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Asynchronous Programming Overview

An asynchronous operation that uses the IAsyncResult design pattern is implemented as two methods named BeginOperationName and EndOperationName that begin and end the asynchronous operation OperationName respectively. For example, the FileStream class provides the BeginRead and EndRead methods to asynchronously read bytes from a file. These methods implement the asynchronous version of the Read method.

After calling BeginOperationName, an application can continue executing instructions on the calling thread while the asynchronous operation takes place on a different thread. For each call to BeginOperationName, the application should also call EndOperationName to get the results of the operation.

Beginning an Asynchronous Operation

The BeginOperationName method begins asynchronous operation OperationName and returns an object that implements the IAsyncResult interface. IAsyncResult objects store information about an asynchronous operation. The following table shows information about an asynchronous operation.




An optional application-specific object that contains information about the asynchronous operation.


A WaitHandle that can be used to block application execution until the asynchronous operation completes.


A value that indicates whether the asynchronous operation completed on the thread used to call BeginOperationName instead of completing on a separate ThreadPool thread.


A value that indicates whether the asynchronous operation has completed.

A BeginOperationName method takes any parameters declared in the signature of the synchronous version of the method that are passed by value or by reference. Any out parameters are not part of the BeginOperationName method signature. The BeginOperationName method signature also includes two additional parameters. The first of these defines an AsyncCallback delegate that references a method that is called when the asynchronous operation completes. The caller can specify null (Nothing in Visual Basic) if it does not want a method invoked when the operation completes. The second additional parameter is a user-defined object. This object can be used to pass application-specific state information to the method invoked when the asynchronous operation completes. If a BeginOperationName method takes additional operation-specific parameters, such as a byte array to store bytes read from a file, the AsyncCallback and application state object are the last parameters in the BeginOperationName method signature.

BeginOperationName returns control to the calling thread immediately. If the BeginOperationName method throws exceptions, the exceptions are thrown before the asynchronous operation is started. If the BeginOperationName method throws exceptions, the callback method is not invoked.

Ending an Asynchronous Operation

The EndOperationName method ends asynchronous operation OperationName. The return value of the EndOperationName method is the same type returned by its synchronous counterpart and is specific to the asynchronous operation. For example, the EndRead method returns the number of bytes read from a FileStream and the EndGetHostByName method returns an IPHostEntry object that contains information about a host computer. The EndOperationName method takes any out or ref parameters declared in the signature of the synchronous version of the method. In addition to the parameters from the synchronous method, the EndOperationName method also includes an IAsyncResult parameter. Callers must pass the instance returned by the corresponding call to BeginOperationName.

If the asynchronous operation represented by the IAsyncResult object has not completed when EndOperationName is called, EndOperationName blocks the calling thread until the asynchronous operation is complete. Exceptions thrown by the asynchronous operation are thrown from the EndOperationName method. The effect of calling the EndOperationName method multiple times with the same IAsyncResult is not defined. Likewise, calling the EndOperationName method with an IAsyncResult that was not returned by the related Begin method is also not defined.


For either of the undefined scenarios, implementers should consider throwing InvalidOperationException.


Implementers of this design pattern should notify the caller that the asynchronous operation completed by setting IsCompleted to true, calling the asynchronous callback method (if one was specified) and signaling the AsyncWaitHandle.

Application developers have several design choices for accessing the results of the asynchronous operation. The correct choice depends on whether the application has instructions that can execute while the operation completes. If an application cannot perform any additional work until it receives the results of the asynchronous operation, the application must block until the results are available. To block until an asynchronous operation completes, you can use one of the following approaches:

Applications that do not need to block while the asynchronous operation completes can use one of the following approaches:

See Also


Calling Synchronous Methods Asynchronously

Using an AsyncCallback Delegate and State Object

Other Resources

Asynchronous Programming Design Patterns