API/TOOHUCardAPI/Controllers/MethodBasedController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using TOOHUCardAPI.Data;

using MethodMap = System.Collections.Generic.Dictionary<string, System.Reflection.MethodInfo>;
namespace TOOHUCardAPI.Controllers
{
    public class MethodBasedController : ControllerBase
    {
        public delegate Task<IActionResult> EndpointHandler(string requestBody);
        private static Dictionary<Type, MethodMap> MethodMapByType = new Dictionary<Type, MethodMap>();
        
        protected Task<IActionResult> InvokeEndpointHandlerForMethod<T>(object _this, string method, string body)
        {
            MethodMap registeredEndpointHandlers = GetMethodMapForType<T>();
            if (registeredEndpointHandlers.ContainsKey(method))
            {
                return ((EndpointHandler) registeredEndpointHandlers[method]
                    .CreateDelegate(typeof(EndpointHandler), _this))(body);
            }
            throw new MissingEndpointHandlerException(method);
        }

        private MethodMap GetMethodMapForType<T>()
        {
            if (!MethodMapByType.ContainsKey(typeof(T)))
            {
                MethodMapByType[typeof(T)] = RegisterEndpointHandlers<T>();
            }
            return MethodMapByType[typeof(T)];
        }

        private static MethodMap RegisterEndpointHandlers<T>()
        {
            MethodInfo[] methods = typeof(T).GetMethods(BindingFlags.NonPublic | BindingFlags.Instance);
            return methods
                .Aggregate(new MethodMap(), (handlers, m) =>
                {
                    Attribute attr = m.GetCustomAttribute(typeof(EndpointHandlerAttribute), false);
                    if (attr != null)
                    {
                        EndpointHandlerAttribute e = (EndpointHandlerAttribute) attr;
                        handlers.Add(e.Method, m);
                    }
                    return handlers;
                });
        }
    }
    
    public class MissingEndpointHandlerException : Exception
    {
        public MissingEndpointHandlerException(string method) : base($"Handler for [{method}] is either missing or incorrectly setup.")
        {
        }
    }
}