Configuration in ASP.NET Core

Rick Anderson, Mark Michaelis, Steve Smith, Daniel Roth, and Luke Latham

The Configuration API provides a way of configuring an app based on a list of name-value pairs. Configuration is read at runtime from multiple sources. The name-value pairs can be grouped into a multi-level hierarchy. There are configuration providers for:

  • File formats (INI, JSON, and XML)
  • Command-line arguments
  • Environment variables
  • In-memory .NET objects
  • An encrypted user store
  • Azure Key Vault
  • Custom providers, which you install or create

Each configuration value maps to a string key. There's built-in binding support to deserialize settings into a custom POCO object (a simple .NET class with properties).

View or download sample code (how to download)

Simple configuration

The following console app uses the JSON configuration provider:

using System;
using System.IO;
using Microsoft.Extensions.Configuration;

public class Program
{
    public static IConfigurationRoot Configuration { get; set; }

    public static void Main(string[] args = null)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json");

        Configuration = builder.Build();

        Console.WriteLine($"option1 = {Configuration["option1"]}");
        Console.WriteLine($"option2 = {Configuration["option2"]}");
        Console.WriteLine(
            $"suboption1 = {Configuration["subsection:suboption1"]}");
        Console.WriteLine();

        Console.WriteLine("Wizards:");
        Console.Write($"{Configuration["wizards:0:Name"]}, ");
        Console.WriteLine($"age {Configuration["wizards:0:Age"]}");
        Console.Write($"{Configuration["wizards:1:Name"]}, ");
        Console.WriteLine($"age {Configuration["wizards:1:Age"]}");
        Console.WriteLine();

        Console.WriteLine("Press a key...");
        Console.ReadKey();
    }
}

The app reads and displays the following configuration settings:

{
  "option1": "value1_from_json",
  "option2": 2,

  "subsection": {
    "suboption1": "subvalue1_from_json"
  },
  "wizards": [
    {
      "Name": "Gandalf",
      "Age": "1000"
    },
    {
      "Name": "Harry",
      "Age": "17"
    }
  ]
}

Configuration consists of a hierarchical list of name-value pairs in which the nodes are separated by a colon. To retrieve a value, access the Configuration indexer with the corresponding item's key:

Console.WriteLine($"option1 = {Configuration["subsection:suboption1"]}");

To work with arrays in JSON-formatted configuration sources, use a array index as part of the colon-separated string. The following example gets the name of the first item in the preceding wizards array:

Console.Write($"{Configuration["wizards:0:Name"]}, ");

Name-value pairs written to the built in Configuration providers are not persisted, however, you can create a custom provider that saves values. See custom configuration provider.

The preceding sample uses the configuration indexer to read values. To access configuration outside of Startup, use the options pattern. The options pattern is shown later in this article.

It's typical to have different configuration settings for different environments, for example, development, test, and production. The CreateDefaultBuilder extension method in an ASP.NET Core 2.x app (or using AddJsonFile and AddEnvironmentVariables directly in an ASP.NET Core 1.x app) adds configuration providers for reading JSON files and system configuration sources:

  • appsettings.json
  • appsettings.<EnvironmentName>.json
  • environment variables

See AddJsonFile for an explanation of the parameters. reloadOnChange is only supported in ASP.NET Core 1.1 and higher.

Configuration sources are read in the order they are specified. In the code above, the environment variables are read last. Any configuration values set through the environment would replace those set in the two previous providers.

The environment is typically set to one of Development, Staging, or Production. See Working with Multiple Environments for more information.

Configuration considerations:

  • IOptionsSnapshot can reload configuration data when it changes. Use IOptionsSnapshot if you need to reload configuration data. See IOptionsSnapshot for more information.
  • Configuration keys are case insensitive.
  • A best practice is to specify environment variables last, so that the local environment can override anything set in deployed configuration files.
  • Never store passwords or other sensitive data in configuration provider code or in plain text configuration files. Don't use production secrets in your development or test environments. Instead, specify secrets outside the project tree, so they cannot be accidentally committed into your repository. Learn more about Working with Multiple Environments and managing safe storage of app secrets during development.
  • If : cannot be used in environment variables in your system, replace : with __ (double underscore).

