How to add "api" prefix to every end point in an asp .net core web API

Make your controller constructor with Route Prefix "api/"

For example lets say your controller class name is CustomerController

[Route("api/[controller]")]
public class CustomerController : ControllerBase
{

}

// This will become api/customer
[HttpGet]
public async Task<ActionResult> GetCustomers()
{
   // Code to get Customers
}

// This will become api/customer/{id}
[HttpGet]
[Route("{id}")]
public async Task<ActionResult> GetCustomerById(int id)
{
   // Code to get Customer by Id
}
    

You can custom MvcOptionsExtensions to set route prefix globally instead of change the route attribute manually.

1.custom MvcOptionsExtensions:

public static class MvcOptionsExtensions
{
    public static void UseRoutePrefix(this MvcOptions opts, IRouteTemplateProvider routeAttribute)
    {
        opts.Conventions.Add(new RoutePrefixConvention(routeAttribute));
    }

    public static void UseRoutePrefix(this MvcOptions opts, string
    prefix)
    {
        opts.UseRoutePrefix(new RouteAttribute(prefix));
    }
}
public class RoutePrefixConvention : IApplicationModelConvention
{
    private readonly AttributeRouteModel _routePrefix;
    public RoutePrefixConvention(IRouteTemplateProvider route)
    {
        _routePrefix = new AttributeRouteModel(route);
    }
    public void Apply(ApplicationModel application)
    {
        foreach (var selector in application.Controllers.SelectMany(c => c.Selectors))
        {
            if (selector.AttributeRouteModel != null)
            {
                selector.AttributeRouteModel = AttributeRouteModel.CombineAttributeRouteModel(_routePrefix, selector.AttributeRouteModel);
            }
            else
            {
                selector.AttributeRouteModel = _routePrefix;
            }
        }
    }
}

2:Register in Startup.cs(version before .Net6) or in Program.cs(version beyond .Net 6):

services.AddControllers(o =>{
    o.UseRoutePrefix("api");
});

Or:

builder.Services.AddControllers(o =>{
    o.UseRoutePrefix("api");
});

we can simply add that in top of the controller like this

[Route("api/[controller]")]
public class TestController : ControllerBase
{
    [HttpGet("version")]
    public IActionResult Get()
    {
        return new OkObjectResult("Version One");
    }
   [HttpGet("Types")]
    public IActionResult GetTypes()
    {
        return new OkObjectResult("Type One");
    }
}

so that you can access like below

....api/Test/version
....api/Test/Types