Question

我不想发布这个，但我找不到解决这个问题的答案。

我正在为朋友制作一个应用程序，但是当我运行它时，它会给我一个警告，并且最近开始崩溃。

我尝试调试问题，它给了我一个完全不同的问题。

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;
    }
}

第一个问题“变量DB_App未声明或从未分配。”

来自这个脚本。

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

我也试过DB_App.OrderID（）;同样的问题。

Answer 1

您的查询无效，因为您错过了WHERE子句

中的列名称

应用快速修正更改

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

到

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

旁注：不要直接从用户输入构建SQL查询字符串;改用参数。它会阻止您的代码进行SQL注入。阅读更多here

据说你的代码看起来像这样

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

变量未申报或从未分配？和数据阅读器不工作？

1 个答案: