How does FromQueryAttribute actually work and why do I need it?

Question

I'm working with some pre-existing code that uses an HttpGet endpoint to get all comments. It takes some default parameters and before each parameter it is decorated with a [FromQuery] attribute. I'm a little confused about why we need this here and what it really does for me.

There isn't much online all I found was this documentation: (https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.fromqueryattribute?view=aspnetcore-2.2)

This doesn't answer my question though...

• What is a query string?
• How is it used in endpoints?
• When is it that I don't need to specify this attribute? I don't see it being used very much.

[HttpGet]
[ProducesResponseType(typeof(FooResponse), 200),
public async Task<ActionResult<FooResponse>> GetAll([FromQuery]string sortColumn = "CommentId",
    [FromQuery]SortDirections sortDirection = SortDirections.Asc, 
    [FromQuery]string filter = "", 
    [FromQuery]int page = 1, 
    [FromQuery]int pageSize = 10)
{
    var data = await _mediator.Send(new GetAllComments(sortColumn, sortDirection, filter, page, pageSize));
.
.
.
    return Ok(data);
}

I suspect that it has something to do with what is passed into the URL but I'm not quite sure...

Answer 1

The usage here is superfluous. The action would function the same without [FromQuery] being applied to all the params, as it only responds to GET requests.

What is a query string?

The query string is the portion of the URI after the ? character is applied. It constitutes a data portion of a URI versus a pure routing portion.

How is it used in endpoints?

The question here is a little tenuous, so it would be more appropriate to ask "How is it used in a request?" The answer to that it is a way to pass data along with a GET request to a particular URI. Unlike other HTTP methods, GET does not really allow a request "body". (Actually, the spec technically does allow this, but it's almost universally not implemented that way.)

When is it that I don't need to specify this attribute? I don't see it being used very much.

Again, a better question is "When do I actually need to specify this attribute?" The answer to that is basically when it's ambiguous where the data is coming from. This will almost invariably be with other HTTP methods such as POST. By default, data is expected to come from the request body there, and depending on the content type of that request body, the binding is expected to be either FromForm or FromBody, which will usually be the default for params there. (Which is the default depends on whether you're dealing with a traditional MVC-style controller or an API-style controller.) If you need to actually get a particular param from the query string instead of the body in such a scenario, then you would apply the [FromQuery] attribute to that particular param.