ASP.NET Core Web App log 404 requests as warning

Question

If I create a new ASP.NET Core Web App (Model-View-Controller) with dotnet new mvc it has already logging configured out of the box.

However, http request error codes are logged as info event.

info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
      Request starting HTTP/1.1 GET https://localhost:5001/doesnotexists
info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
      Request finished in 4.4052ms 404

Is it possible to changed this behaviour to (4xx -> warning, 5xx -> error) .i.e by investigating and rewriting log events.

I already read https://learn.microsoft.com/en-us/aspnet/core/fundamentals/logging/?view=aspnetcore-2.2

this looked promissing but I didn't find a hook for this purpose

    var webHost = new WebHostBuilder()
        .UseKestrel()
        .UseContentRoot(Directory.GetCurrentDirectory())
        .ConfigureLogging((hostingContext, logging) =>
        {
            // investigate and rewrite log event
        })
        .UseStartup<Startup>()
        .Build()

Answer 1

Single line solution

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

using Microsoft.Extensions.Logging;

namespace temp
{
    public class Startup
    {    
        public void Configure(IApplicationBuilder app, IHostEnvironment env, ILogger<Startup> logger)
        {

            ...

            app.UseStatusCodePages(async context =>
            {
                var code = context.HttpContext.Response.StatusCode;
                if (code == 404) {
                    logger.Log(LogLevel.Error, null, "log message");
                }
            });

            ...

        }
    }
}

Answer 2

You need to implement your own ILogger and ILoggerProvider.

A simple demo like:

1. CustomLogger

       public class CustomLogger : ILogger
       {
           public CustomLogger() { }
   
           public IDisposable BeginScope<TState>(TState state)=> NullScope.Instance;
   
           public bool IsEnabled(LogLevel logLevel) => true;
   
           public void Log<TState>(
               LogLevel logLevel,
               EventId eventId,
               TState state,
               Exception exception,
               Func<TState, Exception, string> formatter
           )
           {
   
               var msg = formatter(state, exception);
               if (msg.Contains("404"))
               {
                   logLevel = LogLevel.Warning;
               }
   
               Console.WriteLine($"{ GetLogLevelString(logLevel) } : { eventId } { msg } { exception }");
           }
           private static string GetLogLevelString(LogLevel logLevel)
           {
               switch (logLevel)
               {
                   case LogLevel.Trace:
                       return "trce";
                   case LogLevel.Debug:
                       return "dbug";
                   case LogLevel.Information:
                       return "info";
                   case LogLevel.Warning:
                       return "warn";
                   case LogLevel.Error:
                       return "fail";
                   case LogLevel.Critical:
                       return "crit";
                   default:
                       throw new ArgumentOutOfRangeException(nameof(logLevel));
               }
           }
   
       }
   

2. CustomLoggerProvider

   public class CustomLoggerProvider : ILoggerProvider
   {
       public ILogger CreateLogger(string categoryName)
       {
           return new CustomLogger();
       }
   
       public void Dispose()
       {
   
       }
   }
   

3. Register above code

           public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
               WebHost.CreateDefaultBuilder(args)
                   .UseStartup<Startup>()
                   .ConfigureLogging(config => {
                       config.ClearProviders();
                       config.AddProvider(new CustomLoggerProvider());
                   });
   

   You could check Logging and implement your own log provider.