[electricessence]

Map<T>() on a reader or a raw command, for 11.0.0-rc.4 (API shape). #111 is merged and master is merged in.

What

• reader.Map<T>() returns MappedReader<T>, and command.Map<T>() returns MappedCommand<T>.
• Both are readonly record structs that implement IMappedQuery<T>, with exactly the members MappedQuery<T> has: every terminal in its sync and async form (ValueTask, token last), Read/ReadAsync, and Skip/Take, which return the existing MappedQueryWithSkip<T>/MappedQueryWithTake<T>. Nothing new.
• SingleResult() is on MappedCommand<T> only.
• 5 Map<T> overloads each on IDataReader and IDbCommand, mirroring the IExecuteReader ones (same parameter lists, OverloadResolutionPriority values and guards).

Ownership

• Reader view: the caller owns the reader. It reads the current result set from the current position and never opens, closes or disposes the reader, and never calls NextResult. On a reader view, SingleResult() reached through Skip/Take is a no-op.
• Command view: the same as an expressive command's MappedQuery<T>. It opens the connection only if it is closed and closes (not disposes) only what it opened; it reads through the later result sets to surface late failures unless SingleResult() was called; async terminals open and execute asynchronously on a DbCommand.

Behavior changes in this PR

• A null RowMap<T> passed to Map<T>(map) now maps by default, exactly like Map<T>(), at all three levels. It used to throw ArgumentNullException on IExecuteReader. Noted in whats-new-11.md.
• A default struct of the whole family (MappedQuery<T>, the two staging structs and the two views) now throws InvalidOperationException saying it was not created by Map<T>(), instead of a NullReferenceException.
• A type of your own that implements both IExecuteReader and IDbCommand (or IDataReader) gets an ambiguous Map call (CS0121); cast to pick a level.

Docs

• The Map<T> remarks now say how columns match by default: exactly, then ignoring case, then ignoring case and underscores (author_id feeds AuthorId), with the resolved layout cached per result-set layout.
• mapping.md has a section on mapping a reader or command you already have; whats-new-11.md lists the changes; two new compiled samples are checked in Program.cs.

Checks

• Release and Debug builds: 0 warnings, 0 errors. Tests: 9666 of 9666 pass in both. Samples: 102 of 102 checks pass. Style and analyzers: 0 findings. DocFX: 0 warnings.

Since the review

• Master (Commands take the type in their async terminals (ToArrayAsync<int>(ct)), and Map<int>() reads the first column #111 squashed, rc.4: RetrieveAsync takes a CancellationToken; misnamed parameters renamed #116) is merged in; the diff against master is the views only.
• The command view disposes its reader with ConfigureAwait(false), and the reader view returns a cancelled task for a cancelled token instead of throwing at the call.
• The view remarks say the asynchronous terminals read the rows with Read(), as an expressive command does by default; whats-new notes the CS0121 ambiguity.
• Tests cover parity with MappedQuery<T> for every terminal (sync and async), a counting substitute reader proving the reader view never calls NextResult, Close or Dispose, command-view ownership with open and closed connections, null and empty maps at all three levels, default structs, and compile-only binding.

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