C# - 为什么使用 EventHandler<T> 会出现错误？

Question

制作 Pluralsight 视频，但找不到错误的原因。

此时的错误消息:

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

错误消息:

Cannot implicity convert to type 'System.EventHandler<AB_Events.WorkPerformedEventArgs>' to '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));
        }
    }
}

还有

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

虽然方法和匿名函数可以隐式转换为与其签名匹配的委托(delegate)类型，但委托(delegate)实例不能隐式转换为另一个委托(delegate)类型 em>.

您需要使用 EventHandler<TEventArgs>或您的WorkPerformedHandler ，但不是两者:

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

或者:

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

另外，因为 worker_WorkPerformed可以隐式转换为任一类型，最简洁的语法是这样的:

worker.WorkPerformed += worker_WorkPerformed; // Implicit conversion

---

如果您确实选择 WorkPerformedHandler委托(delegate)，请确保将返回类型更改为 void根据 worker_WorkPerformed 的签名:

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。