Configuration in ASP.NET Core

By Rick Anderson and Kirk Larkin

Application configuration in ASP.NET Core is performed using one or more configuration providers. Configuration providers read configuration data from key-value pairs using a variety of configuration sources:

• Settings files, such as appsettings.json
• Environment variables
• Azure Key Vault
• Azure App Configuration
• Command-line arguments
• Custom providers, installed or created
• Directory files
• In-memory .NET objects

This article provides information on configuration in ASP.NET Core. For information on using configuration in non-ASP.NET Core apps, see .NET Configuration.

For Blazor configuration guidance, which adds to or supersedes the guidance in this node, see ASP.NET Core Blazor configuration.

Application and Host Configuration

ASP.NET Core apps configure and launch a host. The host is responsible for app startup and lifetime management. The ASP.NET Core templates create a WebApplicationBuilder which contains the host. While some configuration can be done in both the host and the application configuration providers, generally, only configuration that is necessary for the host should be done in host configuration.

Application configuration is the highest priority and is detailed in the next section. Host configuration follows application configuration, and is described in this article.

Default application configuration sources

ASP.NET Core web apps created with dotnet new or Visual Studio generate the following code:

var builder = WebApplication.CreateBuilder(args);

WebApplication.CreateBuilder initializes a new instance of the WebApplicationBuilder class with preconfigured defaults. The initialized WebApplicationBuilder (builder) provides default configuration for the app in the following order, from highest to lowest priority:

1. Command-line arguments using the Command-line configuration provider.
2. Non-prefixed environment variables using the Non-prefixed environment variables configuration provider.
3. User secrets when the app runs in the Development environment.
4. appsettings.{Environment}.json using the JSON configuration provider. For example, appsettings.Production.json and appsettings.Development.json.
5. appsettings.json using the JSON configuration provider.
6. A fallback to the host configuration described in the next section.

Note: WebApplication.CreateBuilder(args) should only be called once in apps relying on IIS in-process hosting.

Default host configuration sources

The following list contains the default host configuration sources from highest to lowest priority for WebApplicationBuilder:

1. Command-line arguments using the Command-line configuration provider
2. DOTNET_-prefixed environment variables using the Environment variables configuration provider.
3. ASPNETCORE_-prefixed environment variables using the Environment variables configuration provider.

For the .NET Generic Host and Web Host, the default host configuration sources from highest to lowest priority is:

1. ASPNETCORE_-prefixed environment variables using the Environment variables configuration provider.
2. Command-line arguments using the Command-line configuration provider
3. DOTNET_-prefixed environment variables using the Environment variables configuration provider.

When a configuration value is set in host and application configuration, the application configuration is used.

Host variables

The following variables are locked in early when initializing the host builders and can't be influenced by application config:

• Application name
• Environment name, for example Development, Production, and Staging
• Content root
• Web root
• Whether to scan for hosting startup assemblies and which assemblies to scan for.
• Variables read by app and library code from HostBuilderContext.Configuration in IHostBuilder.ConfigureAppConfiguration callbacks.

Every other host setting is read from application config instead of host config.

URLS is one of the many common host settings that is not a bootstrap setting. Like every other host setting not in the previous list, URLS is read later from application config. Host config is a fallback for application config, so host config can be used to set URLS, but it will be overridden by any configuration source in application config like appsettings.json.

For more information, see Change the content root, app name, and environment and Change the content root, app name, and environment by environment variables or command line

The remaining sections in this article refer to application configuration.

Application configuration providers

The following code displays the enabled configuration providers in the order they were added:

public class Index2Model : PageModel
{
    private IConfigurationRoot ConfigRoot;

    public Index2Model(IConfiguration configRoot)
    {
        ConfigRoot = (IConfigurationRoot)configRoot;
    }

    public ContentResult OnGet()
    {           
        string str = "";
        foreach (var provider in ConfigRoot.Providers.ToList())
        {
            str += provider.ToString() + "\n";
        }

        return Content(str);
    }
}

The preceding list of highest to lowest priority default configuration sources shows the providers in the opposite order they are added to template generated application. For example, the JSON configuration provider is added before the Command-line configuration provider.

