Everyday C#: Null safety tutorials

[BillWagner]

Fixes #52838

If you review commit-by-commit, the 4th commit is updating links, and can be skimmed.

Plan: PR 9 — Null safety: NRT, warnings, migration, tutorial

Consolidate the existing NRT concept article + tutorial into a fundamentals-style concept + tutorial pair, and split nullable-migration-strategies.md into two new fundamentals concept articles. Add redirects, update docs/csharp/toc.yml. Depends on PR 8 (#53509) landing the Null safety toc node first.

Phases

Phase 1 — Concept articles (parallel-safe)

1. fundamentals/null-safety/nullable-reference-types.md — apply template-concept.md. Body from docs/csharp/nullable-references.md, with conceptual prose lifted from the existing tutorial. 4-tier audience tip, ms.topic: concept-article, ai-usage: ai-assisted. All code via :::code::: snippet refs.
2. fundamentals/null-safety/resolve-warnings.md — concept article. Pull "Understand contexts and warnings" / "Address warnings" / "Enable type annotations" / "Attributes extend type annotations" sections from docs/csharp/nullable-migration-strategies.md. Frame as 5 resolution techniques with worked examples (NOT a per-CS86xx catalog). Cross-link to docs/csharp/language-reference/compiler-messages/nullable-warnings.md for per-warning lookup.
3. fundamentals/null-safety/migration-strategies.md — concept article. Pull "Plan your migration" / "Next steps" / intro from docs/csharp/nullable-migration-strategies.md. Cover the four default-context strategies and recommended phased order.

Phase 2 — Tutorial (depends on Phase 1 cross-links)

4. fundamentals/tutorials/nullable-reference-types.md — apply template-tutorial.md. Source: docs/csharp/tutorials/nullable-reference-types.md. Restructure to template (checklist, Prerequisites, numbered task H2s, "Get the code", "Next step"). Trim concept-duplicating prose; replace with cross-links.

Phase 3 — Snippets (parallel-safe with Phases 1–2)

5. New snippet projects under docs/csharp/fundamentals/null-safety/snippets/{nullable-reference-types,resolve-warnings,migration-strategies}/. Each minimal .csproj, <Nullable>enable</Nullable>, latest TFM. Build + execute per docs copilot-instructions.md.
6. Move docs/csharp/tutorials/snippets/NullableIntroduction/ → docs/csharp/fundamentals/tutorials/snippets/NullableIntroduction/. Update tutorial :::code::: source paths.

Phase 4 — TOC + redirects + deletes (depends on Phases 1–3)

7. docs/csharp/toc.yml: remove lines 205–206 (legacy tutorial) and 293–296 (legacy concepts); add 4 new entries inside the Null safety node from PR 8.
8. .openpublishing.redirection.csharp.json: add 3 new redirects (alphabetical insertion); repoint existing redirects on lines 3126, 5389, 5447 (currently target /dotnet/csharp/nullable-migration-strategies) to new fundamentals URL to avoid chains. Run sort-redirects skill.
9. Delete docs/csharp/nullable-references.md, docs/csharp/nullable-migration-strategies.md, docs/csharp/tutorials/nullable-reference-types.md.

Phase 5 — Cross-link cleanup (depends on Phase 4)

10. Repoint inbound links across docs/. Critical: docs/csharp/language-reference/compiler-messages/nullable-warnings.md has multiple links to moved files.

Verification

1. Each new snippet project builds: dotnet build; tutorial sample runs: dotnet run.
2. git grep -n "nullable-references.md\|nullable-migration-strategies.md\|tutorials/nullable-reference-types.md" returns 0 hits in docs/.
3. python -m json.tool .openpublishing.redirection.csharp.json validates JSON.
4. Visual confirmation of TOC ordering against project map.
5. Every new/edited article has ai-usage: ai-assisted; no F1/helpviewer keywords; 4-tier audience tip near top.
6. Spot-check 2–3 samples per article for Everyday C# saturation (file-scoped namespaces, primary constructors, collection expressions, raw strings, var).
7. After PR opens, resolve any OpenPublishing.Build warnings introduced.

Decisions

• Split confirmed: planning content → migration-strategies.md; address-warnings/contexts → resolve-warnings.md. Reference page stays untouched.
• resolve-warnings.md is concept-level (5 techniques) — does not duplicate the per-CS86xx catalog; cross-links to it.
• Move + revise in same PR per overall plan.
• ~14 files total — slightly above the ~10 guideline, acceptable per project guideline ("guideline, not a strict rule").

Further considerations

1. PR 8 merge ordering. Recommendation: wait for PR 8 to merge, then rebase PR 9 onto it (avoids toc conflicts). Alternative: include the Null safety toc node creation in PR 9 (would conflict with PR 8 at merge).
2. Snippet reuse. Recommendation: do NOT share with language-reference/compiler-messages/snippets/null-warnings/. Create fresh, smaller fundamentals-style snippets so reference vs. fundamentals can evolve independently.
3. Tutorial vs concept article overlap. Recommendation: trim concept-duplicating prose from the tutorial and add a short "Before you start, see Nullable reference types" pointer near the top.

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Internal previews

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📄 File	🔗 Preview link
docs/core/compatibility/sdk/6.0/csharp-template-code.md	C# code in templates not supported by earlier versions
docs/core/extensions/windows-service.md	Create Windows Service using BackgroundService
docs/core/whats-new/dotnet-5.md	What's new in .NET 5
docs/core/whats-new/dotnet-core-3-0.md	What's new in .NET Core 3.0
docs/csharp/fundamentals/null-safety/migration-strategies.md	Nullable migration strategies
docs/csharp/fundamentals/null-safety/nullable-reference-types.md	docs/csharp/fundamentals/null-safety/nullable-reference-types
docs/csharp/fundamentals/null-safety/resolve-warnings.md	Resolve nullable warnings
docs/csharp/fundamentals/tutorials/nullable-reference-types.md	Tutorial: Express your design intent with nullable and non-nullable reference types
docs/csharp/language-reference/attributes/nullable-analysis.md	"Attributes interpreted by the compiler: Nullable static analysis"
docs/csharp/language-reference/builtin-types/arrays.md	Arrays
docs/csharp/language-reference/builtin-types/nullable-reference-types.md	docs/csharp/language-reference/builtin-types/nullable-reference-types
docs/csharp/language-reference/compiler-messages/interface-implementation-errors.md	Resolve errors and warnings related to members that implement an interface
docs/csharp/language-reference/compiler-messages/nullable-warnings.md	Resolve nullable warnings
docs/csharp/language-reference/compiler-options/language.md	"Compiler Options - language feature rules"
docs/csharp/language-reference/keywords/where-generic-type-constraint.md	where (generic type constraint) (C# Reference)
docs/csharp/language-reference/operators/null-forgiving.md	docs/csharp/language-reference/operators/null-forgiving
docs/csharp/toc.yml	Taken from https://github.com/dotnet/roslyn/wiki/Samples-and-Walkthroughs
docs/csharp/tour-of-csharp/tips-for-java-developers.md	Tips for Java Developers
docs/csharp/tour-of-csharp/tips-for-javascript-developers.md	Roadmap for JavaScript and TypeScript developers learning C#
docs/csharp/tour-of-csharp/tips-for-python-developers.md	Python
docs/framework/debug-trace-profile/code-contracts.md	docs/framework/debug-trace-profile/code-contracts
docs/fundamentals/code-analysis/style-rules/ide0370.md	Remove unnecessary suppression (IDE0370)
docs/fundamentals/syslib-diagnostics/syslib1026.md	SYSLIB1026 error