about_Pipelines should explain the pipeline's enumeration behavior in more detail

[mklement0]

A core feature of the pipeline is to enumerate collection-like data structures.

The One-At-A-Time Processing section of about_Pipelines touches on that, but could use some more detail, notably with respect to what specific data types are considered collections and are therefore enumerated (sent through the pipeline one by one).

From what I gather, the rules are:

• Instances of data types that implement the IEnumerable interface are automatically enumerated, but there are exceptions:
  • Types that also implement IDictionary, such as hashtables, are not enumerated, and neither are XmlNode instances.
• Conversely, instances of DataTable (which doesn't implement IEnumerable) are enumerated (as the elements of their .Rows collection)

See the source code.

If that is considered too detailed / technical, it's at least worth mentioning that [hashtable] instances are not enumerated by default, and that doing so requires a call to .GetEnumerator() (a fact that already is mentioned in the hash-table-specific topic, about_Hash_Tables).

Additionally it's worth mentioning that (stdout) output from external programs is enumerated line by line in the pipeline.

Version(s) of document impacted

• Impacts 6.next document
• Impacts 6 document
• Impacts 5.1 document
• Impacts 5.0 document
• Impacts 4.0 document
• Impacts 3.0 document

[Halkcyon]

I feel like many of these details are addressed in other articles on those specific items inadvertently, but it would be nice to see them in a single location when talking about the pipeline to redirect newer users to learn as suggested by this request.