Using Options and configuration objects

The options pattern uses custom options classes to represent a group of related settings. We recommended that you create decoupled classes for each feature within your app. Decoupled classes follow:

The options class must be non-abstract with a public parameterless constructor. For example:

namespace UsingOptions.Models
{
    public class MyOptions
    {
        public MyOptions()
        {
            // Set default value.
            Option1 = "value1_from_ctor";
        }
        public string Option1 { get; set; }
        public int Option2 { get; set; } = 5;
    }
}

In the following code, the JSON configuration provider is enabled. The MyOptions class is added to the service container and bound to configuration.

public class Startup
{
    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();

        // Register the IConfiguration instance which MyOptions binds against.
        services.Configure<MyOptions>(Configuration);

        // Add framework services.
        services.AddMvc();
    }

The following controller uses constructor Dependency Injection on IOptions<TOptions> to access settings:

public class HomeController : Controller
{
    private readonly MyOptions _options;

    public HomeController(IOptions<MyOptions> optionsAccessor)
    {
        _options = optionsAccessor.Value;
    }

    public IActionResult Index()
    {
        var option1 = _options.Option1;
        var option2 = _options.Option2;
        return Content($"option1 = {option1}, option2 = {option2}");
    }
}

With the following appsettings.json file:

{
  "option1": "value1_from_json",
  "option2": 2
}

The HomeController.Index method returns option1 = value1_from_json, option2 = 2.

Typical apps won't bind the entire configuration to a single options file. Later on I'll show how to use GetSection to bind to a section.

In the following code, a second IConfigureOptions<TOptions> service is added to the service container. It uses a delegate to configure the binding with MyOptions.

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

    // Register the ConfigurationBuilder instance which MyOptions binds against.
    services.Configure<MyOptions>(Configuration);

    // Registers the following lambda used to configure options.
    services.Configure<MyOptions>(myOptions =>
    {
        myOptions.Option1 = "value1_from_action";
    });

    // Add framework services.
    services.AddMvc();
}

You can add multiple configuration providers. Configuration providers are available in NuGet packages. They are applied in order they are registered.

Each call to Configure<TOptions> adds an IConfigureOptions<TOptions> service to the service container. In the preceding example, the values of Option1 and Option2 are both specified in appsettings.json -- but the value of Option1 is overridden by the configured delegate.

When more than one configuration service is enabled, the last configuration source specified "wins" (sets the configuration value). In the preceding code, the HomeController.Index method returns option1 = value1_from_action, option2 = 2.

When you bind options to configuration, each property in your options type is bound to a configuration key of the form property[:sub-property:]. For example, the MyOptions.Option1 property is bound to the key Option1, which is read from the option1 property in appsettings.json. A sub-property sample is shown later in this article.

In the following code, a third IConfigureOptions<TOptions> service is added to the service container. It binds MySubOptions to the section subsection of the appsettings.json file:

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

    // Configure with Microsoft.Extensions.Options.ConfigurationExtensions
    // Binding the whole configuration should be rare, subsections are more typical.
    services.Configure<MyOptions>(Configuration);

    // Configure MyOptions using code.
    services.Configure<MyOptions>(myOptions =>
    {
        myOptions.Option1 = "value1_from_action";
    });

    // Configure using a sub-section of the appsettings.json file.
    services.Configure<MySubOptions>(Configuration.GetSection("subsection"));

    // Add framework services.
    services.AddMvc();
}

Note: This extension method requires the Microsoft.Extensions.Options.ConfigurationExtensions NuGet package.

Using the following appsettings.json file:

{
  "option1": "value1_from_json",
  "option2": -1,
  "subsection": {
    "suboption1": "subvalue1_from_json",
    "suboption2": 200
  }
}

The MySubOptions class:

public class MySubOptions
{
    public MySubOptions()
    {
        // Set default values.
        SubOption1 = "value1_from_ctor";
        SubOption2 = 5;
    }
    public string SubOption1 { get; set; }
    public int SubOption2 { get; set; }
}

With the following Controller:

public class HomeController : Controller
{
    private readonly MySubOptions _subOptions;

    public HomeController(IOptions<MySubOptions> subOptionsAccessor)
    {
        _subOptions = subOptionsAccessor.Value;
    }

    public IActionResult Index()
    {
        var subOption1 = _subOptions.SubOption1;
        var subOption2 = _subOptions.SubOption2;
        return Content($"subOption1 = {subOption1}, subOption2 = {subOption2}");
    }
}

subOption1 = subvalue1_from_json, subOption2 = 200 is returned.

