Concept of handling SqlCommand in C#

问题:

I'm trying to select a list of users from the database and send email to each of every users based on a condition isSent == false. After send email to them, the value of this false should be update to true. Below code is the way i retrieve list of users from database and call method sendEmail() to each of them.

           myConnection.Open();
            //*******
            try
            {
                SqlDataReader myReader = null;
                string sql = "SELECT * FROM testTable where isSent = false";
                SqlCommand myCommand = new SqlCommand(sql, myConnection);
                myReader = myCommand.ExecuteReader();

                while (myReader.Read())
                {
                    sendEmail(myReader["emailAdd"].ToString(), 
                              myReader["UserID"].ToString());
                }

Second part:

public static void sendEmail(string emailAdd,string userID){
    .
    .
    .
    try
    {
        smtpClient.Send(mail);
        try
            {
                string sql = "UPDATE testTable SET isSent = 1 WHERE  UserID = " + userID;
                SqlCommand myCommand = new SqlCommand(sql, myConnection);
                int rows = myCommand.ExecuteNonQuery();
                .
                .
                .
            }
    }
}

The problem I'm facing is that since from main method I already have SqlDataReader being hold to read and so I cant update now. Any work around for me? The error message that I get is as below:

There is already an open DataReader associated with this Command which must be closed first.

回答1:

The problem I'm facing is that since from main method I already have SqlDataReader being hold to read and so I cant update now. Any work around for me?

That's only a problem because you're sharing the connection (myConnection). Don't do that. Create a new SqlConnection every time you want to perform a database operation, and let the connection pool infrastructure handle making it efficient. Also, use using statements for database-related resources:

using (var connection = new SqlConnection(...))
{
    using (var command = new SqlCommand(...))
    {
        using (var reader = command.ExecuteReader(...))
        {
            ...
        }
    }
}

回答2:

As another alternative. You could add this to your connection string for the database.

MultipleActiveResultSets=True

There is a small performance degradation. And its not supported in older SQL Servers. I think pre 2005

回答3:

Try this code..`

 public static void sendEmail(string emailAdd,string userID){
  try
    {
        SqlConnection con = new SqlConnection(@"");
        con.Open();

        MailMessage mail = new MailMessage("[e-mail]", emailAdd);
        SmtpClient smtpClient = new SmtpClient();
        NetworkCredential nc = new NetworkCredential("[e-mail]", "test");

        smtpClient.Port = 587;
        smtpClient.Host = "smtp.gmail.com";
        smtpClient.EnableSsl = true;
        smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
        smtpClient.UseDefaultCredentials = false;
       // smtpClient.Host = "smtp.google.com";
        mail.Subject = "this is a test email.";
        mail.Body = "this is my test email body";
        smtpClient.Credentials = nc;
        smtpClient.Send(mail);

        using (SqlCommand cmd = con.CreateCommand())
        {
            string sql = "UPDATE TestTable SET IsSent = 'true' WHERE  UserID = " + userID;
            SqlCommand myCommand = new SqlCommand(sql, con);
            int rows = myCommand.ExecuteNonQuery();
        }
    }
    catch(Exception e)
    {
        string message = e.Message;
    }`
 }