变量未声明或从未分配？ 和数据读取器无法正常工作？

Question

I didn't want to post this but I couldn't find an answer to solve this problem.

Im making an application for a friend, but when I run it, it gives me a warning, and recently started crashing.

I tried debugging the problem, and it gave me a completely different problem.

class DBApplication : DBInfo
{
    private MySqlConnection connection = new MySqlConnection();
    private int customer_id;
    public string lastname;

    //constructor
    public DBApplication()
    {
        OrderID();
    }

    public string OrderID()
    {
        customer_id = 8;
        //add the DataBase query to the string.
        string query = "SELECT * FROM wdb_customer WHERE=" + customer_id;

        if (OpenConnection() == true)
        {
            MySqlCommand myComm = connection.CreateCommand();
            MySqlDataReader Reader;
            myComm.CommandText = query;
            /*when I debugged, it said this was the problem? 
              but i dont understand why.*/*
            Reader = myComm.ExecuteReader(); 

            while (Reader.Read())
            {
                lastname = Reader["customer_name"].ToString();
            }
            Reader.Close();
        }
        return lastname;
    }
}

the first problem "the variable DB_App is either undeclared or was never assigned."

thats from this script.

partial class Form1 {
    ...
    private DBApplication DB_App = new DBApplication();
    ...
    private void InitializeComponent()
    {
       this.OrderID.Text = DB_App.lastname;
    }

I have also tried DB_App.OrderID(); same problem.

Answer 1

Your query is invalid since you miss a column name in WHERE clause

To apply a quick fix change

string query = "SELECT * FROM wdb_customer WHERE=" + customer_id;

to

string query = "SELECT * FROM wdb_customer WHERE customer_id = " + customer_id;
                                                 ^^^ use real column name here

On a side note: do not build SQL query strings directly from user input; use parameters instead. It prevents your code from SQL injections . Read more here

This being said your code might look like this

...
myComm.CommandText = "SELECT * FROM wdb_customer WHERE customer_id = @customer_id";
myComm.Parameters.AddWithValue("@customer_id", customer_id);
Reader = myComm.ExecuteReader(); 
...