# ASP.NET Configuration

## `.csproj`

```xml
<PropertyGroup>
  <!-- enable documentation comments (can be used for swagger) -->
  <GenerateDocumentationFile>true</GenerateDocumentationFile>

  <!-- do not warn public classes w/o documentation comments -->
  <NoWarn>$(NoWarn);1591</NoWarn>
</PropertyGroup>
```

## `Program.cs`

```cs
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;

namespace App
{
    public class Program
    {
        public static void Main(string[] args)
        {
            CreateHostBuilder(args).Build().Run();  // start and config ASP.NET Core App

            // or start Blazor WASM Single Page App
            var builder = WebAssemblyHostBuilder.CreateDefault(args);
            builder.RootComponents.Add<App>("#app");

            builder.Services.AddScoped(sp => new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) });

            await builder.Build().RunAsync();
        }

        // for MVC, Razor Pages and Blazor Server
        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>();  // config handled in Startup.cs
                });
    }
}
```

## `Startup.cs`

```cs
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace App
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the DI container.
        public void ConfigureServices(IServiceCollection services)
        {
            // set db context for the app using the connection string
            services.AddDbContext<AppDbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

            // Captures synchronous and asynchronous Exception instances from the pipeline and generates HTML error responses.
            services.AddDatabaseDeveloperPageExceptionFilter();

            // use Razor Pages, runtime compilation needs Microsoft.AspNetCore.Mvc.Razor.RuntimeCompilation pkg
            services.AddRazorPages().AddRazorRuntimeCompilation();
            // or
            services.AddControllers();  // controllers w/o views
            //or
            services.AddControllersWithViews();  // MVC Controllers
            //or
            services.AddServerSideBlazor();  // needs Razor Pages

            services.AddSignalR();

            // set dependency injection lifetimes
            services.AddSingleton<ITransientService, ServiceImplementation>();
            services.AddScoped<ITransientService, ServiceImplementation>();
            services.AddTransient<ITransientService, ServiceImplementation>();

            // add swagger
            services.AddSwaggerGen(options => {

                // OPTIONAL: use xml comments for swagger documentation
                var xmlFilename = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
                options.IncludeXmlComments(Path.Combine(AppContext.BaseDirectory, xmlFilename));
            });

        }

        // 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();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();

            app.UseRouting();

            app.UseAuthorization();

            app.UseSwagger();
            app.UseSwaggerUI();

            app.UseEndpoints(endpoints =>
            {
                // MVC routing
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}"
                );
                // or
                endpoints.MapControllers();  // map controllers w/o views
                // or
                endpoints.MapRazorPages();
                // or
                endpoints.MapBlazorHub();  // SignalR Hub for Blazor Server

                endpoints.MapHub("/hub/endpoint");  // SignalR Hub
                endpoints.MapFallbackToPage("/_Host");  // fallback for razor server
            });
        }
    }
}
```

## Application Settings

App settings are loaded (in order) from:

1. `appsettings.json`
2. `appsettings.<Environment>.json`
3. User Secrets

The environment is controlled by the env var `ASPNETCORE_ENVIRONMENT`. If a setting is present in multiple locations, the last one is used and overrides the previous ones.

### User Secrets

User secrets are specific to each machine and can be initialized with `dotnet user-secrets init`. Each application is linked with it's settings by a guid.

The settings are stored in:

- `%APPDATA%\Microsoft\UserSecrets\<user_secrets_id>\secrets.json` (Windows)
- `~/.microsoft/usersecrets/<user_secrets_id>/secrets.json` (Linux/macOS)

Setting a value is done with `dotnet user-secrets set <key> <value>`, keys can be nested by separating each level with `:` or `__`.

## Options Pattern

The *options pattern* uses classes to provide strongly-typed access to groups of related settings.

```json
{
    "SecretKey": "Secret key value",
    "TransientFaultHandlingOptions": {
        "Enabled": true,
        "AutoRetryDelay": "00:00:07"
    },
    "Logging": {
        "LogLevel": {
            "Default": "Information",
            "Microsoft": "Warning",
            "Microsoft.Hosting.Lifetime": "Information"
        }
    }
}
```

```cs
// options model for binding
public class TransientFaultHandlingOptions
{
    public bool Enabled { get; set; }
    public TimeSpan AutoRetryDelay { get; set; }
}
```

```cs
// setup the options
builder.Services.Configure<TransientFaultHandlingOptions>(builder.Configuration.GetSection<TransientFaultHandlingOptions>(nameof(Options)));
builder.Services.Configure<TransientFaultHandlingOptions>(builder.Configuration.GetSection<TransientFaultHandlingOptions>(key));
```

```cs
class DependsOnOptions
{
  private readonly IOptions<TransientFaultHandlingOptions> _options;

  public DependsOnOptions(IOptions<TransientFaultHandlingOptions> options) => _options = options;
}
```

### [Options interfaces](https://docs.microsoft.com/en-us/dotnet/core/extensions/options#options-interfaces)

`IOptions<TOptions>`:

- Does not support:
  - Reading of configuration data after the app has started.
  - Named options
- Is registered as a Singleton and can be injected into any service lifetime.

`IOptionsSnapshot<TOptions>`:

- Is useful in scenarios where options should be recomputed on every injection resolution, in scoped or transient lifetimes.
- Is registered as Scoped and therefore cannot be injected into a Singleton service.
- Supports named options

`IOptionsMonitor<TOptions>`:

- Is used to retrieve options and manage options notifications for `TOptions` instances.
- Is registered as a Singleton and can be injected into any service lifetime.
- Supports:
  - Change notifications
  - Named options
  - Reloadable configuration
  - Selective options invalidation (`IOptionsMonitorCache<TOptions>`)