How to fix ASP.NET Core WEB API ? localhost error

Question

I am trying to run an API test first but then whenever i tried to run it it gives me this error localhost cannot be found even on the localhost:5125 still wont run

Answer 1

I guess you have created your Web API app with the target framework of .NET 9.0. Your app is probably working and will return JSON string as expected when you request the WeatherForecast as shown below:

Answer 2

Hi @LOGAN PANUCAT,

404 means the requested url cannot be found on the server. You need check if your app actually contains such endpoint with requested route /.

Note: If you use default WEB API project template, there is no route for / which is quite different with MVC Project.

Default WEB API project with target framework .net 8 or lower, it will integrate with Swagger, you can access the url:https://localhost:7211/swagger/index.html.

Default WEB API project with target framework .net 9, it will integrate with OpenAPI, you can access the url:http://localhost:7211/openapi/v1.json.

Only if you define the endpoint like the following examples, you can access the url / .

For Example:

Open your Program.cs (or Startup.cs if using an older version) and check if you have the following code.

app.MapGet("/", () => "Hello, World!");

Or check your controller if define the route like below:

[ApiController]
[Route("api/[controller]")]  // This is the default route for this controller
public class TestController : ControllerBase
{
    [HttpGet("/")]  // This explicitly maps to the root `/`
    public IActionResult Root() => Ok("Welcome to my API!");
    [HttpGet] // This will respond to `/api/test`
    public IActionResult Get() => Ok("This is the Test API.");
}

More details you could refer to the official document for better understanding:

Get Started with ASP.NET Core WEB API

Attribute routing for REST APIs

---

If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Best regards,

Rena

Answer 3

Usually, you will have to call a controller in order to get a response back. Also, if you are using swagger, you should be able to access the swagger ui using https://localhost:7211/swagger

If you still get a 404, you should share the code of the controller as well as the startup code so that we can help you.

Answer 4

If this is an ASP.NET Core 9 Web API project and you did not configure a root route then a 404 is expected. Try accessing one of your HTTP GET actions or setup a root route.

    [Route("api/[controller]")]
    [ApiController]
    public class ValuesController : ControllerBase
    {
        private ILogger<ValuesController> _logger;
        public ValuesController(ILogger<ValuesController> logger)
        {
            _logger = logger;
        }

        [HttpGet("/")]
        public IActionResult Get()
        {
            _logger.LogInformation("Hello from the root route.");
            return Ok(new { message = "Hello from the root route." });
        }
    }

If this is an If this is an ASP.NET Core 9 Web API project and you expect to see a documentation GUI in the browser (Open API) and have not configured one then see the official documentation.

https://learn.microsoft.com/en-us/aspnet/core/fundamentals/openapi/aspnetcore-openapi?view=aspnetcore-9.0&tabs=visual-studio