Configuration providers that are added later have higher priority and override previous key settings. For example, if MyKey is set in both appsettings.json and the environment, the environment value is used. Using the default configuration providers, the Command-line configuration provider overrides all other providers.

For more information on CreateBuilder, see Default builder settings.

appsettings.json

Consider the following appsettings.json file:

{
  "Position": {
    "Title": "Editor",
    "Name": "Joe Smith"
  },
  "MyKey": "My appsettings.json Value",
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
}

The following code from the sample download displays several of the preceding configurations settings:

public class TestModel : PageModel
{
    // requires using Microsoft.Extensions.Configuration;
    private readonly IConfiguration Configuration;

    public TestModel(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {
        var myKeyValue = Configuration["MyKey"];
        var title = Configuration["Position:Title"];
        var name = Configuration["Position:Name"];
        var defaultLogLevel = Configuration["Logging:LogLevel:Default"];


        return Content($"MyKey value: {myKeyValue} \n" +
                       $"Title: {title} \n" +
                       $"Name: {name} \n" +
                       $"Default Log Level: {defaultLogLevel}");
    }
}

The default JsonConfigurationProvider loads configuration in the following order:

1. appsettings.json
2. appsettings.{Environment}.json : For example, the appsettings.Production.json and appsettings.Development.json files. The environment version of the file is loaded based on the IHostingEnvironment.EnvironmentName. For more information, see Use multiple environments in ASP.NET Core.

appsettings.{Environment}.json values override keys in appsettings.json. For example, by default:

• In development, appsettings.Development.json configuration overwrites values found in appsettings.json.
• In production, appsettings.Production.json configuration overwrites values found in appsettings.json. For example, when deploying the app to Azure.

If a configuration value must be guaranteed, see GetValue. The preceding example only reads strings and doesn’t support a default value.

Using the default configuration, the appsettings.json and appsettings.{Environment}.json files are enabled with reloadOnChange: true. Changes made to the appsettings.json and appsettings.{Environment}.json file after the app starts are read by the JSON configuration provider.

Comments in appsettings.json

Comments in appsettings.json and appsettings.{Environment}.json files are supported using JavaScript or C# style comments.

Some integrated development environments (IDE) display errors when editing a JSON file that contains comments. You can generally ignore comment errors and warnings, but you can also usually disable them with a setting in the IDE. In Visual Studio Code, for example, add the following to the settings.json file to disable the errors:

"files.associations": {
  "appsettings*.json": "jsonc"
}

For other IDEs, check the tool's documentation and other product support channels to determine how to silence the errors.

Bind hierarchical configuration data using the options pattern

The preferred way to read related configuration values is using the options pattern. For example, to read the following configuration values:

  "Position": {
    "Title": "Editor",
    "Name": "Joe Smith"
  }

Create the following PositionOptions class:

public class PositionOptions
{
    public const string Position = "Position";

    public string Title { get; set; } = String.Empty;
    public string Name { get; set; } = String.Empty;
}

An options class:

• Must be non-abstract with a public parameterless constructor.
• All public read-write properties of the type are bound.
• Fields are not bound. In the preceding code, Position is not bound. The Position field is used so the string "Position" doesn't need to be hard coded in the app when binding the class to a configuration provider.

The following code:

• Calls ConfigurationBinder.Bind to bind the PositionOptions class to the Position section.
• Displays the Position configuration data.

public class Test22Model : PageModel
{
    private readonly IConfiguration Configuration;

    public Test22Model(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {
        var positionOptions = new PositionOptions();
        Configuration.GetSection(PositionOptions.Position).Bind(positionOptions);

        return Content($"Title: {positionOptions.Title} \n" +
                       $"Name: {positionOptions.Name}");
    }
}

In the preceding code, by default, changes to the JSON configuration file after the app has started are read.