You can also supply options in a view model or inject IOptions<TOptions> directly into a view:

@using Microsoft.Extensions.Options
@using UsingOptions.Models
@model MyOptions
@inject IOptions<MyOptions> OptionsAccessor

<!DOCTYPE html>
<!-- This view works with HomeController3.cs -->
<html lang="en">
<head>
    <title>Using Options sample app</title>
</head>
<body>
    <h1>Options</h1>

    <h2>Options provided by the model</h2>
    <p><b>Option1:</b> @Model.Option1</p>
    <p><b>Option2:</b> @Model.Option2</p>

    <h2>Options injected into the view</h2>
    <p><b>Option1:</b> @OptionsAccessor.Value.Option1</p>
    <p><b>Option2:</b> @OptionsAccessor.Value.Option2</p>
</body>
</html>

IOptionsSnapshot

Requires ASP.NET Core 1.1 or higher.

IOptionsSnapshot supports reloading configuration data when the configuration file has changed. It has minimal overhead. Using IOptionsSnapshot with reloadOnChange: true, the options are bound to IConfiguration and reloaded when changed.

The following sample demonstrates how a new IOptionsSnapshot is created after config.json changes. Requests to the server will return the same time when config.json has not changed. The first request after config.json changes will show a new time.

public class TimeOptions
{
    // Records the time when the options are created.
    public DateTime CreationTime { get; set; } = DateTime.Now;

    // Bound to config. Changes to the value of "Message"
    // in config.json will be reflected in this property.
    public string Message { get; set; }
}

public class Controller
{
    public readonly TimeOptions _options;

    public Controller(IOptionsSnapshot<TimeOptions> options)
    {
        _options = options.Value;
    }

    public Task DisplayTimeAsync(HttpContext context)
    {
        return context.Response.WriteAsync(_options.Message + _options.CreationTime);
    }
}

public class Startup
{
    public Startup(IHostingEnvironment env)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            // reloadOnChange: true is required for config changes to be detected.
            .AddJsonFile("config.json", optional: false, reloadOnChange: true)
            .AddEnvironmentVariables();
        Configuration = builder.Build();
    }

    public IConfigurationRoot Configuration { get; set; }

    public void Configure(IApplicationBuilder app)
    {
        // Simple mockup of a simple per request controller that writes
        // the creation time and message of TimeOptions.
        app.Run(DisplayTimeAsync);
    }

    public void ConfigureServices(IServiceCollection services)
    {
        // Simple mockup of a simple per request controller.
        services.AddScoped<Controller>();

        // Binds config.json to the options and setups the change tracking.
        services.Configure<TimeOptions>(Configuration.GetSection("Time"));
    }

    public Task DisplayTimeAsync(HttpContext context)
    {
        context.Response.ContentType = "text/plain";
        return context.RequestServices.GetRequiredService<Controller>().DisplayTimeAsync(context);
    }

    public static void Main(string[] args)
    {
        var host = new WebHostBuilder()
            .UseKestrel()
            .UseIISIntegration()
            .UseStartup<Startup>()
            .Build();
        host.Run();
    }
}

The following image shows the server output:

browser image showing "Last Updated: 11/22/2016 4:43 PM"

Refreshing the browser doesn't change the message value or time displayed (when config.json has not changed).

