Global Error Handling in ASP.NET Core Web API using NLog

using CustomExceptionHandlerPOC.Models;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace CustomExceptionHandlerPOC.CustomDB
{
    public class PurchaseOrderDB
    {
        public static List<PurchaseDetails> GetPurchaseOrders()
        {
            return new List<PurchaseDetails>
            {
               new PurchaseDetails { ProductName="Laptop", Rate=80000, Qty=2, Amount=160000},
               new PurchaseDetails { ProductName="Dekstop", Rate=40000, Qty=1, Amount=40000},
               new PurchaseDetails { ProductName="Hard Disk", Rate=4000, Qty=10, Amount=40000},
               new PurchaseDetails { ProductName="Pen Drive", Rate=600, Qty=10, Amount=6000},
            };
        }
    }
} 

Step 8. Now let’s create middleware. Create a “ExceptionHandlerMiddleware” folder in the application. Inside “ExceptionHandlerMiddleware” folder create a class “ExceptionMiddleware”.

using CustomExceptionHandlerPOC.Logger;
using CustomExceptionHandlerPOC.Models;
using Microsoft.AspNetCore.Http;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Threading.Tasks;

namespace CustomExceptionHandlerPOC.ExceptionHandlerMiddleware
{
    public class ExceptionMiddleware
    {
        private readonly RequestDelegate _next;
        private readonly ILoggerService _logger;
        public ExceptionMiddleware(RequestDelegate next, ILoggerService logger)
        {
            _logger = logger;
            _next = next;
        }
        public async Task Invoke(HttpContext httpContext)
        {
            try
            {
                await _next(httpContext);
            }
            catch (Exception ex)
            {
                _logger.LogError($"Something went wrong: {ex}");
                await HandleException(httpContext, ex);
            }
        }
        private async Task HandleException(HttpContext context, Exception exception)
        {
            context.Response.ContentType = "application/json";
            context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
            await context.Response.WriteAsync(new ErrorInfo()
            {
                StatusCode = context.Response.StatusCode,
                Message = "Internal Server Error from the custom middleware."
            }.ToString());
        }
    }
}

Step 9. Now let’s register the middleware and configure the “nlog.config”.

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using NLog;
using System.IO;
using CustomExceptionHandlerPOC.Logger;
using CustomExceptionHandlerPOC.ExceptionHandlerMiddleware;
namespace CustomExceptionHandlerPOC
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            LogManager.LoadConfiguration(string.Concat(Directory.GetCurrentDirectory(), "/nlog.config"));
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddCors();

            // Configure Logger service. 
            services.AddSingleton<ILoggerService, LoggerService >();

            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();
            }
            //Configure Exception Middelware
            app.UseMiddleware<ExceptionMiddleware>();

            app.UseRouting();

            app.UseAuthorization();

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

Step 10. Now let’s create a PurchaseOrder Controller inside Controllers folder.

using CustomExceptionHandlerPOC.CustomDB;
using CustomExceptionHandlerPOC.Logger;
using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace CustomExceptionHandlerPOC.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class PurchaseOrderController : ControllerBase
    {
        private ILoggerService _logger;
        public PurchaseOrderController(ILoggerService logger)
        {
            _logger = logger;
        }
        [HttpGet("GetPurchaseOrder")]
        public IActionResult GetPurchaseOrder()
        {
            _logger.LogInfo("Fetching All Purchase orders");
            // Get Purchase data from  PurchaseOrder DB
            var purchaseOrders = PurchaseOrderDB.GetPurchaseOrders();

         // Uncomment below two lines code to get exception.
            //var num1 = 100;
            //dynamic num = num1 / 0;

            _logger.LogInfo($"Total purchase order count: {purchaseOrders.Count}");
            return Ok(purchaseOrders);
        }
    }
}

Step 11. Now run the application. Open the Postman tool and test the API. The API will return the Purchase order data. Please see the below scree shot.

Now open the logs folder. You will get a file name (DDDD-MM-YY_logfile.txt) which logs details.

Step 12. Now let’s break the code and try to generate error. For this uncomment the two lines of code in GetPurchaseOrder() endpoint.

var num1 = 100;

dynamic num = num1 / 0;

Open the Postman tool and test the API. You will get error in response as below.

Now check the logfile, the error info will be written in the logfile as below.

Summary

We learned how to handle expectation globally using NLog in ASP.NET core web API. I hope this article will help you. Happy learning!!