How many models of Asynchronous development in .NET?

Thread.Start - brutal

delegate.BeginInvoke/EndInvoke - 'old' standard

ThreadPool.QueueUserWorkItem - smart

TaskFactory.StartNew - the only way to do it correct (according to Patterns of parallel programming book | i recommend you to read it first for disambiguation)

There's a lot that can be caught in the term "asynchronous development."

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For one, you could want to execute code on a background thread. I recently updated a blog post of mine contrasting several common approaches to executing code in the background. Here's the list, in order from most desirable to least:

1. Task (as used by async/await).
2. Task (as used by the Task Parallel Library).
3. BackgroundWorker.
4. Delegate.BeginInvoke.
5. ThreadPool.QueueUserWorkItem.
6. Thread

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On another hand, you could want to represent an asynchronous operation (which may or may not be actual code executing on a background thread). In that case, there are several approaches, in order from most desirable to least:

1. Task (in the style of the Task-based Asynchronous Pattern (TAP))
2. IAsyncResult with Begin*/End* methods (which has the unfortunate name Asynchronous Programming Model (APM)).
3. A component written using the Event-based Asynchronous Pattern (EAP).

(As a side note, BackgroundWorker is EAP, and Delegate.BeginInvoke is APM).

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On another hand, you could mean asynchronous programming in general, which can be interpreted to mean a reactive approach. In this case, there are only two approaches that I know of:

1. Reactive Extensions (Rx).
2. Event-based Asynchronous Pattern (EAP).

However, you could make a case that any event-driven program is reactive to some extent, so just handling UI events is a (simple) form of "asynchronous programming."

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Also, these are only the common models. Any platform or library can add more. Here's some off the top of my head:

• The Socket class has a special form of APM that can be used to minimize memory allocations. It works very similarly to APM but does not fit the pattern.
• The WinRT runtime (coming in Windows 8) has its own representations of asynchronous operations (IAsyncOperation<TResult> and IAsyncInfo).
• Windows Phone has specific support for a background agent, which permits you to run code in the background even if your app isn't currently running.