如何存储mysql查询的结果

Question

How do I store the results from a mysql query for use in other classes most efficiently?

I've tried the following code, which executes properly and stores all data in reader as it should. Reading the DataReader here works fine if I want to!

public class DatabaseHandler
{
    public void MySqlGetUserByName(string input_username, MySqlDataReader reader)
    {
        try
        {
            _database.Open();
            string query = "SELECT * FROM users WHERE username = '@input'";
            MySqlParameter param = new MySqlParameter(); param.ParameterName = "@input"; param.Value = input_username;
            MySqlCommand command = new MySqlCommand(query, _database);
            command.Parameters.Add(param);
            reader = command.ExecuteReader();
            _database.Close();
        }
        catch(Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }
    }
}

But when I try to read the same DataReader here, it is null and throws an exception (right after Debug6).

public class LoginHandler
{
    public static void UserAuth(Client user, string input_username, string input_password)
    {
        DatabaseHandler dataBase = new DatabaseHandler();
        MySqlDataReader dataReader = null; 
        dataBase.MySqlGetUserByName(input_username, dataReader);

        Console.WriteLine("Debug6");
        if (!dataReader.HasRows)
        {
            user.SendChatMessage("No match found.");
            return;
        }

        while (dataReader.Read())
        {
            user.SetData("ID", (int)dataReader[0]);
            user.SetData("username", (string)dataReader[1]);
            user.SetData("email", (string)dataReader[2]);
            user.SetData("password", (string)dataReader[3]);
        }
        dataReader.Close();
    }
}

Please let me know how to make this work, or if there is a more efficient way of doing this without limiting the function of MySqlGetUserByName. The purpose of it is to input a name and a place to store all info from the match in the database.

Also, feel free to drop in any other suggestions that could make the code more efficient.

Answer 1

You could change your MySqlGetUserByName to return a User instance if all goes well, otherwise you return a null instance to the caller (Or you can thrown an exception, or you can set a global error flag in the DatabaseHandler class..., but to keep things simple I choose to return a null)

public class DatabaseHandler
{
    public User MySqlGetUserByName(string input_username)
    {
        User result = null;
        try
        {
            string query = "SELECT * FROM users WHERE username = @input";
            using(MySqlConnection cnn = new MySqlConnection(......))
            using(MySqlCommand command = new MySqlCommand(query, cnn))
            {
                cnn.Open();
                command.Parameters.AddWithValue("@input", input_username);
                using(MySqlDataReader dataReader = command.ExecuteReader())
                {
                   if (dataReader.Read())
                   {
                       result = new User();
                       result.ID = Convert.ToInt32(dataReader[0]);
                       ..... and so on with the other user properties ....
                   }
                }
            }
        }
        catch(Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }
        // Return the user to the caller. If we have not found the user we return null
        return result;
    }
}

In the same way the caller handles the situation

public class LoginHandler
{
    public static void UserAuth(string input_username, string input_password)
    {
        DatabaseHandler dataBase = new DatabaseHandler();
        User result = dataBase.MySqlGetUserByName(input_username);

        // If we have not found the user we have a null in the variable 
        if(result == null)
        {
            // Send your message using a static method in the user class
            // User.SendMessage("User with username {input_username} not found!");
        }
        else
        {
            // User ok. return it? or do something with its data?
        }
    }
}