ASP NET Core Define API Controller within class - asp.net-core

Question

I'm currently switching from .net framework to .net core 3.1.
Defining Api Controllers inside the namespace is all fine and works.
Now I have the case, that I need to declare the Api Controllers within another class, like this:
namespace Api.Controllers
{
public class MainClass : BaseClass
{
public MainClass()
{
}
[ApiController]
[Route("Test")]
public class TestController : ControllerBase
{
[HttpGet]
public int GetResult()
{
return 0;
}
}
}
}
The result is, that the Api Controller can't be found after calling the "AddControllers" method inside the "ConfigureServices" method of the startup class.
The MainClass is instantiated before the Startup class will be called.
I've tried to change the global route and defining an area like "{area=Test}/{controller=Test}", or set the ApiController attribute above the MainClass, but none of them worked.
Is there a way to get this working?

Answer 1

Looks like the default ControllerFeatureProvider does not treat nested controller types as controller. You can add (don't need to replace) your custom provider to change that behavior, like this:
public class NestedControllerFeatureProvider : ControllerFeatureProvider
{
protected override bool IsController(TypeInfo typeInfo)
{
if(!typeInfo.IsClass) return false;
if(typeInfo.IsAbstract) return false;
var isNestedType = typeInfo.DeclaringType != null;
var isPublic = true;
var parentType = typeInfo.DeclaringType;
while(parentType != null){
isPublic = parentType.IsPublic;
parentType = parentType.DeclaringType; ​
​ }
​ return isNestedType && isPublic; ​
}
}
Then add that provider to the ApplicationPartManager in Startup.ConfigureServices like this:
​​services
.AddMvc()
​​ .ConfigureApplicationPartManager(appPart => {
​​appPart.FeatureProviders.Add(new NestedControllerFeatureProvider());
});
If you want to replace the default ControllerFeatureProvider, just find it in the FeatureProviders and remove it. Of course then you need to ensure that your custom one should handle everything just like what done by the default logic, something like this:
​​//for IsController
return base.IsController(typeInfo) || <...your-custom-logic...>;
NOTE: You can refer to the default implementation of ControllerFeatureProvider to learn some standard logic to implement your own logic correctly. The code above is just a basic example. To me, as long as the classes inherits from ControllerBase and not abstract, they can work fine as a controller to serve requests. There would be no serious troubles except some weird conventions (e.g: class name not ending with Controller is still a controller or some standard attributes applied on the controller class are not working ...).
We should not use nested controller classes. Each controller class should be put in a separate file (as a good practice). However the point of this answer (the most interesting part that I'm pretty sure not many know about, is the use of ControllerFeatureProvider which can help you customize the features set in other scenarios). And really if you really have to stick with your design somehow, you of course have to use this solution, no other way.