Why we need events for callback?

Question

We can call callback methods using delegates. For eg,

public delegate bool ContinueProcessing();

// later in code we can write ,

ContinueProcessing cp = new ContinueProcessing(IsDataAvailable);

cp += new ContinueProcessing(IsTransactionComplete);

//later in code defination of methods

bool IsDataAvailable() { return true; }

bool IsTransactionComplete() { return true; }

cp.Invoke() ;

The above call will call two boolean method one after the other. Why we need "Events" ? What is the purpose of "Events" ?

Answer 1

Events are callbacks where you can have multiple subscribers who don't interfere with each other and can't call each other.

Delegates provide the encapsulation of "this is the action I want to take" and events provide the encapsulation of the pub/sub model.

See my article on events for more information.

Answer 2

Delegates are very generic. They can be used in any context: within the same class, between classes, in static methods etc.

Events are more specific -- they are specifically designed to be used for one class to subscribe to an event raised by another class.

Delegate is a (kind of) type-safe pointer to a function, whereas event is wrapper around delegate that provides standard interface to Subscribe/Unsubscribe/Raise an event.

Answer 3

看起来'event'关键字是委托声明的修饰符，允许它包含在接口中，从声明它的类中约束它的调用，为它提供一对可自定义的访问器（添加和删除）并强制委托的签名（在.NET框架中使用时）。