.NET Core console application, how to configure appSettings per environment?

This is how we do it in our .netcore console app. The key here is to include the right dependencies on your project namely (may be not all, check based on your needs) and copy to output the appSetting.json as part of your buildoptions

  {
    "buildOptions": {
    "emitEntryPoint": true,
    "copyToOutput": {
       "include": [
       "appsettings*.json",
       "App*.config"
                 ]
          }
},

using Microsoft.Extensions.Configuration;
namespace MyApp
{
    public static void Main(string[] args)
    {
        var environmentName = Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT");
       

        var builder = new ConfigurationBuilder()
            .AddJsonFile($"appsettings.json", true, true)
            .AddJsonFile($"appsettings.{environmentName}.json", true, true)
            .AddEnvironmentVariables();
        var configuration = builder.Build();
        var myConnString= configuration.GetConnectionString("SQLConn");
    }
}

For those who are using .NET Core version 2.1.0+ and Microsoft.Extensions.Hosting to host your console app, you can use the following code (according to the Feiyu Zhou's answer in another thread):

var hostBuilder = new HostBuilder()
    .ConfigureHostConfiguration(config =>
    {
        if (args != null)
        {
            // enviroment from command line
            // e.g.: dotnet run --environment "Staging"
            config.AddCommandLine(args);
        }
    })
    .ConfigureAppConfiguration((context, builder) =>
    {
        builder.SetBasePath(AppContext.BaseDirectory)
            .AddJsonFile("appsettings.json", optional: false)
            .AddJsonFile($"appsettings.{context.HostingEnvironment.EnvironmentName}.json", optional: true);
    })

There are two IHostingEnvironment interfaces that you should use. One is for ASP.NET Core Applications, the other one is for .NET Core Console applications. You can use this code example for both:

using System;
using System.IO;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.FileProviders;
using Microsoft.Extensions.Hosting.Internal;

namespace MyApplication.Common
{
    public static class ConfigurationFactory
    {
        /// <summary>
        /// Use for ASP.NET Core Web applications.
        /// </summary>
        /// <param name="config"></param>
        /// <param name="env"></param>
        /// <returns></returns>
        public static IConfigurationBuilder Configure(IConfigurationBuilder config, IHostingEnvironment env)
        {
            return Configure(config, env.EnvironmentName);
        }

        /// <summary>
        /// Use for .NET Core Console applications.
        /// </summary>
        /// <param name="config"></param>
        /// <param name="env"></param>
        /// <returns></returns>
        private static IConfigurationBuilder Configure(IConfigurationBuilder config, Microsoft.Extensions.Hosting.IHostingEnvironment env)
        {
            return Configure(config, env.EnvironmentName);
        }

        private static IConfigurationBuilder Configure(IConfigurationBuilder config, string environmentName)
        {
            return config
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
                .AddJsonFile($"appsettings.{environmentName}.json", optional: true, reloadOnChange: true)
                .AddEnvironmentVariables();
        }

        /// <summary>
        /// Use for .NET Core Console applications.
        /// </summary>
        /// <returns></returns>
        public static IConfiguration CreateConfiguration()
        {
            var env = new HostingEnvironment
            {
                EnvironmentName = Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT") ?? "Production",
                ApplicationName = AppDomain.CurrentDomain.FriendlyName,
                ContentRootPath = AppDomain.CurrentDomain.BaseDirectory,
                ContentRootFileProvider = new PhysicalFileProvider(AppDomain.CurrentDomain.BaseDirectory)
            };

            var config = new ConfigurationBuilder();
            var configured = Configure(config, env);
            return configured.Build();
        }
    }
}

As of Net Core 3.1++, the generic Host class Microsoft.Extensions.Hosting.Host uses DOTNET_ENVIRONMENT environment variable instead of ASPNETCORE_ENVIRONMENT.

Setting DOTNET_ENVIRONMENT="Development" in Project Debug settings will work without any additional coding.

Or you can add your own prefix like this:

// works for WPF but should work similar for Console Apps
public partial class App : Application
{
    private readonly IHost _host;

    public App()
    {
        _host = Host.CreateDefaultBuilder()
            .ConfigureHostConfiguration(configHost => {
                configHost.AddEnvironmentVariables(prefix: "PREFIX_");
            })
            .ConfigureServices((context, services) =>
            {
                ConfigureServices(context.Configuration, services);
            })
            .Build();
    }

    private void ConfigureServices(
        IConfiguration configuration,
        IServiceCollection services)
    {
       // ...
    }

    // ...
}

If like me, you're simply trying to have a different configuration file for Release and Development mode, just add a appsettings.Development.json file with CopyToOutputDirectory setting set to true in the file's property window.

Now, to access the file depending on the build configuration, you can use the #if DEBUG preprocessor directive.

Here's an example :

static void Main(string[] args)
{

#if DEBUG
    var builder = new ConfigurationBuilder()
            .AddJsonFile($"appsettings.Development.json", true, true);
#else
    var builder = new ConfigurationBuilder()
            .AddJsonFile($"appsettings.json", true, true);
#endif

    var configuration = builder.Build();

    // ... use configuration
}