Question

我正在开发一个类来管理Mysql数据库上的操作。我有以下代码：

 using System;
using MySql.Data.MySqlClient;

public class MysqlAccess
{
    private MySqlConnection pCnn;
    public enum OperationType {Select = 1,Insert = 2,Update = 3,Delete = 4};

    public MysqlAccess()
    {

        MySqlConnection Connection=new MySqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["LocalMySqlServer"].ConnectionString);


    }

   public MySqlConnection Connection { get {return pCnn;} set {pCnn=value;} }


    public int Connect()
    {
        try
        {

            Connection.Open();

            if (Connection.State == System.Data.ConnectionState.Open)
            {
                return 1;
            }
            else
            {
                return 0;
            }

        }
        catch (Exception e)
        {
            return -1;

        }
    }



    }

}

页面加载代码

 protected void Page_Load(object sender, EventArgs e)
    {

        MysqlAccess DBManager = new MysqlAccess();


        DBManager.Connect();
        Response.Write("Connection State:" + DBManager.Connection.State.ToString());
    }

当我执行response.write连接为空时，为什么？

提前致谢！

Answer 1

嗯，它是null，因为你从未真正初始化Connection属性，在初始化之前它将为null。所以而不是：

public MysqlAccess()
{
    // Here you are initializing a local variable 
    // that is subject to GC and goes into the void of forgetfulness
    MySqlConnection Connection=new MySqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["LocalMySqlServer"].ConnectionString);
}

初始化属性：

public MysqlAccess()
{
    var connectionString = ConfigurationManager.ConnectionStrings["LocalMySqlServer"].ConnectionString;
    // The 'this' keyword is optional here and its usage is a 
    // matter of personal preference
    this.Connection = new MySqlConnection(connectionString);
}

虽然这可能会修复你所获得的NullReferenceException，但你应该知道MySqlConnection实现了IDisposable，这意味着你应该确保调用Dispose方法来避免泄漏连接或在每个请求上创建新连接，这在Web应用程序中可能特别具有灾难性。

Answer 2

在构造函数中，您没有设置属性Connection。您正在声明一个局部变量Connection。所以，你从来没有初始化属性Connection。

使用以下代码更正代码：

this.Connection=new MySqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["LocalMySqlServer"].ConnectionString);

Answer 3

因为您在此处将Connection声明为局部变量：

public MysqlAccess()
{

    MySqlConnection Connection=new MySqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["LocalMySqlServer"].ConnectionString);


}

尝试

public MysqlAccess()
{

    Connection=new MySqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["LocalMySqlServer"].ConnectionString);


}

Answer 4

将构造函数更改为：

public MysqlAccess()
{
    Connection=new MySqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["LocalMySqlServer"].ConnectionString);
}

Answer 5

在MysqlAccess构造函数中，您指定一个新的局部变量：

MySqlConnection Connection=new MySqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["LocalMySqlServer"].ConnectionString);

...稍后您将引用一个属性，该属性恰好与本地变量同名：

Response.Write("Connection State:" + DBManager.Connection.State.ToString());

将构造函数中的MySqlConnection Connection更改为Connection应修复它。

C＃连接变为空

5 个答案: