The standard event pattern uses the EventArgs object to include fields that event subscribers can use to communicate cancel. Two different patterns could be used, based on the semantics of the Cancel contract. In both cases, you'll add a boolean field to the EventArguments for the found file event.

In the event handler for the MethodNameCompleted event, the MethodNameCompletedEventArgs object contains a Cancelled field, which clients can use to determine whether the cancellation occurred. Abide by the cancellation semantics described in Best Practices for Implementing the Event-based Asynchronous Pattern.

Use the EventHandler delegate for all events that don't include event data. Use the EventHandler<TEventArgs> delegate for events that include data about the event. These delegates have no return type value and take two parameters (an object for the source of the event and an object for event data).

A class that supports the Event-based Asynchronous Pattern will have one or more methods named MethodNameAsync. These methods may mirror synchronous versions, which perform the same operation on the current thread. The class may also have a MethodNameCompleted event and it may have a MethodNameAsyncCancel (or simply CancelAsync) method.