ConfigurationBinder.Get<T> binds and returns the specified type. ConfigurationBinder.Get<T> may be more convenient than using ConfigurationBinder.Bind. The following code shows how to use ConfigurationBinder.Get<T> with the PositionOptions class:

public class Test21Model : PageModel
{
    private readonly IConfiguration Configuration;
    public PositionOptions? positionOptions { get; private set; }

    public Test21Model(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {            
        positionOptions = Configuration.GetSection(PositionOptions.Position)
                                                     .Get<PositionOptions>();

        return Content($"Title: {positionOptions.Title} \n" +
                       $"Name: {positionOptions.Name}");
    }
}

In the preceding code, by default, changes to the JSON configuration file after the app has started are read.

An alternative approach when using the options pattern is to bind the Position section and add it to the dependency injection service container. In the following code, PositionOptions is added to the service container with Configure and bound to configuration:

using ConfigSample.Options;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Services.Configure<PositionOptions>(
    builder.Configuration.GetSection(PositionOptions.Position));

var app = builder.Build();

Using the preceding code, the following code reads the position options:

public class Test2Model : PageModel
{
    private readonly PositionOptions _options;

    public Test2Model(IOptions<PositionOptions> options)
    {
        _options = options.Value;
    }

    public ContentResult OnGet()
    {
        return Content($"Title: {_options.Title} \n" +
                       $"Name: {_options.Name}");
    }
}

In the preceding code, changes to the JSON configuration file after the app has started are not read. To read changes after the app has started, use IOptionsSnapshot.

Using the default configuration, the appsettings.json and appsettings.{Environment}.json files are enabled with reloadOnChange: true. Changes made to the appsettings.json and appsettings.{Environment}.json file after the app starts are read by the JSON configuration provider.

See JSON configuration provider in this document for information on adding additional JSON configuration files.

Combining service collection

Consider the following which registers services and configures options:

using ConfigSample.Options;
using Microsoft.Extensions.DependencyInjection.ConfigSample.Options;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Services.Configure<PositionOptions>(
    builder.Configuration.GetSection(PositionOptions.Position));
builder.Services.Configure<ColorOptions>(
    builder.Configuration.GetSection(ColorOptions.Color));

builder.Services.AddScoped<IMyDependency, MyDependency>();
builder.Services.AddScoped<IMyDependency2, MyDependency2>();

var app = builder.Build();

Related groups of registrations can be moved to an extension method to register services. For example, the configuration services are added to the following class:

using ConfigSample.Options;
using Microsoft.Extensions.Configuration;

namespace Microsoft.Extensions.DependencyInjection
{
    public static class MyConfigServiceCollectionExtensions
    {
        public static IServiceCollection AddConfig(
             this IServiceCollection services, IConfiguration config)
        {
            services.Configure<PositionOptions>(
                config.GetSection(PositionOptions.Position));
            services.Configure<ColorOptions>(
                config.GetSection(ColorOptions.Color));

            return services;
        }

        public static IServiceCollection AddMyDependencyGroup(
             this IServiceCollection services)
        {
            services.AddScoped<IMyDependency, MyDependency>();
            services.AddScoped<IMyDependency2, MyDependency2>();

            return services;
        }
    }
}

The remaining services are registered in a similar class. The following code uses the new extension methods to register the services:

using Microsoft.Extensions.DependencyInjection.ConfigSample.Options;

var builder = WebApplication.CreateBuilder(args);

builder.Services
    .AddConfig(builder.Configuration)
    .AddMyDependencyGroup();

builder.Services.AddRazorPages();

var app = builder.Build();

Note: Each services.Add{GROUP_NAME} extension method adds and potentially configures services. For example, AddControllersWithViews adds the services MVC controllers with views require, and AddRazorPages adds the services Razor Pages requires.