Change and save the config.json and then refresh the browser:

browser image showing "Last Updated to,e: 11/22/2016 4:53 PM"

In-memory provider and binding to a POCO class

The following sample shows how to use the in-memory provider and bind to a class:

using System;
using System.Collections.Generic;
using Microsoft.Extensions.Configuration;

public class Program
{   
    public static IConfigurationRoot Configuration { get; set; }

    public static void Main(string[] args = null)
    {
        var dict = new Dictionary<string, string>
            {
                {"Profile:MachineName", "Rick"},
                {"App:MainWindow:Height", "11"},
                {"App:MainWindow:Width", "11"},
                {"App:MainWindow:Top", "11"},
                {"App:MainWindow:Left", "11"}
            };

        var builder = new ConfigurationBuilder();
        builder.AddInMemoryCollection(dict);

        Configuration = builder.Build();

        Console.WriteLine($"Hello {Configuration["Profile:MachineName"]}");

        var window = new MyWindow();
        Configuration.GetSection("App:MainWindow").Bind(window);
        Console.WriteLine($"Left {window.Left}");
        Console.WriteLine();

        Console.WriteLine("Press any key...");
        Console.ReadKey();
    }
}

Configuration values are returned as strings, but binding enables the construction of objects. Binding allows you to retrieve POCO objects or even entire object graphs. The following sample shows how to bind to MyWindow and use the options pattern with a ASP.NET Core MVC app:

namespace WebConfigBind
{
    public class MyWindow
    {
        public int Height { get; set; }
        public int Width { get; set; }
        public int Top { get; set; }
        public int Left { get; set; }
    }
}
{
  "AppConfiguration": {
    "MainWindow": {
      "Height": "400",
      "Width": "600",
      "Top": "5",
      "Left": "11"
    }
  }
}

Bind the custom class in ConfigureServices when building the host:

.ConfigureServices(services =>
{
    services.Configure<MyWindow>(
        Configuration.GetSection("AppConfiguration:MainWindow"));
    services.AddMvc();
})

Display the settings from the HomeController:

using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Options;

namespace WebConfigBind
{
    public class HomeController : Controller
    {
        private readonly IOptions<MyWindow> _optionsAccessor;

        public HomeController(IOptions<MyWindow> optionsAccessor)
        {
            _optionsAccessor = optionsAccessor;
        }

        public IActionResult Index()
        {
            var height = _optionsAccessor.Value.Height;
            var width = _optionsAccessor.Value.Width;
            var left = _optionsAccessor.Value.Left;
            var top = _optionsAccessor.Value.Top;

            return Content($"height = {height}, width = {width}, " + 
                $"left = {left}, top = {top}");
        }
    }
}

GetValue

The following sample demonstrates the GetValue extension method:

using System;
using System.Collections.Generic;
using Microsoft.Extensions.Configuration;

public class Program
{   
    public static IConfigurationRoot Configuration { get; set; }

    public static void Main(string[] args = null)
    {
        var dict = new Dictionary<string, string>
            {
                {"Profile:MachineName", "Rick"},
                {"App:MainWindow:Height", "11"},
                {"App:MainWindow:Width", "11"},
                {"App:MainWindow:Top", "11"},
                {"App:MainWindow:Left", "11"}
            };

        var builder = new ConfigurationBuilder();
        builder.AddInMemoryCollection(dict);

        Configuration = builder.Build();

        Console.WriteLine($"Hello {Configuration["Profile:MachineName"]}");

        // Show GetValue overload and set the default value to 80
        // Requires NuGet package "Microsoft.Extensions.Configuration.Binder"
        var left = Configuration.GetValue<int>("App:MainWindow:Left", 80);
        Console.WriteLine($"Left {left}");

        var window = new MyWindow();
        Configuration.GetSection("App:MainWindow").Bind(window);
        Console.WriteLine($"Left {window.Left}");
        Console.WriteLine();

        Console.WriteLine("Press a key...");
        Console.ReadKey();
    }
}

The ConfigurationBinder's GetValue<T> method allows you to specify a default value (80 in the sample). GetValue<T> is for simple scenarios and does not bind to entire sections. GetValue<T> gets scalar values from GetSection(key).Value converted to a specific type.

