Question

我有一个提升事件的课程。我希望订阅者能够修改EventArgs中传递的值。

在提升事件的班级中：

    class Factory
    {
    public event EventHandler<MessageReceivedEventArgs> MessageReceived;

    private IServerLib _myObject;

    public void Connect()
    {
        _myObject = new ServerLib();
        _myObject.AddMessageReceivedHandler((short terminal, ref string message, ref short functionNo) =>
        {
            MessageReceivedEventArgs args = new MessageReceivedEventArgs { Terminal = terminal, Message = message, FunctionNo = functionNo };
            MessageReceivedEvent(ref args);
        });
    }

    private void MessageReceivedEvent(ref MessageReceivedEventArgs args)
    {
        EventHandler<MessageReceivedEventArgs> handler = MessageReceived;
        if (handler != null)
        {
            handler(this, args);
        }
    }

    public class MessageReceivedEventArgs : EventArgs
    {
        public short Terminal { get; set; }
        public string Message { get; set; }
        public short FunctionNo { get; set; }
    }
}


interface IServerLib
    {
        void AddMessageReceivedHandler(MessageReceivedEventHandler action);
    }
    public delegate void MessageReceivedEventHandler(short terminal, ref string message, ref short functionNo);

订阅者（恰好是VB）如下所示：

Dim WithEvents _va As MyAssembly.MyClass

Private Sub _va_MessageReceived(sender As Object, e As Factory.MessageReceivedEventArgs) Handles _va.MessageReceived
    Debug.WriteLine($"Message: {e.Message} Terminal: {e.Terminal} Function: {e.FunctionNo}")
    If e.Message = "1" Then
        e.Message = ""
        e.FunctionNo = 0
        Debug.WriteLine("Cancelled")
    End If
End Sub

这会引发事件，但设置e.Message和e.Function似乎没有设置值。我做错了吗？

Answer 1

问题在于使用这一行：

var args = new MessageReceivedEventArgs
           { Terminal = terminal, Message = message, FunctionNo = functionNo };

它将所有变量复制到事件args类。更改它不会在另一端（您添加ref的位置）自动更改它。这不是一个很好的解决方案，但要向您证明这是问题，请在handler(this, args)之后添加：

message = args.Message;
functionNo = args.FunctionNo;

这将导致ref s覆盖这些值。

Answer 2

看来你认为你的＆＃34; ref＆＃34;参数应受上述代码的影响，但事实并非如此。是的，您通过message功能引用MessageReceivedEvent，但随后将其分配给MessageReceivedEventArgs.Message，这是按值而不是通过引用发生的。

结果，当您在VB代码中修改MessageReceivedEventArgs.Message时，message变量不会受到影响（但MessageReceivedEventArgs.Message 当然受到影响），尽管如此你通过引用传递它，因为它应该是。

你应该做的是将MessageReceivedEventArgs的实例直接传递给你的函数（不在该函数中创建）：

private void MessageReceivedEvent(MessageReceivedEventArgs args)
{
    EventHandler<MessageReceivedEventArgs> handler = MessageReceived;
    if (handler != null)
    {
        handler(this, args);
    }
}

Answer 3

首先，您不需要ref才能更改MessageReceivedEventArgs的属性。对象变量只是指向内存中实际对象的指针，因此您已经在更改其属性。如果您需要订户更改引用本身 - 即将指针重置为另一个对象 - 那么您需要使用ref。其次，这种设计很糟糕，因此我不确定我是否理解这个问题。无论如何，我将提供一种正确的方法来提升事件并在此之后消耗更改的值。

class Factory 
{
    public event EventHandler<MessagereceivedEventArgs> MessageReceived;

    void ReceiveMessage(string Message)
    {
        // Do something with the Message

        // Then let your subscribers know that the message has been processed:
        if (MessageReceived != null)
        {
            var ea = new MessageReceivedEventArgs();
            ea.Message = Message;
            // Set ea properties as appropriate 
            MessageReceived(this, ea);
            // Check ea properties for change
            if (ea.Message != Message)
            {
                // A subscriber has changed the message in the MessageReceivedEventArgs
            }
        }
    }
}

似乎无法通过参考传递EventArgs

3 个答案: