HAL

A .NET 9 library for implementing the Hypertext Application Language (HAL) and HAL-Forms in web APIs and clients.

Purpose

HAL is a standard for building discoverable, self-describing REST APIs using hypermedia links. This library helps you:

• Expose resources with links and forms for easy client navigation
• Support both standard and OData-based endpoints
• Integrate HAL in .NET and Angular applications

Use HAL if you want your API clients to discover available actions and related resources dynamically, following the HATEOAS principle.

Overview

This project provides:

• Core HAL resource and link models
• JSON serialization support
• ASP.NET Core integration for easy HAL responses
• OData support for list endpoints and paging
• .NET and Angular client libraries

Specifications

• IETF HAL Draft
• Informal HAL Documentation
• HAL-Forms Specification

Packages

1. HAL.Common which contains the Resource, Resource<T>, FormsResource, FormsResource<T> and Link implementations and the converters needed for serialization with System.Text.Json.
2. HAL.AspNetCore adds IResourceFactory, IFormFactory and ILinkFactory which can be used in your controllers to easily generate resources from your models. It also comes with a HalControllerBase class which can be used for all Controllers which return HAL.
3. HAL.AspNetCore.OData adds IODataResourceFactory and IODataFormFactory which can be used in your controllers to easily generate list endoints with paging from OData $filter, $skip and $top syntax.
4. Hal.Client.Net is a client library to consume HAL APIs in .Net applications. When using it, you should call app.Services.AddHalClientFactoy() to inject the IHalClientFactory which can then be resolved in your application.
5. Hal.Client.Angular/@wertzui/ngx-hal-client is a client library to consume HAL APIs in Angular applications. It exposes the HalClientModule which then provides the HalClient and a FormService.

Installation

Add the desired package(s) via NuGet:

# Example for HAL.AspNetCore
 dotnet add package HAL.AspNetCore

Angular Client Installation

To use the Angular HAL client, install the npm package:

npm install @wertzui/ngx-hal-client

Usage

Without OData

Use this approach for standard REST endpoints.

Configure services in Program.cs to enable HAL serialization:

builder.Services
    .AddControllers()
    .AddHal()
    .AddJsonOptions(o =>
    {
        o.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault;
    });

Return HAL resources from your controller:

[Route("[controller]")]
public class MyController : HalControllerBase
{
    private readonly IResourceFactory _resourceFactory;

    public MyController(IResourceFactory resourceFactory)
    {
        _resourceFactory = resourceFactory ?? throw new ArgumentNullException(nameof(resourceFactory));
    }

    [HttpGet]
    [ApiConventionMethod(typeof(DefaultApiConventions), nameof(DefaultApiConventions.Get))]
    public ActionResult<IResource> GetList()
    {
        var models = new[]
        {
            new MyModelListDto {Id = 1, Name = "Test1"},
            new MyModelListDto {Id = 2, Name = "Test2"},
        };

        var result = _resourceFactory.CreateForListEndpoint(models, _ => "items", m => m.Id);

        return Ok(result);
    }

    [HttpGet("{id}")]
    public ActionResult<IResource<ModelFullDto>> Get(int id)
    {
        var model = new ModelFullDto { Id = id, Name = $"Test{id}", Description = "Very important!" };

        var result = _resourceFactory.CreateForGetEndpoint(model);

        return Ok(result);
    }

    // PUT, POST, ...
}

With OData

Use this approach if your API supports OData queries for filtering, paging, and sorting.

Configure services in Program.cs to enable OData and HAL:

builder.Services
    .AddControllers(options => // or .AddMvc()
    {
        options.OutputFormatters.RemoveType<ODataOutputFormatter>();
        options.InputFormatters.RemoveType<ODataInputFormatter>();
    })
    .AddOData()
    .AddHALOData()
    .AddJsonOptions(o => // not neccessary, but creates a much nicer output
    {
        o.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault;
    });

var app = builder.Build();

app.UseRouting();

// ...

app.UseEndpoints(_ => { });
app.MapControllers();

Return HAL resources with OData support from your controller:

[Route("[controller]")]
public class MyController : HalControllerBase
{
    private readonly IODataResourceFactory _resourceFactory;

    public MyController(IODataResourceFactory resourceFactory)
    {
        _resourceFactory = resourceFactory ?? throw new ArgumentNullException(nameof(resourceFactory));
    }

    [HttpGet]
    [ApiConventionMethod(typeof(DefaultApiConventions), nameof(DefaultApiConventions.Get))]
    public ActionResult<Resource> GetList(
            // The SwaggerIgnore attribute and all parameters beside the options are just here to give you a nice swagger experience.
            // If you do not need that, you can remove everything except the options parameter.
            // If you are using RESTworld, you can also remove everything except the options parameter, because there is a custom Swagger filter for that.
            [SwaggerIgnore] ODataQueryOptions<TEntity> options,
        [FromQuery(Name = "$filter")] string? filter = default,
        [FromQuery(Name = "$orderby")] string? orderby = default,
        [FromQuery(Name = "$top")] long? top = default,
        [FromQuery(Name = "$skip")] long? skip = default)
    {
        var models = new[]
        {
            new MyModelListDto { Id = 1, Name = "Test1" },
            new MyModelListDto { Id = 2, Name = "Test2" },
        };

        // Apply the OData filtering
        models = options.Apply(models.AsQueryable()).Cast<MyModelListDto>().ToArray()

        var result = _resourceFactory.CreateForOdataListEndpointUsingSkipTopPaging(models, _ => "items", m => m.Id, options);

        return Ok(result);
    }

    // GET, PUT, POST, ...
}

When to use HAL

• You want your API to be self-describing and discoverable by clients
• You want to support hypermedia-driven navigation and forms
• You want to enable dynamic client behavior based on available links/actions

Contributing

Contributions are welcome! Please open issues or submit pull requests.

License

This project is licensed under the Unlicense.