---
title: Validation
description: Annotate parameters (or [AsParameters] members) with standard System.ComponentModel.DataAnnotations attributes, optionally combined with Nullean.Argh...
url: https://docs-v3-preview.elastic.dev/argh/features/validation
---

# 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

```csharp
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

<dropdown title="Full attribute reference table">
  | 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` | file extension      | `[extensions: json\|yaml]`            |
  | `[UriScheme("https")]`                     | `Uri`      | URI scheme          | `[schemes: https]`                    |
</dropdown>


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

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

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


## Filesystem path validation

These attributes apply to `FileInfo` / `DirectoryInfo` parameters (including on `[AsParameters]` members). Incompatible combinations (such as `[Existing]` with `[NonExisting]`) are diagnosed at compile time.

| Attribute               | Applies to                    | Help token           |
|-------------------------|-------------------------------|----------------------|
| `[Existing]`            | `FileInfo`                    | `[existing]`         |
| `[Existing]`            | `DirectoryInfo`               | `[existing]`         |
| `[NonExisting]`         | `FileInfo` or `DirectoryInfo` | `[unused path]`      |
| `[RejectSymbolicLinks]` | `FileInfo` or `DirectoryInfo` | `[no symlinks]`      |
| `[ExpandUserProfile]`   | `FileInfo` or `DirectoryInfo` | `[expand ~ profile]` |

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


### Example

```csharp
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).

## 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.