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Asynchronous programming patterns

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.NET provides three patterns for performing asynchronous operations:

  • Task-based Asynchronous Pattern (TAP), which uses a single method to represent the initiation and completion of an asynchronous operation. TAP was introduced in .NET Framework 4. It's the recommended approach to asynchronous programming in .NET. The async and await keywords in C# and the Async and Await operators in Visual Basic add language support for TAP. For more information, see Task-based Asynchronous Pattern (TAP).

  • Event-based Asynchronous Pattern (EAP), which is the event-based legacy model for providing asynchronous behavior. It requires a method that has the Async suffix and one or more events, event handler delegate types, and EventArg-derived types. EAP was introduced in .NET Framework 2.0. It's no longer recommended for new development. For more information, see Event-based Asynchronous Pattern (EAP).

  • Asynchronous Programming Model (APM) pattern (also called the IAsyncResult pattern), which is the legacy model that uses the IAsyncResult interface to provide asynchronous behavior. In this pattern, asynchronous operations require Begin and End methods (for example, BeginWrite and EndWrite to implement an asynchronous write operation). This pattern is no longer recommended for new development. For more information, see Asynchronous Programming Model (APM).

Comparison of patterns

For a quick comparison of how the three patterns model asynchronous operations, consider a Read method that reads a specified amount of data into a provided buffer starting at a specified offset:

public class MyClass  
{  
    public int Read(byte [] buffer, int offset, int count);  
}

The TAP counterpart of this method would expose the following single ReadAsync method:

public class MyClass  
{  
    public Task<int> ReadAsync(byte [] buffer, int offset, int count);  
}

The EAP counterpart would expose the following set of types and members:

public class MyClass  
{  
    public void ReadAsync(byte [] buffer, int offset, int count);  
    public event ReadCompletedEventHandler ReadCompleted;  
}

The APM counterpart would expose the BeginRead and EndRead methods:

public class MyClass  
{  
    public IAsyncResult BeginRead(  
        byte [] buffer, int offset, int count,
        AsyncCallback callback, object state);  
    public int EndRead(IAsyncResult asyncResult);  
}

See also