动作<对象，EventArgs>无法转换为EventHandler？ [英] Action<object, EventArgs> could not be cast to EventHandler?

问题描述

我正在整理一个使用lambda的事件，该事件需要在触发后自行删除。我无法通过将lambda内联到+ =事件（没有可用于删除事件的可访问变量）来做到这一点，所以我设置了 Action< object，EventArgs> 变量并将lambda移到那里。主要错误是它无法将 Action< object，EventArgs> 转换为EventHandler。我以为lambda表达式可以隐式转换为事件处理程序，这为什么不起作用？

I was wiring up an event to use a lambda which needed to remove itself after triggering. I couldn't do it by inlining the lambda to the += event (no accessable variable to use to remove the event) so i set up an Action<object, EventArgs> variable and moved the lambda there. The main error was that it could not convert an Action<object, EventArgs> to an EventHandler. I thought lambda expressions were implicitly convertable to event handlers, why doesn't this work?

推荐答案

Lambda可以隐式转换为委托类型具有正确的形状，但是两个相同形状的委托类型不能隐式地相互转换。

Lambdas are implicitly convertible to delegate types with the right shape, but two same-shaped delegate types are not implicitly convertible to one another. Just make the local variable have type EventHandler instead.

EventHandler h = (o, ea) => { ... };
e += h;
...
e -= h;

（以防万一：

Action<object, EventArgs> a = (o, ea) => { }; 
EventHandler e = a;  // not allowed
EventHandler e2 = (o,ea) => a(o,ea);  // ok

）

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