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Async in C# 5.0 by Alex Davies

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Chapter 6. The Task-Based Asynchronous Pattern

The Task-based Asynchronous Pattern (TAP) is a set of recommendations from Microsoft for writing asynchronous APIs in .NET using Task. The document by Stephen Toub from the parallel programming team at Microsoft has good examples and is worth a read.

The pattern makes APIs that can be consumed using await, and while using async produces methods that follow the pattern, it’s often useful to use Task manually. In this chapter, I’ll explain the pattern, and techniques to work with it.

What the TAP Specifies

I’ll assume we already know how to design a good method signature for synchronous C# code:

  • It should have a few parameters, or maybe none. ref and out parameters should be avoided if possible.

  • It should have a return type, if it makes sense, which really expresses the result of the code inside the method, as opposed to a success indicator like in some C++ code.

  • It should have a name that explains the behavior of the method, without extra notation.

  • Common or expected failures should be part of the return type, while unexpected failures should throw exceptions.

Here is a well designed synchronous method, which is part of the Dns class:

public static IPHostEntry GetHostEntry(string hostNameOrAddress)

The TAP gives the same level of guidelines on designing an asynchronous method, based on your existing skills with synchronous methods. Here they are:

  • It should have the same parameters as an equivalent synchronous method would. ref and out parameters ...

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