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.

Example

		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]
		
	

DataAnnotations reference

Attribute	Applies to	Validates	Help token
`[Range(min, max)]`	numeric	value within bounds	`[range: min-max]`
`[StringLength(max)]`	`string`	string length	`[max-length: n]`
`[StringLength(max, MinimumLength = min)]`	`string`	string length	`[length: min-max]`
`[MinLength(n)]` / `[MaxLength(n)]`	`string`	string length	`[min-length: n]` / `[max-length: n]`
`[MinLength(n)]` / `[MaxLength(n)]`	collection	item count	`[min-count: n]` / `[max-count: n]`
`[Length(min, max)]` (.NET 8)	`string`	string length	`[length: min-max]`
`[Length(min, max)]` (.NET 8)	collection	item count	`[count: min-max]`
`[RegularExpression(pattern)]`	`string`	regex match	`[pattern: …]`
`[AllowedValues(v1, v2, …)]` (.NET 8)	any	value in set	`[allowed: v1\\|v2\\|…]`
`[DeniedValues(v1, v2, …)]` (.NET 8)	any	value not in set	`[denied: v1\\|v2\\|…]`
`[EmailAddress]`	`string`	`user@host` shape	`[email]`
`[Url]`	`string`	absolute URL	`[url]`
`[Url]`	`Uri`	http or https	`[schemes: http\\|https]`
`[FileExtensions(Extensions="json,yaml")]`	`FileInfo` (incl. collections)	file extension	`[extensions: json\\|yaml]`
`[UriScheme("https")]`	`Uri`	URI scheme	`[schemes: https]`

Collection validation

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) { … }

max 5 --file flags
2-10 positional items

Note

Enum parameters automatically show [allowed: Member1|Member2] in help. The enum type itself enforces the constraint with no extra attribute needed.

Filesystem path validation

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.

Attribute	Applies to	Help token
`[Existing]`	`FileInfo` or `DirectoryInfo`	`[existing]`
`[NonExisting]`	`FileInfo` or `DirectoryInfo`	`[unused path]`
`[RejectSymbolicLinks]`	`FileInfo` or `DirectoryInfo`	`[no symlinks]`
`[ExpandUserProfile]`	`FileInfo` or `DirectoryInfo`	`[expand ~ profile]`
`[FileExtensions(Extensions="…")]`	`FileInfo` only	`[extensions: …]`

Important

[RejectSymbolicLinks] runs before existence checks. A symlink pointing to a real path still fails when symlink rejection is enabled.

Example

		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).

Collections of FileInfo / DirectoryInfo

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.

Schema integration

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.