c#-为什么此委托分配不起作用以及如何解决呢？

我有以下示例代码：

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.del = this.EventHandler; <-- This does not build
    }

    public void EventHandler(object sender, PropertyChangedEventArgs e)
    {

    }

    public delegate void A(object sender, EventArgs e);

    private A del;
}

它抱怨委托分配.我怀疑这是由于协方差/协方差问题引起的,说实话,这是我不完全了解的概念.

有什么办法可以构建此代码？我需要一个具有任何事件args的事件的通用处理程序(继承自EventArgs).

解决方法:

想象一下,该行已编译：

        this.del = this.EventHandler;

然后,这是一个问题：

        // since del must accept ANY EventArgs descendant, this should be possible:
        del.Invoke(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add));
        // since del's signature IS (object, PropertyChangedEventArgs), we must convert 
        // NotifyCollectionChangedEventArgs to PropertyChangedEventArgs
        // OOOPS...

I need a generic handler for any event with any event args

使用具有此签名的处理程序：

public void EventHandler(object sender, EventArgs e)
{

}


// compiles just fine
this.Loaded += EventHandler;
this.Closing += EventHandler;
this.Drop += EventHandler;

标签：delegates,covariance,contravariance,c,net
来源： https://codeday.me/bug/20191120/2040326.html