Binding to an object graph

You can recursively bind to each object in a class. Consider the following AppOptions class:

public class AppOptions
{
    public Window Window { get; set; }
    public Connection Connection { get; set; }
    public Profile Profile { get; set; }
}

public class Window
{
    public int Height { get; set; }
    public int Width { get; set; }
}

public class Connection
{
    public string Value { get; set; }
}

public class Profile
{
    public string Machine { get; set; }
}

The following sample binds to the AppOptions class:

using System;
using System.IO;
using Microsoft.Extensions.Configuration;

public class Program
{
    public static void Main(string[] args = null)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json");

        var config = builder.Build();

        var appConfig = new AppOptions();
        config.GetSection("App").Bind(appConfig);

        Console.WriteLine($"Height {appConfig.Window.Height}");
        Console.WriteLine();

        Console.WriteLine("Press a key...");
        Console.ReadKey();
    }
}

ASP.NET Core 1.1 and higher can use Get<T>, which works with entire sections. Get<T> can be more convienent than using Bind. The following code shows how to use Get<T> with the sample above:

var appConfig = config.GetSection("App").Get<AppOptions>();

Using the following appsettings.json file:

{
  "App": {
    "Profile": {
      "Machine": "Rick"
    },
    "Connection": {
      "Value": "connectionstring"
    },
    "Window": {
      "Height": "11",
      "Width": "11"
    }
  }
}

The program displays Height 11.

The following code can be used to unit test the configuration:

[Fact]
public void CanBindObjectTree()
{
    var dict = new Dictionary<string, string>
        {
            {"App:Profile:Machine", "Rick"},
            {"App:Connection:Value", "connectionstring"},
            {"App:Window:Height", "11"},
            {"App:Window:Width", "11"}
        };
    var builder = new ConfigurationBuilder();
    builder.AddInMemoryCollection(dict);
    var config = builder.Build();

    var options = new AppOptions();
    config.GetSection("App").Bind(options);

    Assert.Equal("Rick", options.Profile.Machine);
    Assert.Equal(11, options.Window.Height);
    Assert.Equal(11, options.Window.Width);
    Assert.Equal("connectionstring", options.Connection.Value);
}

Basic sample of Entity Framework custom provider

In this section, a basic configuration provider that reads name-value pairs from a database using EF is created.

Define a ConfigurationValue entity for storing configuration values in the database:

public class ConfigurationValue
{
    public string Id { get; set; }
    public string Value { get; set; }
}

Add a ConfigurationContext to store and access the configured values:

public class ConfigurationContext : DbContext
{
    public ConfigurationContext(DbContextOptions options) : base(options)
    {
    }

    public DbSet<ConfigurationValue> Values { get; set; }
}

Create an class that implements IConfigurationSource:

using System;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;

namespace CustomConfigurationProvider
{
    public class EFConfigSource : IConfigurationSource
    {
        private readonly Action<DbContextOptionsBuilder> _optionsAction;

        public EFConfigSource(Action<DbContextOptionsBuilder> optionsAction)
        {
            _optionsAction = optionsAction;
        }

        public IConfigurationProvider Build(IConfigurationBuilder builder)
        {
            return new EFConfigProvider(_optionsAction);
        }
    }
}

Create the custom configuration provider by inheriting from ConfigurationProvider. The configuration provider initializes the database when it's empty:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;

namespace CustomConfigurationProvider
{
    public class EFConfigProvider : ConfigurationProvider
    {
        public EFConfigProvider(Action<DbContextOptionsBuilder> optionsAction)
        {
            OptionsAction = optionsAction;
        }

        Action<DbContextOptionsBuilder> OptionsAction { get; }

        // Load config data from EF DB.
        public override void Load()
        {
            var builder = new DbContextOptionsBuilder<ConfigurationContext>();
            OptionsAction(builder);

            using (var dbContext = new ConfigurationContext(builder.Options))
            {
                dbContext.Database.EnsureCreated();
                Data = !dbContext.Values.Any()
                    ? CreateAndSaveDefaultValues(dbContext)
                    : dbContext.Values.ToDictionary(c => c.Id, c => c.Value);
            }
        }

        private static IDictionary<string, string> CreateAndSaveDefaultValues(
            ConfigurationContext dbContext)
        {
            var configValues = new Dictionary<string, string>
                {
                    { "key1", "value_from_ef_1" },
                    { "key2", "value_from_ef_2" }
                };
            dbContext.Values.AddRange(configValues
                .Select(kvp => new ConfigurationValue { Id = kvp.Key, Value = kvp.Value })
                .ToArray());
            dbContext.SaveChanges();
            return configValues;
        }
    }
}

The highlighted values from the database ("value_from_ef_1" and "value_from_ef_2") are displayed when the sample is run.

