ASP.net: How to log from within CreateHostBuilder()?

user_33404 0
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I have an ASP.net Core 3.1 web application. In the Program.cs class, I have implemented the CreateHostBuilder() static method similar to this:

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host
        .CreateDefaultBuilder(args)
        .ConfigureAppConfiguration((context, config) =>
        {
            try 
            {
                // do some configuration work that might fail
            }
            catch (Exception ex)
            {
                // how can I write to a log from here?
            }
        })
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseStartup<Startup>();
        });
}

If an exception is thrown when configuring the app, how can I log it using an ILogger when I catch it?

asp.net-core logging exception configuration .net
2 Answers

Microsoft's "Logging in .NET Core and ASP.NET Core" documentation article says:

Logging during host construction isn't directly supported. However, a separate logger can be used.

They go on to give this example using Serilog:

public static IHostBuilder CreateHostBuilder(string[] args)
{
    var builtConfig = new ConfigurationBuilder()
        .AddJsonFile("appsettings.json")
        .AddCommandLine(args)
        .Build();

    Log.Logger = new LoggerConfiguration()
        .WriteTo.Console()
        .WriteTo.File(builtConfig["Logging:FilePath"])
        .CreateLogger();

    try
    {
        return Host.CreateDefaultBuilder(args)
            .ConfigureServices((context, services) =>
            {
                services.AddRazorPages();
            })
            .ConfigureAppConfiguration((hostingContext, config) =>
            {
                config.AddConfiguration(builtConfig);
            })
            .ConfigureLogging(logging =>
            {   
                logging.AddSerilog();
            })
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });
    }
    catch (Exception ex)
    {
        Log.Fatal(ex, "Host builder error");

        throw;
    }
    finally
    {
        Log.CloseAndFlush();
    }
}
user_334040

Place your logger outside the Main method to make it accessible from CreateHostBuilder

using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;
using NLog.Common;
using System;
using System.IO;

namespace WebApplication1
{
    public class Program
    {
        // Place your logger outside Main to make it accessible from CreateHostBuilder
        private static NLog.Logger logger;

        public static void Main(string[] args)
        {
            // Manually setup your desired logger.
            // I've used NLog for demonstration purposes but you can use your own preferred logger.
            var today = DateTime.Today.ToString("yyyy-MM-dd");
            var binDirectory = GetBinDirectory<Program>();
            var nLogInternalFullPath = Path.Combine(binDirectory, "NLog", $"{today}-internal.log");

            InternalLogger.LogFile = nLogInternalFullPath;

            // Assign logger. Make sure you have the file NLog.config
            // https://github.com/NLog/NLog/wiki/File-target
            logger = NLog.LogManager.LoadConfiguration("NLog.config").GetCurrentClassLogger();

            CreateHostBuilder(args).Build().Run();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureAppConfiguration((context, config) =>
                {
                    try
                    {
                        // do some configuration work that might fail
                    }
                    catch (Exception ex)
                    {
                        // Log errors
                        logger.Error(ex, "Unhandled error in ConfigureAppConfiguration.");
                    }
                })
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>();
                });

        public static string GetBinDirectory<T>()
        {
            return Path.GetDirectoryName(typeof(T).Assembly.Location);
        }
    }
}
user_85199250
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