What is "name" property in the constructor for HttpGetAttribute?

As I can see the biggest advantage of the Name property of the HttpMethodAttribute (which is the base class of HttpGetAttribute) is that you can distinguish method overloads:

[HttpGet(Name="ById"]
public IActionResult GetBy(int id)
{

}

[HttpGet(Name="ByExternalId"]
public IActionResult GetBy(Guid id)
{

}  

So, this can contribute in the route selection.

EDIT: I've revised answer

The above sample code would result in an AmbiguousMatchException, where it is stating the same Template has been registered for different action.

I had put together another sample and used the following RouteDebugger to get insight. In the Configure method I've called the app.UseRouteDebugger() method to be able to see the registered routes in json format under the /route-debugger url.

[Route("api/[controller]")]
[ApiController]
public class TestController : ControllerBase
{
    [HttpGet()] 
    public IActionResult GetA(string a)
    {
        return Ok(nameof(GetA));
    }

    [HttpGet(template: "GetB")]
    public IActionResult GetB(string b)
    {
        return Ok(nameof(GetB));
    }

    [HttpGet(template: "GetC", Name= "GetD")]
    public IActionResult GetD(string d, string e)
    
    {
        return CreatedAtRoute(routeName:"GetC", routeValues: new { c = "v"}, value: null);
    }

    [HttpGet(template: "GetC/{c}", Name = "GetC")]
    public IActionResult GetC(string c)
    {
        return Ok(nameof(GetC));
    }
}

The route table would look like this:

[
   {
      "Action":"GetA",
      "Controller":"Test",
      "Name":null,
      "Template":"api/Test",
      "Contraint":[{}]
   },
   {
      "Action":"GetB",
      "Controller":"Test",
      "Name":null,
      "Template":"api/Test/GetB",
      "Contraint":[{}]
   },
   {
      "Action":"GetD",
      "Controller":"Test",
      "Name":"GetD",
      "Template":"api/Test/GetC",
      "Contraint":[{}]
   },
   {
      "Action":"GetC",
      "Controller":"Test",
      "Name":"GetC",
      "Template":"api/Test/GetC/{c}",
      "Contraint":[{}]
   }
]

As you seen the following happened:

GetA method

• It is exposed under the controller route because not Template has been specified.
• The route itself does not have a Name so you can't refer to this route via its name inside ActionLink or CreatedAtRoute, etc.

GetB method

• It is exposed under the api/test/getb, because the controller's and the action's Template properties are combined.
• The route itself does not have a Name so you can't refer to this route via its name inside ActionLink or CreatedAtRoute, etc.

GetC method

• It is exposed under the api/test/getc/{c}, because the controller's and the action's Template properties are combined. The c parameter can accept any value. If it is not provided then GetD will be called, because that was registered first.
• The route has a Name (GetC) so you can refer to this route via its name inside ActionLink or CreatedAtRoute, etc. Like as we did it inside GetD

GetD method

• It is exposed under the api/test/getc, because the controller's and the action's Template properties are combined. Because it was registered prior GetC method's route that's why it will called if no further path is provided.
• The route has a Name (GetD). In the CreatedAtRoute we are referring to GetC via its name not through its route. If we are replacing the Name to GetC then it would throw the following exception at runtime:

InvalidOperationException: The following errors occurred with attribute routing information:

Error 1: Attribute routes with the same name 'GetC' must have the same template: Action: 'Controllers.TestController.GetD ()' - Template: 'api/Test/GetC' Action: 'Controllers.TestController.GetC ()' - Template: 'api/Test/GetC/{c}

Summary

1. If you register two routes with the same Template it will throw an AmbiguousMatchException at runtime when you make a call against that route. (So, other routes will work perfectly fine)
2. If you register two routes with the same Name it will throw an InvalidOperationException at runtime when you make any call. (So, other routes will not work)
3. Route's Name provides the ability to refer them easily without knowing the exact Template. This separation allows us to change the Template without affecting referring links.

Name is the route name, as distinct from the template. See the docs page.

The docs state:

Route names can be used to generate a URL based on a specific route. Route names:

• Have no impact on the URL matching behavior of routing.
• Are only used for URL generation.

Route names must be unique application-wide.

You can use the name to generate URLs for named routes using IUrlHelper. For example, if you named a route "Konrad", you could generate a link like so:

string url = urlHelper.Link("Konrad", new { id = 5, query = "test" });

Where id and query are the route parameters.

By the way, the issue you had with the documentation is that HttpGet is an Attribute. The attribute syntax allows you to specify values for the attributes' properties by name after any positional constructor values.

Considering the following attribute, you can see that the constructor accepts int a, but there's also a string propery: B.

public class TestAttribute : System.Attribute
{
    public TestAttribute(int a)
    {
    }

    public string B {get;set;}
}

To use such an attribute, you could apply it in the following ways:

[TestAttribute(5)] // B is null
[TestAttribute(5, B = "hello")] // B is "hello"

or simply as Test:

[Test(5)] // B is null
[Test(5, B = "hello")] // B is "hello"

It says they can be used to generate a URL based on a specific route. I don't think I understand what it means (despite understanding the words as such). Please elaborate

Here are my two cents:

When designing a proper API, the level 3 of maturity model talks about HATEOAS constraint (Hypermedia As Transfer Engine Of Application State). More information about it can be found at : Maturity models of Rest API

In order to traverse through a resource and to know the actions that are available with that resource we generate links for the resource.

The generation of links is done with the help of URLHelper library, which takes in a name parameter to define the link. The name we associate with in the URLHelper library, is the name defined for HTTP GET/POST/PUT etc action using the attribute Name parameter.

In simple terms, it is an identifier for your Route.

P.S

A complete working example of a web api with HATEOAS can be found on github:

Web Api with HATEOAS example

I think this and this could help.

Route name

The following code defines a route name of Products_List:

[ApiController]
public class Products2ApiController : ControllerBase
{
    [HttpGet("/products2/{id}", Name = "Products_List")]
    public IActionResult GetProduct(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

Route names can be used to generate a URL based on a specific route. Route names:

Have no impact on the URL matching behavior of routing.
Are only used for URL generation.
Route names must be unique application-wide.

Contrast the preceding code with the conventional default route, which defines the id parameter as optional ({id?}). The ability to precisely specify APIs has advantages, such as allowing /products and /products/5 to be dispatched to different actions.

In asp.net core source code I've found only this usage

[HttpGet("{id}", Name = "FindPetById")]
[ProducesResponseType(typeof(Pet),  StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status401Unauthorized)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public async Task<ActionResult<Pet>> FindById(int id)
{
     var pet = await DbContext.Pets
                .Include(p => p.Category)
                .Include(p => p.Images)
                .Include(p => p.Tags)
                .FirstOrDefaultAsync(p => p.Id == id);
     if (pet == null)
     {
         return new NotFoundResult();
     }

     return pet;
}