Return datareader from method

Question

I have the following method

    public static SqlDataReader MenuDataReader(string url)
    {
        using (SqlConnection con = new SqlConnection(connectionString))
        {
            using (SqlCommand cmd = new SqlCommand("spR_GetChildMenus", con))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@PageUrl", url);
                cmd.Parameters.AddWithValue("@MenuId", ParameterDirection.Output);
                cmd.Parameters.AddWithValue("@ParentId", ParameterDirection.Output);
                cmd.Parameters.AddWithValue("@TitleText", ParameterDirection.Output);
                cmd.Parameters.AddWithValue("@ExternalUrl", ParameterDirection.Output);
                cmd.Parameters.AddWithValue("@FullUrl", ParameterDirection.Output);
                cmd.Parameters.AddWithValue("@ChildCount", ParameterDirection.Output);
                con.Open();
                SqlDataReader reader = cmd.ExecuteReader();
                if (reader.HasRows)
                {
                    //return reader;

                    while (reader.Read())
                    {
                        return reader;
                    }

                }
            }
        }
        return null;
    }

which im calling like this

        SqlDataReader reader = MenuDataReader(url);
        if (reader.HasRows)
        {
            while (reader.Read())
            { }}

however im getting the error message

Invalid attempt to call HasRows when reader is closed.

can anyone help me out

thanks

Answer 1

Do you really need the reader, or do you just need some way to iterate over the rows inside it? I suggest an iterator block. You can iterate over your rows inside the source method, and yield each row in turn to the caller.

There is a twist with this technique: because you're yielding the same object with each iteration, there are cases where this can cause a problem, and so you're best off also asking for a delegate to copy the contents of the row somewhere. I also like to abstract this to a generic method that can be used for any query, and use the same delegate technique to handle parameter data, like so:

private IEnumerable<T> GetRows<T>(string sql, Action<SqlParameterCollection> addParameters, Func<IDataRecord, T> copyRow)
{
     using (var cn = new SqlConnection("Connection string here"))
     using (var cmd = new SqlCommand(sql, cn)
     {
         cmd.CommandType = CommandType.StoredProcedure;
         addParameters(cmd.Parameters);
         cn.Open();
         using (var rdr = cmd.ExecuteReader())
         {
             while (rdr.Read())
             {
                 yield return copyRow(rdr);
             }
             rdr.Close();
         }
     }
}

public IEnumerable<MenuItem> GetChildMenus(string url)
{
     return GetRows<MenuItem>("spR_GetChildMenus", p =>
     {
         //these lines are copied from your question, but they're almost certainly wrong
         p.AddWithValue("@PageUrl", url);
         p.AddWithValue("@MenuId", ParameterDirection.Output);
         p.AddWithValue("@ParentId", ParameterDirection.Output);
         p.AddWithValue("@TitleText", ParameterDirection.Output);
         p.AddWithValue("@ExternalUrl", ParameterDirection.Output);
         p.AddWithValue("@FullUrl", ParameterDirection.Output);
         p.AddWithValue("@ChildCount", ParameterDirection.Output);
     }, r =>
     {
         return new MenuItem( ... );
     }
 }

Answer 2

When you return inside the using statement the code calls Dispose on the SqlConnection. This closes the DataReader, causing the error.

Answer 3

As seen in https://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand(v=vs.110).aspx :

  public static SqlDataReader ExecuteReader(String connectionString, String commandText,
      CommandType commandType, params SqlParameter[] parameters) {
     SqlConnection conn = new SqlConnection(connectionString);

     using (SqlCommand cmd = new SqlCommand(commandText, conn)) {
        cmd.CommandType = commandType;
        cmd.Parameters.AddRange(parameters);

        conn.Open();
        // When using CommandBehavior.CloseConnection, the connection will be closed when the 
        // IDataReader is closed.
        SqlDataReader reader = cmd.ExecuteReader(CommandBehavior.CloseConnection);

        return reader;
     }
  }

Answer 4

I would not return the reader - the Dispose of your connection and command are closing the connection. I would instead return a representative model of your data.