Request.Validation is a lightweight validation library for C#, built around composable validators, fluent rules, and
structured validation results.

## Features

- **Composable validators:** Build validators by inheriting from `Validator<T>` and defining rules with `RuleFor` and
  `RuleForEach`.
- **Built-in and custom rules:** Use helpers like `NotEmpty`, `Length`, `Between`, and `Matches`, or define custom
  predicates with `Must`.
- **Structured validation output:** Each failure is returned as a `Problem` with a property path, message, severity,
  code, and attempted value.
- **Nested validation:** Reuse validators for complex object graphs with `SetValidator`, including DI-based resolution.

## Getting Started

### Install the NuGet package:

```shell
dotnet add package Geekeey.Request.Validation
```

You may need to add our NuGet feed to your nuget.config this can be done by running the following command:

```shell
dotnet nuget add source -n geekeey https://code.geekeey.de/api/packages/geekeey/nuget/index.json
```

### Usage

```csharp
using Geekeey.Request.Validation;

public sealed record Address(string? Street);

public sealed record CreateUserRequest(
  string? Name,
  int Age,
  Address? Address,
  IReadOnlyList<string> Tags);

public sealed class AddressValidator : Validator<Address>
{
  public AddressValidator()
  {
    RuleFor(address => address.Street)
      .NotEmpty();
  }
}

public sealed class CreateUserRequestValidator : Validator<CreateUserRequest>
{
  public CreateUserRequestValidator()
  {
    RuleFor(request => request.Name)
      .NotEmpty()
      .Length(2, 100)
      .WithCode("NAME_INVALID");

    RuleFor(request => request.Age) 
      .Between(18, 120);

    RuleFor(request => request.Address)
      .SetValidator(new AddressValidator());

    RuleForEach(request => request.Tags)
      .NotEmpty()
      .WithSeverity(Severity.Warning);
  }
}

var validator = new CreateUserRequestValidator();
var validation = validator.Validate(new CreateUserRequest(
  Name: "",
  Age: 16,
  Address: new Address(""),
  Tags: ["", "admin"]));

foreach (var problem in validation.Problems)
{
  Console.WriteLine($"{problem.PropertyName}: {problem.Message}");
}
```

The resulting `Problem` entries include full property paths like `Address.Street` and `Tags[0]`, making it easy to
surface validation errors back to callers or APIs.