Id request from the database

I'm trying to write the id request from the database. This is how I wrote it:

public int QueryId(String query)
{
    var temp = this.connection;
    MySqlCommand verifica = new MySqlCommand(query, connection);
    var queryResult = verifica.ExecuteScalar();
    return Convert.ToInt32(verifica.ExecuteScalar());
} 

This is how I make use of the function:

        MySqlDataReader reader = cmd.ExecuteReader();
        while (reader.Read())
        {
            nomeCorrente = reader.GetString("nome");
            cognomeCorrente = reader.GetString("cognome");
            idCorrente = db.QueryId("SELECT id FROM thewishlist.user WHERE email='" + user.Text + "'");
        }

        reader.Close();
        db.CloseConnection();

It does not generate errors, but when I run the project and log out the user gives me the following error:

> MySql.Data.MySqlClient.MySqlException There is already an open DataReader associated with this Connection which must be closed first.

The error is pretty clear. I suggest you make use of using statement and also since you're only returning one column you and use ExcecuteScalar instead of ExecuteReader. So your code will look something like:

var id = 0;
var query = "SELECT ID FROM thewishlist.user WHERE email = @email";
using (var con = new SqlConnection(this.connection))
{
   using (var cmd = new SqlCommand(query, con))
   {
       con.Open();
       cmd.Parameters.Add("@email", SqlDbType.NVarChar).Value = user.Text;

       id = (int)cmd.ExecuteScalar();
    }
}//connection will auto close here and object will get disposed

return id;

Also to prevent sql injection you should always use paramertised sql queries.

As Jason said that, you should close the reader firstly, then call db.QueryId to execute the new query, I modifed your code as follows:

 using (MySqlDataReader reader = cmd.ExecuteReader())
        {               
            while (reader.Read())
            {
                nomeCorrente = reader.GetString("nome");
                cognomeCorrente = reader.GetString("cognome");                 
            }
            reader.Close();              
        }
        idCorrente = db.QueryId("SELECT id FROM thewishlist.user WHERE email='" + user.Text + "'");
        db.CloseConnection();