Validation

Annotate parameters (or [AsParameters] members) with standard System.ComponentModel.DataAnnotations attributes, optionally combined with Nullean.Argh filesystem attributes. The source generator reads the attributes at build time and emits inline validation checks. No reflection, no Validator.ValidateObject call, AOT-safe.

Constraint hints appear in --help after the description. Failures print to stderr and exit with code 2.

public static void Deploy(
    [Range(1, 65535)]                          int port,
    [StringLength(64, MinimumLength = 2)]      string name,
    [AllowedValues("dev", "staging", "prod")]  string env,
    [RegularExpression(@"^[a-z0-9\-]+$")]      string slug,
    [UriScheme("https")]                       Uri endpoint)
{ … }

$ myapp deploy --port 99999
Error: --port: value must be between 1 and 65535.
Run 'myapp deploy --help' for usage.

$ myapp deploy --help
Options:
  --port <int>       [required] [range: 1-65535]
  --name <string>    [required] [length: 2-64]
  --env <string>     [required] [allowed: dev|staging|prod]
  --slug <string>    [required] [pattern: ^[a-z0-9\-]+$]
  --endpoint <uri>   [required] [schemes: https]

When [MinLength] / [MaxLength] / [Length] is applied to a collection parameter (T[], List<T>, etc.), it validates the number of items, not string length:

public static void Process([MaxLength(5)] List<string> files) { … }

public static void Archive([Argument][MinLength(2)][MaxLength(10)] string[] files) { … }

1. max 5 --file flags
2. 2-10 positional items

These attributes apply to FileInfo / DirectoryInfo parameters (including on [AsParameters] members and [Argument] positionals). Incompatible combinations (such as [Existing] with [NonExisting]) are diagnosed at compile time, as is applying [FileExtensions] to a DirectoryInfo.

public static Task<int> Lint(
    [Existing][FileExtensions(Extensions="json")][RejectSymbolicLinks] FileInfo manifest,
    [ExpandUserProfile][Existing] DirectoryInfo outDir)
{ … }

Failures produce stderr messages such as file does not exist or path must not be a symbolic link (exit code 2).

All five filesystem attributes also work on a collection of FileInfo / DirectoryInfo — List<FileInfo>, FileInfo[], DirectoryInfo[], etc. — including a variadic [Argument] collection (FileInfo[]/params FileInfo[]). Each attribute is applied per item, and every failing item is reported together in a single error, not just the first one encountered:

public static void Ingest(
    [Existing][FileExtensions(Extensions = "json,yaml")] List<FileInfo> configs) { … }

public static void Archive(
    [Argument][Existing][FileExtensions(Extensions = "zip")] FileInfo[] files) { … }

$ myapp ingest --configs a.json --configs missing.txt --configs b.png
Error: --configs: /path/missing.txt: file does not exist.
Error: --configs: /path/b.png: extension must be one of: json, yaml.
Run 'myapp ingest --help' for usage.

[MinLength] / [MaxLength] / [Length] on the same collection continue to validate item count (see Collection validation above) and combine with the per-item filesystem checks.

Validations include JSON kind values in __schema output: existing, nonExisting, rejectSymbolicLinks, expandUserProfile.

Validation also runs through the TryParseArgh static extension emitted for [AsParameters] DTOs, so unit tests can assert constraints without spawning a subprocess.

For rules attributes cannot express (a port that must be free of conflicts with another flag, rules that come from a domain model), register a validator for an [AsParameters] or global/namespace options type. It runs right after the built-in checks, and a failure prints exactly like one of them: the error, the flag's help rows, the Run '...' for usage. line, and exit code 2.

using Nullean.Argh.Validation;

ArghValidation.Register<ServeOptions>(o =>
    o.Port > 1024 ? [] : [new ArghValidationError("must be above 1024", nameof(ServeOptions.Port))]);

$ myapp serve --port 80
Error: --port: must be above 1024

The member name in ArghValidationError is turned into its flag (--port); an error without a member is printed as Error: <message>. Validators are found by the object's runtime type, then its base types. Register them at startup, before RunAsync. With none registered the generated code skips the check entirely. TryParseArgh runs the registered validator too and returns false on failure, so tests cover it without a subprocess.