为什么使用 CreatedAtAction 在 WebAPI POST 方法中返回 object？

Question

When writing a POST method for an API in Asp.NET Core there is the possibility to use CreatedAtAction

Following example from the documentation

[HttpPost]
[Consumes(MediaTypeNames.Application.Json)]
[ProducesResponseType(StatusCodes.Status201Created)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<IActionResult> CreateAsync(Product product)
{
    if (product.Description.Contains("XYZ Widget"))
    {
        return BadRequest();
    }

    await _repository.AddProductAsync(product);

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

The CreatedAtAction action result has following effects:

1. The Response Status Code will be 201
2. A "Location" response Header will be included with the URI of the new resource
3. The created object will be returned in the response body

The first two seem reasonable to me. However I wonder why the third effect would be desired? At first it seems to be an unnecessary increase of the response size, yet I am curious to understand why that pattern is presented in the documentation as it is, including the created object. What is the benefit of returning the created resource, which was sent as request anyway? Or could it be that the (only) reason for including the object in the response is to include values which are generated server side, as for example a create date or a primary key?

Answer 1

The server can generate new data that the client will not know about, such as an ID or timestamps for creation or modification. If the client requires this data after the initial request, you save an unnecessary round trip by including the created resource in the response body.

It's not always the best choice. You have weigh the benefit of having immediate access against the increased bandwidth use. Some consideration is warranted before returning large resources this way.