Overview

• ASP.NET Core offers the following three options for Web API controller action return types: Specific Type, IActionResult, and ActionResult<T>.

Specific Type

• It returns a primitive or complex data type.
  • Without known conditions to safeguard against during action execution, returning a specific type could suffice.
• The preceding action accepts no parameters, so parameter constraints validation isn't needed.
• When known conditions need to be accounted for in an action, multiple return paths are introduced.
  • In such a case, it's common to mix an ActionResult return type with the primitive or complex return type.
  • Either IActionResult or ActionResult<T> are necessary to accommodate this type of action.

[HttpGet]
public IEnumerable<Product> Get() => _repository.GetProducts()

IActionResult Type

• The IActionResult return type is appropriate when multiple ActionResult return types are possible in an action.
• The ActionResult types represent various HTTP status codes.
• Because there are multiple return types and paths in the action, liberal use of the ProducesResponseType attribute is necessary.
• This attribute produces more descriptive response details for API help pages generated by tools like Swagger.
• It indicates the known types and HTTP status codes to be returned by the action.

Synchronous Action

[HttpGet("{id}")]
[ProducesResponseType(200, Type = typeof(Product))]
[ProducesResponseType(404)]
public IActionResult GetById(int id)
{
    if (!_repository.TryGetProduct(id, out var product))
        return NotFound();
    return Ok(product);
}

Asynchronous Action

• The preceding action's other known return code is a 201, which is generated by the CreatedAtRoute helper method.

[HttpPost]
[ProducesResponseType(201, Type = typeof(Product))]
[ProducesResponseType(400)]
public async Task<IActionResult> CreateAsync([FromBody] Product product)
{
    if (!ModelState.IsValid)
        return BadRequest(ModelState);

    await _repository.AddProductAsync(product);
    return CreatedAtAction(nameof(GetById), new { id = product.Id }, product);
}

ActionResult<T> Type

• The ActionResult<T> enables you to return a type deriving from ActionResult or return a specific type.