Security and user secrets

Configuration data guidelines:

• Never store passwords or other sensitive data in configuration provider code or in plain text configuration files. The Secret Manager tool can be used to store secrets in development.
• Don't use production secrets in development or test environments.
• Specify secrets outside of the project so that they can't be accidentally committed to a source code repository.
• Production apps should use the most secure authentication flow available. For more information, see Secure authentication flows.

By default, the user secrets configuration source is registered after the JSON configuration sources. Therefore, user secrets keys take precedence over keys in appsettings.json and appsettings.{Environment}.json.

For more information on storing passwords or other sensitive data:

• Use multiple environments in ASP.NET Core
• Safe storage of app secrets in development in ASP.NET Core: Includes advice on using environment variables to store sensitive data. The Secret Manager tool uses the File configuration provider to store user secrets in a JSON file on the local system.
• Azure Key Vault safely stores app secrets for ASP.NET Core apps. For more information, see Azure Key Vault configuration provider in ASP.NET Core.

Non-prefixed environment variables

Non-prefixed environment variables are environment variables other than those prefixed by ASPNETCORE_ or DOTNET_. For example, the ASP.NET Core web application templates set "ASPNETCORE_ENVIRONMENT": "Development" in launchSettings.json. For more information on ASPNETCORE_ and DOTNET_ environment variables, see:

• List of highest to lowest priority default configuration sources including non-prefixed, ASPNETCORE_-prefixed and DOTNETCORE_-prefixed environment variables.
• DOTNET_ environment variables used outside of Microsoft.Extensions.Hosting.

Using the default configuration, the EnvironmentVariablesConfigurationProvider loads configuration from environment variable key-value pairs after reading appsettings.json, appsettings.{Environment}.json, and user secrets. Therefore, key values read from the environment override values read from appsettings.json, appsettings.{Environment}.json, and user secrets.

The : separator doesn't work with environment variable hierarchical keys on all platforms. For example, the : separator is not supported by Bash. The double underscore, __, is:

• Supported by all platforms.
• Automatically replaced by a colon, :.

The following commands:

• Set the environment keys and values of the preceding example on Windows.
• Test the settings when using the sample download. The dotnet run command must be run in the project directory.

set MyKey="My key from Environment"
set Position__Title=Environment_Editor
set Position__Name=Environment_Rick
dotnet run

The preceding environment settings:

• Are only set in processes launched from the command window they were set in.
• Won't be read by browsers launched with Visual Studio.

The following setx commands can be used to set the environment keys and values on Windows. Unlike set, setx settings are persisted. /M sets the variable in the system environment. If the /M switch isn't used, a user environment variable is set.

setx MyKey "My key from setx Environment" /M
setx Position__Title Environment_Editor /M
setx Position__Name Environment_Rick /M

To test that the preceding commands override appsettings.json and appsettings.{Environment}.json:

• With Visual Studio: Exit and restart Visual Studio.
• With the CLI: Start a new command window and enter dotnet run.

Call AddEnvironmentVariables with a string to specify a prefix for environment variables:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Configuration.AddEnvironmentVariables(prefix: "MyCustomPrefix_");

var app = builder.Build();

In the preceding code:

• builder.Configuration.AddEnvironmentVariables(prefix: "MyCustomPrefix_") is added after the default configuration providers. For an example of ordering the configuration providers, see JSON configuration provider.
• Environment variables set with the MyCustomPrefix_ prefix override the default configuration providers. This includes environment variables without the prefix.

The prefix is stripped off when the configuration key-value pairs are read.

The following commands test the custom prefix:

set MyCustomPrefix_MyKey="My key with MyCustomPrefix_ Environment"
set MyCustomPrefix_Position__Title=Editor_with_customPrefix
set MyCustomPrefix_Position__Name=Environment_Rick_cp
dotnet run

The default configuration loads environment variables and command line arguments prefixed with DOTNET_ and ASPNETCORE_. The DOTNET_ and ASPNETCORE_ prefixes are used by ASP.NET Core for host and app configuration, but not for user configuration. For more information on host and app configuration, see .NET Generic Host.

