C# - 为什么使用 EventHandler<T> 会出错？答案

【问题标题】：C# - Why is there an error using EventHandler<T>?C# - 为什么使用 EventHandler<T> 会出错？
【发布时间】：2020-07-28 08:13:03
【问题描述】：

制作 Pluralsight 视频，但找不到错误的原因。

关于这一点的错误信息：

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

错误信息：

不能隐式转换为类型 'System.EventHandler' 到 '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();
    }
}

【问题讨论】：

worker.WorkPerformed += worker_WorkPerformed; ?
Pluralsight 视频中的那个人让我像我一样，我想在这个方向上有一个方法
我碰巧订阅了 PluralSight。你能分享你正在学习的课程的链接吗？
模块 3，演示：实例化代表.. app.pluralsight.com/…

标签： c# events event-handling

【解决方案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类型，但委托实例不能隐式转换为另一个委托类型 em>。

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

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，所以最简洁的语法是这样的：

worker.WorkPerformed += worker_WorkPerformed; // Implicit conversion

如果您确实选择了 WorkPerformedHandler 代表，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。

【讨论】：

【解决方案2】：

我所做的是将WorkPerformedHandler 替换为EventHandler<WorkPerformedEventArgs>

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);

【讨论】：