You can add an EFConfigSource extension method for adding the configuration source:

using System;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;

namespace CustomConfigurationProvider
{
    public static class EntityFrameworkExtensions
    {
        public static IConfigurationBuilder AddEntityFrameworkConfig(
            this IConfigurationBuilder builder, Action<DbContextOptionsBuilder> setup)
        {
            return builder.Add(new EFConfigSource(setup));
        }
    }
}

The following code shows how to use the custom EFConfigProvider:

using System;
using System.IO;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using CustomConfigurationProvider;

public static class Program
{
    public static void Main()
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json");

        var connectionStringConfig = builder.Build();

        var config = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            // Add "appsettings.json" to bootstrap EF config.
            .AddJsonFile("appsettings.json")
            // Add the EF configuration provider, which will override any
            // config made with the JSON provider.
            .AddEntityFrameworkConfig(options =>
                options.UseSqlServer(connectionStringConfig.GetConnectionString(
                    "DefaultConnection"))
            )
            .Build();

        Console.WriteLine("key1={0}", config["key1"]);
        Console.WriteLine("key2={0}", config["key2"]);
        Console.WriteLine("key3={0}", config["key3"]);
        Console.WriteLine();

        Console.WriteLine("Press a key...");
        Console.ReadKey();
    }
}

Note the sample adds the custom EFConfigProvider after the JSON provider, so any settings from the database will override settings from the appsettings.json file.

Using the following appsettings.json file:

{
  "ConnectionStrings": {
    "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=CustomConfigurationProvider;Trusted_Connection=True;MultipleActiveResultSets=true"
  },
  "key1": "value_from_json_1",
  "key2": "value_from_json_2",
  "key3": "value_from_json_3"
}

The following is displayed:

key1=value_from_ef_1
key2=value_from_ef_2
key3=value_from_json_3

CommandLine configuration provider

The CommandLine configuration provider receives command-line argument key-value pairs for configuration at runtime.

View or download the CommandLine configuration sample

Setting up the provider

To activate command-line configuration, call the AddCommandLine extension method on an instance of ConfigurationBuilder:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Extensions.Configuration;

public class Program
{
    public static IConfigurationRoot Configuration { get; set; }

    public static void Main(string[] args = null)
    {
        var dict = new Dictionary<string, string>
            {
                {"Profile:MachineName", "MairaPC"},
                {"App:MainWindow:Left", "1980"}
            };

        var builder = new ConfigurationBuilder();

        builder.AddInMemoryCollection(dict)
            .AddCommandLine(args);

        Configuration = builder.Build();

        Console.WriteLine($"MachineName: {Configuration["Profile:MachineName"]}");
        Console.WriteLine($"Left: {Configuration["App:MainWindow:Left"]}");
        Console.WriteLine();

        Console.WriteLine("Press a key...");
        Console.ReadKey();
    }
}

Running the code, the following output is displayed:

MachineName: MairaPC
Left: 1980

Passing argument key-value pairs on the command line changes the values of Profile:MachineName and App:MainWindow:Left:

dotnet run Profile:MachineName=BartPC App:MainWindow:Left=1979

The console window displays:

MachineName: BartPC
Left: 1979

To override configuration provided by other configuration providers with command-line configuration, call AddCommandLine last on ConfigurationBuilder:

var config = new ConfigurationBuilder()
    .SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
    .AddEnvironmentVariables()
    .AddCommandLine(args)
    .Build();

Arguments

Arguments passed on the command line must conform to one of two formats shown in the following table.

Argument format Example
Single argument: a key-value pair separated by an equals sign (=) key1=value
Sequence of two arguments: a key-value pair separated by a space /key1 value1

Single argument

The value must follow an equals sign (=). The value can be null (for example, mykey=).

The key may have a prefix.

Key prefix Example
No prefix key1=value1
Single dash (-)† -key2=value2
Two dashes (--) --key3=value3
Forward slash (/) /key4=value4

†A key with a single dash prefix (-) must be provided in switch mappings, described below.

