Adding an API controller to an ASP.NET Core project

I've just fired up VS and did a new .net core Web Api project from template and it works as you would expect to work:

[ApiController]
[Route("api/TheApi")]
public class WeatherForecastController : ControllerBase
{
    private static readonly string[] Summaries = new[]
    {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

    private readonly ILogger<WeatherForecastController> _logger;

    public WeatherForecastController(ILogger<WeatherForecastController> logger)
    {
        _logger = logger;
    }

    [HttpGet]
    public IEnumerable<WeatherForecast> Get()
    {
        var rng = new Random();
        return Enumerable.Range(1, 5).Select(index => new WeatherForecast
        {
            Date = DateTime.Now.AddDays(index),
            TemperatureC = rng.Next(-20, 55),
            Summary = Summaries[rng.Next(Summaries.Length)]
        })
        .ToArray();
    }
    [HttpGet("OtherGet")]
    public IEnumerable<WeatherForecast> OtherGet()
    {
        var rng = new Random();
        return Enumerable.Range(1, 2).Select(index => new WeatherForecast
        {
            Date = DateTime.Now.AddDays(index),
            TemperatureC = rng.Next(-20, 55),
            Summary = Summaries[rng.Next(Summaries.Length)]
        })
        .ToArray();
    }
}

Valid urls:

• https://localhost:44308/api/TheApi -->calls the Get method
• https://localhost:44308/api/TheApi/Get -->calls the Get method
• https://localhost:44308/api/TheApi/OtherGet -->calls the OtherGet method, that have the OtherGet route name in the HttpGet annotation.

Startup.cs looks like this:

public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseHttpsRedirection();

            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }

UPDATE: Also keep in mind that you can not have multiple GET/POST etc methods in one controller, only if you use the annotation for the others (or for all of them) like I did with OtherGet: HttpGet("OtherGet")]. If you don't use this annotation and have multiple GET, POST etc methods you will have exception: AmbiguousMatchException: The request matched multiple endpoints.

After a lot of very unpleasant trial and error I can now report that it was indeed Startup.cs that needed to be updated -- scaffolding the webAPI controller had not updated it.

In my

public void Configure(IApplicationBuilder app, IWebHostEnvironment env, IServiceProvider serviceProvider)

I had

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapRazorPages();
            });

and it needed to be changed to

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapRazorPages();
                endpoints.MapControllers(); // Added for webAPI.
            });

By default, Web API routes are defined like so: api/{controller}. Notice there is no {action} present.

They are this way to conform to the REST (Representational state transfer) way of doing things. It is based on the notion of resources and verbs that are applied to them; commonly GET, UPDATE, POST, DELETE.

So with the default route, this would match:

GET api/TheApi

namespace TheAspNetCoreProject
{
    [Route("api/TheApi")]
    [ApiController]
    public class TheApiController : ControllerBase
    {
        public IActionResult Get()
        {
            return Ok("Foo");
        }
    }
}

Of course, the route can always be changed to be more RPC-like.