#42 validate options

[matthewdevenny]

Closes #42

Problem

A missing or whitespace BucketName threw a NullReferenceException from the NatsCache constructor. That surfaced as a confusing error at first cache use rather than a clear configuration failure at startup.

Change

Validate NatsCacheOptions through the options pattern so a bad BucketName fails fast, and replace the NullReferenceException with clear, typed exceptions.

Library (src/NatsDistributedCache)

• NatsDistributedCacheExtensions.cs: AddNatsDistributedCache now registers AddOptions<NatsCacheOptions>().Configure(...).Validate(o => !string.IsNullOrWhiteSpace(o.BucketName), ...).ValidateOnStart(). With the Generic Host, an empty/whitespace BucketName fails at startup with a descriptive OptionsValidationException. Even without host startup validation, resolving the cache reads IOptions.Value, which runs the same validation before the constructor is reached. AddNatsHybridCache inherits this automatically (it delegates to AddNatsDistributedCache).
• NatsCache.cs: removed the NullReferenceException throw. The constructor keeps a guard that now throws ArgumentException — this is the only backstop for the direct new NatsCache(...) path, which bypasses the options-validation pipeline (NatsCacheOptions is its own IOptions<>, so a raw instance skips .Validate()).
• NatsCacheOptions.cs: the failure message lives in a single shared internal const referenced by both the DI validator and the constructor guard, so the two paths can't drift.

Tests (test/UnitTests)

• AddNatsCache_MissingBucketName_FailsValidationOnStart — null/empty/whitespace fail fast via IStartupValidator.Validate() (the mechanism the host invokes), with the descriptive message.
• AddNatsCache_ValidBucketName_PassesValidationOnStart — a valid bucket name passes startup validation.
• NatsCache_DirectConstruction_MissingBucketName_ThrowsArgumentException — the direct-construction path throws ArgumentException, not NullReferenceException.

Acceptance criteria

• ✅ Empty/whitespace BucketName fails fast at startup with a descriptive OptionsValidationException.
• ✅ No NullReferenceException path remains.

Verification

• dotnet test test/UnitTests → 69/69 passing on net8.0 + net10.0.
• dotnet build -p:TreatWarningsAsErrors=true → 0 warnings (net8.0 + net10.0).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Implements fail-fast configuration validation for NatsCacheOptions.BucketName using the options pattern so misconfiguration surfaces as an OptionsValidationException at startup rather than a NullReferenceException at first cache use.

Changes:

• Added Validate(...).ValidateOnStart() to AddNatsDistributedCache for BucketName validation.
• Replaced the NullReferenceException path in NatsCache with an ArgumentException guard.
• Added unit tests covering startup validation behavior and direct constructor behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
test/UnitTests/Extensions/NatsDistributedCacheExtensionsTests.cs	Adds tests asserting startup options validation fails fast for missing BucketName, and direct construction throws a clear exception.
src/NatsDistributedCache/NatsDistributedCacheExtensions.cs	Registers NatsCacheOptions with predicate validation and ValidateOnStart().
src/NatsDistributedCache/NatsCacheOptions.cs	Introduces a shared validation message constant for consistent error reporting.
src/NatsDistributedCache/NatsCache.cs	Replaces the NullReferenceException for missing BucketName with an ArgumentException guard.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[mtmk]

LGTM