Why I get there is already an open datareader associated with this connection which must be closed first error?

Question

try
{
        string log = "SELECT * from rfidprototype.account where username ='" + USERNAME + "' and password = '" + PASS + "';";

        MySqlCommand cmd = new MySqlCommand(log, SQLconn);
        MySqlDataReader dRead;

        dRead = cmd.ExecuteReader();

        if (DRead.Read())
        {
            MenuHere form = new MenuHere();

            form.Show();
            form.ManageTile.Enabled = false;

            SQLconn.Close();
            DRead.Close();
        }
        else
        {
            MessageBox.Show("Incorrect Username or Password!");
            DiriLogin form = new DiriLogin();
            form.ShowDialog();
        }
    }
    catch(Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}

Answer 1

In your case, you try to close your connection while you read your reader.

Instead of using Close or Dispose methods manually, use using statement to dispose your connection, command and reader automatically.

But more important, you should always use parameterized queries. This kind of string concatenations are open for SQL Injection attacks.

And do not store your passwords as a plain text. Read: Best way to store password in database

using(var SQLconn = new MySqlConnection(conString))
using(var cmd = SQLconn.CreateCommand())
{
    // Set your CommandText property.
    using(var dRead = cmd.ExecuteReader())
    {
        // Do your stuff
    }
}

As a last thing, PASSWORD is a reserved keyword in MySQL. You might need to use it as between ` characters depending your database manager is case sensitive or not.