Derived from Dispatch #633 which can surely only be chained with a Zip, BatchBy combine Dispatch and Zip to builds up buckets that are dictionaries.

IEnumerable<Dictionary<TKey, TValue>> BatchBy<TKey, TValue>(
  this IEnumerable<TValue> source,
  Func<TValue, TKey> keySelector,
  IEnumerable<TKey> acceptedKeys);

An example of use:

Let says that you have the logs of a connection between a client and a server.
All client request start with Q and all server response start with A.
There is also some noise that start with #.

  Q> Hy
  A> Hello
  Q> What's your name ?
  #> the fuzz is buzzing
  Q> And city ?
  A> My name is Bob
  Q> And age
  #> the buzz is fuzzing
  A> I live in Detroit
  A> I'm 42
  ...

That is the input of type IEnumerable<string>
We want to match each Q with it's corresponding A.

We can do it with BatchBy like this:

var keys = new[] {'Q', 'A'};
var batches = intput.BatchBy(s => s.FirstOrDefault(), keys);

With the example data bellow, batches will yield those dictionaries:

  {{Key: 'Q', Value: "Q> Hy"}, {Key: 'A', Value: "A> Hello"}},
  {{Key: 'Q', Value: "Q> What's your name ?"}, {Key: 'A', Value: "A> My name is Bob"}},
  {{Key: 'Q', Value: "Q> And city ?"}, {Key: 'A', Value: "A> I live in Detroit"}},
  {{Key: 'Q', Value: "Q> And age"}, {Key: 'A', Value: "A> I'm 42"}},
  ...

remark
In the example bellow Q always come before A, it may be otherwise, the batches are yield when they have a matching value for each key.

Overload with a static number of keys are possible:

IEnumerable<TResult> BatchBy<TKey, TValue, TResult>(
  this IEnumerable<TValue> source,
  Func<TValue, TKey> keySelector,
  TKey key1,
  ...,
  TKey keyN,
  Func<TKey, ..., TKey, TResult> resultSelector);

And can be used like this:

var batches = intput.BatchBy(s => s.FirstOrDefault(), 'Q', 'A', (Question, Answer) => (Question, Answer));

ValueTuple overload are also possibles:

IEnumerable<(TValue, ..., TValue)> BatchBy<TKey, TValue>(
  this IEnumerable<TValue> source,
  Func<TValue, TKey> keySelector,
  TKey key1,
  ...,
  TKey keyN);

To discuss

• Arguments order
• Case of N = 1 (witch is Where(v => keySelector(v) == v).Select(resultSelector))