Action<object, EventArgs> could not be casted to EventHandler?

问题

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?

回答1:

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;

(in case it helps:

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

)

回答2:

Action<Object, EventArgs> a = (o, ea) => { };
EventHandler e = a.Invoke;

回答3:

In general, delegates can't be cast because they have no inheritance tree defining which casts are valid. To that end, you have two choices:

1. Use a variable of type EventHandler instead of the Action<T1, T2>

2. Use an inline declaration.

   // option 1: local variable
   EventHandler eh = (o, ea) => { /* [snip] */ };
   obj.event += eh;
   obj.event -= eh;
   
   // option 2: inline declaration
   obj.event += (o, ea) => { /* [snip] */ };
   

回答4:

Declare your event as

public event Action<object, EventArgs> e;

Then you can directly add your action:

Action<object, EventArgs> a = something;
e += a;

回答5:

You can use an anonymous method instead:

Event += (sender, e) =>
{
     // Multiple lines
     // of code here
};

来源：https://stackoverflow.com/questions/1551953/actionobject-eventargs-could-not-be-casted-to-eventhandler