C# Language Task Parallel Library (TPL) Dataflow Constructs TransformManyBlock

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(SelectMany, 1-m: The results of this mapping are “flattened”, just like LINQ’s SelectMany)

TransformManyBlock<TInput, TOutput> is very similar to TransformBlock<TInput, TOutput>.
The key difference is that whereas a TransformBlock<TInput, TOutput> produces one and only one output for each input, TransformManyBlock<TInput, TOutput> produces any number (zero or more) outputs for each input. As with ActionBlock and TransformBlock<TInput, TOutput>, this processing may be specified using delegates, both for synchronous and asynchronous processing.

A Func<TInput, IEnumerable> is used for synchronous, and a Func<TInput, Task<IEnumerable>> is used for asynchronous. As with both ActionBlock and TransformBlock<TInput, TOutput>, TransformManyBlock<TInput, TOutput> defaults to sequential processing, but may be configured otherwise.

The mapping delegate retuns a collection of items, which are inserted individually into the output buffer.

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Asynchronous Web Crawler

var downloader = new TransformManyBlock<string, string>(async url =>
    Console.WriteLine(“Downloading “ + url);
        return ParseLinks(await DownloadContents(url)); 
    return Enumerable.Empty<string>();

Expanding an Enumerable Into Its Constituent Elements

var expanded = new TransformManyBlock<T[], T>(array => array);

Filtering by going from 1 to 0 or 1 elements

public IPropagatorBlock<T> CreateFilteredBuffer<T>(Predicate<T> filter)
    return new TransformManyBlock<T, T>(item =>
        filter(item) ? new [] { item } : Enumerable.Empty<T>());

Introduction to TPL Dataflow by Stephen Toub

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