C# 构造函数事件

Question

我创建了一个类，当我通过表单创建一个员工对象时，我想给一个消息；

这是我的班级、活动和代表

public delegate void ctorDel(); 

class Employee
{
    private int empID;
    private string empName;

    public event ctorDel myEvent;

    public Employee(int empID,string empName)
    {
        this.empID = empID;
        this.empName = empName;

        **if (myEvent != null)
        {
            myEvent();
        }**
    }

形式上

  int id = Convert.ToInt16(textBox1.Text);
            string name = textBox2.Text;

            Employee emp = new Employee(id, name);
            emp.myEvent += new ctorDel(showMessage);

和功能

 public void showMessage()
        {
            MessageBox.Show("An employee is created");
        }

Answer 1

你想要完成什么？您尝试的原因不起作用是因为您在 ctor 之后附加了您的代表。一旦您调用了“新员工”，该事件早就被解雇了。

如果你真的需要这样的事件，创建一个工厂类：

public delegate void EmpCreated();
public EmployeeFactory {
  public event EmpCreated myEvent;
  public Employee Create(int empId, string empName){
    var result = new Employee(empId, empName);
    if(myEvent != null) myEvent();
    return result;
  }
}

在工厂类上订阅事件，您将获得该事件。

Answer 2

您在构造函数已经运行之后附加事件。

Answer 3

在构造函数中引发实例事件是没有意义的，因为由于实例的初始化还没有完成，所以不能有任何处理程序附加到事件......

但是，您可以创建一个静态事件：

public static event ctorDel myEvent;

...

Employee.myEvent += new ctorDel(showMessage);

（但不要在每次创建Employee 时都订阅该事件，否则处理程序将被调用多次...）

Answer 4

解决方案

这是解决问题的通用方法

public class EventFactory
{
    public U Create<U, V>(V constructorArgs)
    {
        var instance = (U)Activator.CreateInstance(typeof(U), constructorArgs);
        OnCreated?.Invoke();
        return instance;
    }

    public delegate void CreatedEventHandler();
    public event CreatedEventHandler OnCreated;
}

你可以这样做

var ef = new EventFactory();
ef.OnCreated += myEventHandler; 
var instance = ef.Create<Employee>(employeeArgs);

.. 更进一步

当您需要传递事件参数或构造函数无参数时，可以调整我的代码以提供更大的灵活性。我还没有测试过它，但它应该看起来像

public class EventFactory<T>
{
    public U Create<U, V>(V constructorArgs, T eventArgs)
    {
        var instance = (U)Activator.CreateInstance(typeof(U), constructorArgs);
        OnCreated?.Invoke(eventArgs);
        return instance;
    }

    public U Create<U>(T eventArgs)
    {
        return Create<U, object>(null, eventArgs);
    }

    public delegate void CreatedEventHandler(T args);
    public event CreatedEventHandler OnCreated;
}

public class EventFactory
{
    public U Create<U, V>(V constructorArgs)
    {
        var instance = (U)Activator.CreateInstance(typeof(U), constructorArgs);
        OnCreated?.Invoke();
        return instance;
    }

    public U Create<U>() where U : new()
    {
        var instance = new U();
        OnCreated?.Invoke();
        return instance;
    }

    public delegate void CreatedEventHandler();
    public event CreatedEventHandler OnCreated;
}

Answer 5

创建Employee时可以传递handler：

private Employee(ctorDel construcEvent)
{
    if (construcEvent != null)
        this.myEvent += construcEvent;
}

public Employee(int empID,string empName, ctorDel construcEvent)
    : this(construcEvent)
{
    this.empID = empID;
    this.empName = empName;

    if (myEvent != null)
    {
        myEvent();
    }
}

然后：

Employee emp = new Employee(id, name, new ctorDel(showMessage));

Answer 6

当您订阅此事件时，实例已经构建完毕。 我建议使用工厂模式来隐藏构造函数。

class EmployeeFactory
{
     public Employee Create(int id, string name)
     {
         Employee instance = new Employee(id, name);

         var handler = EmployeeCreated;
         if (handler != null)
         {
              EmployeeEventArgs e = new EmployeeEventArgs(instance);
              handler(e);    
         }

         return instance;
     }

     public event EventHandler<EmployeeEventArgs>  EmployeeCreated;
}

事件订阅：

  factory.EmployeeCreated += MyHandler;

实例构造：

  var emp = factory.Create(id, name);