C# - 为什么使用 EventHandler 时出错<t> ？</t>

Question

做一个 Pluralsight 视频，找不到为什么不对。

关于这一点的错误消息：

worker.WorkPerformed += new EventHandler<WorkPerformedEventArgs>(worker_WorkPerformed);

错误信息：

无法将类型“System.EventHandler<AB_Events.WorkPerformedEventArgs>”隐式转换为“AB_Events.WorkPerformedHandler”

片段

public delegate int WorkPerformedHandler(object sender,WorkPerformedEventArgs e);
public class Worker
{
    public event WorkPerformedHandler WorkPerformed;
    protected virtual void OnWorkPerformed(int hours, WorkType workType)
    {  
        var del = WorkPerformed as WorkPerformedHandler;
        if (del != null)
        {
            del(this, new WorkPerformedEventArgs(hours, workType));
        }
    }
}

和

class Program
{
    static void Main(string[] args)
    {
        var worker = new Worker();

        worker.WorkPerformed += new EventHandler<WorkPerformedEventArgs>(worker_WorkPerformed);

        Console.ReadKey();
    }

    public static void worker_WorkPerformed(object sender, WorkPerformedEventArgs e)
    {
        throw new NotImplementedException();
    }
}

Answer 1

尽管方法和匿名函数可以隐式转换为与其签名匹配的委托类型，但委托实例不能隐式转换为另一种委托类型。

您需要使用EventHandler<TEventArgs>或您的WorkPerformedHandler ，但不能同时使用：

public class Worker
{
    public event WorkPerformedHandler WorkPerformed;
    
    //...
}

要么：

public class Worker
{
    public event EventHandler<WorkPerformedEventArgs> WorkPerformed;
    
    //...
}

此外，因为worker_WorkPerformed可以隐式转换为任何一种类型，最简洁的语法是这样的：

worker.WorkPerformed += worker_WorkPerformed; // Implicit conversion

---

如果您使用WorkPerformedHandler委托执行 go，请确保根据worker_WorkPerformed的签名将返回类型更改为void ：

public delegate void WorkPerformedHandler(object sender, WorkPerformedEventArgs e);

通常不建议从事件处理程序返回，因为可能有多个订阅者。

Answer 2

我所做的是用EventHandler<WorkPerformedEventArgs>替换WorkPerformedHandler

public delegate int WorkPerformedHandler(object sender, WorkPerformedEventArgs e);
public class Worker
{
    public event EventHandler<WorkPerformedEventArgs> WorkPerformed;
    protected virtual void OnWorkPerformed(WorkPerformedEventArgs e)
    {
        WorkPerformed?.Invoke(this, e);
    }
}

活动订阅：

var worker = new Worker();
worker.WorkPerformed += new EventHandler<WorkPerformedEventArgs>(worker_WorkPerformed);