Example command:

dotnet run key1=value1 -key2=value2 --key3=value3 /key4=value4

Note: If -key1 isn't present in the switch mappings given to the configuration provider, a FormatException is thrown.

Sequence of two arguments

The value can't be null and must follow the key separated by a space.

The key must have a prefix.

Key prefix Example
Single dash (-)† -key1 value1
Two dashes (--) --key2 value2
Forward slash (/) /key3 value3

†A key with a single dash prefix (-) must be provided in switch mappings, described below.

Example command:

dotnet run -key1 value1 --key2 value2 /key3 value3

Note: If -key1 isn't present in the switch mappings given to the configuration provider, a FormatException is thrown.

Duplicate keys

If duplicate keys are provided, the last key-value pair is used.

Switch mappings

When manually building configuration with ConfigurationBuilder, you can optionally provide a switch mappings dictionary to the AddCommandLine method. Switch mappings allow you to provide key name replacement logic.

When the switch mappings dictionary is used, the dictionary is checked for a key that matches the key provided by a command-line argument. If the command-line key is found in the dictionary, the dictionary value (the key replacement) is passed back to set the configuration. A switch mapping is required for any command-line key prefixed with a single dash (-).

Switch mappings dictionary key rules:

  • Switches must start with a dash (-) or double-dash (--).
  • The switch mappings dictionary must not contain duplicate keys.

In the following example, the GetSwitchMappings method allows your command-line arguments to use a single dash (-) key prefix and avoid leading subkey prefixes.

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Extensions.Configuration;

public class Program
{
    public static IConfigurationRoot Configuration { get; set; }

    public static Dictionary<string, string> GetSwitchMappings(
        IReadOnlyDictionary<string, string> configurationStrings)
    {
        return configurationStrings.Select(item =>
            new KeyValuePair<string, string>(
                "-" + item.Key.Substring(item.Key.LastIndexOf(':') + 1),
                item.Key))
                .ToDictionary(
                    item => item.Key, item => item.Value);
    }

    public static void Main(string[] args = null)
    {
        var dict = new Dictionary<string, string>
            {
                {"Profile:MachineName", "RickPC"},
                {"App:MainWindow:Left", "1980"}
            };

        var builder = new ConfigurationBuilder();

        builder.AddInMemoryCollection(dict)
            .AddCommandLine(args, GetSwitchMappings(dict));

        Configuration = builder.Build();

        Console.WriteLine($"MachineName: {Configuration["Profile:MachineName"]}");
        Console.WriteLine($"Left: {Configuration["App:MainWindow:Left"]}");
        Console.WriteLine();

        Console.WriteLine("Press a key...");
        Console.ReadKey();
    }
}

Without providing command-line arguments, the dictionary provided to AddInMemoryCollection sets the configuration values. Run the app with the following command:

dotnet run

The console window displays:

MachineName: RickPC
Left: 1980

Use the following to pass in configuration settings:

dotnet run /Profile:MachineName=DahliaPC /App:MainWindow:Left=1984

The console window displays:

MachineName: DahliaPC
Left: 1984

After the switch mappings dictionary is created, it contains the data shown in the following table.

Key Value
-MachineName Profile:MachineName
-Left App:MainWindow:Left

To demonstrate key switching using the dictionary, run the following command:

dotnet run -MachineName=ChadPC -Left=1988

The command-line keys are swapped. The console window displays the configuration values for Profile:MachineName and App:MainWindow:Left:

MachineName: ChadPC
Left: 1988

The web.config file

A web.config file is required when you host the app in IIS or IIS-Express. web.config turns on the AspNetCoreModule in IIS to launch your app. Settings in web.config enable the AspNetCoreModule in IIS to launch your app and configure other IIS settings and modules. If you are using Visual Studio and delete web.config, Visual Studio will create a new one.

Additional notes

  • Dependency Injection (DI) is not set up until after ConfigureServices is invoked.
  • The configuration system is not DI aware.
  • IConfiguration has two specializations:
    • IConfigurationRoot Used for the root node. Can trigger a reload.
    • IConfigurationSection Represents a section of configuration values. The GetSection and GetChildren methods return an IConfigurationSection.

Additional resources