Read appsettings.json in Main Program.cs

Question

First of all my main purpose is to setup the IP and Port for my application dynamically.

I'm using IConfiguration to inject a json config file, like some tutorial mentioned.

However, I can't retrieve the configuration in Program.cs, because my WebHostBuilder will use the StartUp and Url at the same time.

So at the time the host build up, there is nothing in my configuration.

WebProtocolSettings settings_Web = new WebProtocolSettings();
var host = new WebHostBuilder()
                .UseIISIntegration()
                .UseKestrel()
                .UseContentRoot(Directory.GetCurrentDirectory())
                .UseStartup<Startup>()
                .UseUrls(settings_Web.Url + ":" + settings_Web.Port)
                .Build();

In Startup.cs

public Startup(IHostingEnvironment env)
{
    // Set up configuration sources.
    var builder = new ConfigurationBuilder()
        .SetBasePath(env.ContentRootPath)
        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true);

    Configuration = builder.Build();
}

public IConfigurationRoot Configuration { get; set; }

public void ConfigureServices(IServiceCollection services)
{
    // Adds services required for using options.
    services.AddOptions();

    var _WebProtocolSettings = Configuration.GetSection("WebProtocolSettings");

    // Register the IConfiguration instance
    services.Configure<WebProtocolSettings>(_WebProtocolSettings);
}

My appsettings.json:

{
    "WebProtocolSettings": {
        "Url": "127.0.0.1",
        "Port": 5050
    }
}

My WebProtocolSettings.cs:

public class WebProtocolSettings
{
    public string Url { get; set; }
    public int Port { get; set; }
}

Answer 1

Update .Net 6

It's now easy to get any settings from the ConfigurationManager by calling the GetValue<type>(string key) extension method. You can also use Index(string key) to return a string. See this answer.

---

You must build a configuration in your main method, get the section and bind it to your model. No way around it.

public static void Main(string[] args)
{
    var config = new ConfigurationBuilder()
        .AddJsonFile("appsettings.json", optional: false)
        .Build();

    WebProtocolSettings settings_Web = new WebProtocolSettings();
    config.GetSection("WebProtocolSettings").Bind(settings_Web);

    var host = new WebHostBuilder()
            .UseIISIntegration()
            .UseKestrel()
            .UseContentRoot(Directory.GetCurrentDirectory())
            .UseStartup<Startup>()
            .UseUrls(settings_Web.Url + ":" + settings_Web.Port)
            .Build()

    host.Run();
}

##Update

An alternative way of doing it is by passing the configuration to UseConfiguration as described in the

public static void Main(string[] args)
{
    var config = new ConfigurationBuilder()
        .SetBasePath(Directory.GetCurrentDirectory())
        .AddJsonFile("hosting.json", optional: true)
        .AddCommandLine(args)
        .Build();

    var host = new WebHostBuilder()
        .UseUrls("http://*:5000")
        .UseConfiguration(config)
        .UseKestrel()
        .Configure(app =>
        {
            app.Run(context => 
                context.Response.WriteAsync("Hello, World!"));
        })
        .Build();

    host.Run();
}

or in ASP.NET Core > 2.0

public static void Main(string[] args)
{
    BuildWebHost(args).Run();
}

public static IWebHost BuildWebHost(string[] args)
{
    var config = new ConfigurationBuilder()
        .SetBasePath(Directory.GetCurrentDirectory())
        .AddJsonFile("hosting.json", optional: true)
        .AddCommandLine(args)
        .Build();

    return WebHost.CreateDefaultBuilder(args)
        .UseUrls("http://*:5000")
        .UseConfiguration(config)
        .Configure(app =>
        {
            app.Run(context => 
                context.Response.WriteAsync("Hello, World!"));
        })
        .Build();
}

Answer 2

In .NET 6

var builder = WebApplication.CreateBuilder(args);

// Using the GetValue<type>(string key) method
var configValue = builder.Configuration.GetValue<string>("Authentication:CookieAuthentication:LoginPath");

// or using the index property (which always returns a string)
var configValue = builder.Configuration["Authentication:CookieAuthentication:LoginPath"];