Completes the asynchronous operation started in BeginExists.
true if the file exists.
Before calling BeginExists, you need to create a callback method that implements the AsyncCallback delegate. This callback method executes in a separate thread and is called by the system after BeginExists returns. The callback method must accept the IAsyncResult returned by the BeginExists method as a parameter. This parameter may then be passed to the EndExists method to complete the operation.