TOP 1 * FROM Table ORDER BY Id DESC", mySQLConnection, mySQLConnection);
   mySQLCommand.Connection.Open();
   using(mySQLDataReader = mySQLCommand.ExecuteReader())
   {
      while (mySQLDataReader.Read())
      {
        //Perform Logic : If the last record being returned meets some condition then call the below method
         MethodCalled();
      }            
   }
}

If this version runs fine, we can then dig the problem better.

Answer 2

If there is no data to iterate, while loop will not execute at all. Do you need to check for HasRows as such? Also, you should use CommandBehavior.CloseConnection when you are creating data reader. This will make sure that underlying connection is closed once you have read through it.

Should if call SqlDataReader.HasRows if I am calling SqlReader.Read

SQLDataReader Source Code

using (SqlConnection mySQLConnection = new SqlConnection(myConnectionString))
{
using (SqlCommand mySQLCommand = new SqlCommand("SELECT TOP 1 * FROM Table ORDER BY Id DESC", mySQLConnection))
  {
    mySQLConnection.Open();

    SqlDataReader mySQLDataReader = mySQLCommand.ExecuteReader(CommandBehavior.CloseConnection);
     while (mySQLDataReader.Read())
       {
          //Code logic here
        }
        // this call to mySQLDataReader.Close(); will close the underlying connection
         mySQLDataReader.Close();
    }
     MessageBox.Show("Connection state: " + mySQLConnection.State);
}