Раздел «Технологии программирования».AsyncCallback:

References the callback method to be called when the asynchronous operation is completed.

[Serializable]
public delegate void AsyncCallback(
   IAsyncResult ar
);

Parameters

The declaration of your callback method must have the same parameters as the AsyncCallback delegate declaration.

ar
The result of the asynchronous operation.

Remarks

AsyncCallback provides a way for client applications to complete an asynchronous operation. This callback delegate is supplied to the client when the asynchronous operation is initiated. The event handler referenced by AsyncCallback contains program logic to finish processing the asynchronous task for the client.

AsyncCallback uses the IAsyncResult interface to obtain the status of the asynchronous operation.

-- AndreyUstyuzhanin - 06 Apr 2004