On Azure App Service, select New application setting on the Settings > Configuration page. Azure App Service application settings are:

• Encrypted at rest and transmitted over an encrypted channel.
• Exposed as environment variables.

For more information, see Azure Apps: Override app configuration using the Azure Portal.

See Connection string prefixes for information on Azure database connection strings.

Naming of environment variables

Environment variable names reflect the structure of an appsettings.json file. Each element in the hierarchy is separated by a double underscore (preferable) or a colon. When the element structure includes an array, the array index should be treated as an additional element name in this path. Consider the following appsettings.json file and its equivalent values represented as environment variables.

appsettings.json

{
    "SmtpServer": "smtp.example.com",
    "Logging": [
        {
            "Name": "ToEmail",
            "Level": "Critical",
            "Args": {
                "FromAddress": "MySystem@example.com",
                "ToAddress": "SRE@example.com"
            }
        },
        {
            "Name": "ToConsole",
            "Level": "Information"
        }
    ]
}

environment variables

setx SmtpServer smtp.example.com
setx Logging__0__Name ToEmail
setx Logging__0__Level Critical
setx Logging__0__Args__FromAddress MySystem@example.com
setx Logging__0__Args__ToAddress SRE@example.com
setx Logging__1__Name ToConsole
setx Logging__1__Level Information

Environment variables set in generated launchSettings.json

Environment variables set in launchSettings.json override those set in the system environment. For example, the ASP.NET Core web templates generate a launchSettings.json file that sets the endpoint configuration to:

"applicationUrl": "https://localhost:5001;http://localhost:5000"

Configuring the applicationUrl sets the ASPNETCORE_URLS environment variable and overrides values set in the environment.

Escape environment variables on Linux

On Linux, the value of URL environment variables must be escaped so systemd can parse it. Use the linux tool systemd-escape which yields http:--localhost:5001

groot@terminus:~$ systemd-escape http://localhost:5001
http:--localhost:5001

Display environment variables

The following code displays the environment variables and values on application startup, which can be helpful when debugging environment settings:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

foreach (var c in builder.Configuration.AsEnumerable())
{
    Console.WriteLine(c.Key + " = " + c.Value);
}

Command-line

Using the default configuration, the CommandLineConfigurationProvider loads configuration from command-line argument key-value pairs after the following configuration sources:

• appsettings.json and appsettings.{Environment}.json files.
• App secrets in the Development environment.
• Environment variables.

By default, configuration values set on the command-line override configuration values set with all the other configuration providers.

Command-line arguments

The following command sets keys and values using =:

dotnet run MyKey="Using =" Position:Title=Cmd Position:Name=Cmd_Rick

The following command sets keys and values using /:

dotnet run /MyKey "Using /" /Position:Title=Cmd /Position:Name=Cmd_Rick

The following command sets keys and values using --:

dotnet run --MyKey "Using --" --Position:Title=Cmd --Position:Name=Cmd_Rick

The key value:

• Must follow =, or the key must have a prefix of -- or / when the value follows a space.
• Isn't required if = is used. For example, MySetting=.

Within the same command, don't mix command-line argument key-value pairs that use = with key-value pairs that use a space.

Switch mappings

Switch mappings allow key name replacement logic. Provide a dictionary of switch replacements to the AddCommandLine method.

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 is passed back to set the key-value pair into the app's 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 - or --.
• The switch mappings dictionary must not contain duplicate keys.

To use a switch mappings dictionary, pass it into the call to AddCommandLine:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

var switchMappings = new Dictionary<string, string>()
         {
             { "-k1", "key1" },
             { "-k2", "key2" },
             { "--alt3", "key3" },
             { "--alt4", "key4" },
             { "--alt5", "key5" },
             { "--alt6", "key6" },
         };

builder.Configuration.AddCommandLine(args, switchMappings);

var app = builder.Build();

Run the following command works to test key replacement:

dotnet run -k1 value1 -k2 value2 --alt3=value2 /alt4=value3 --alt5 value5 /alt6 value6

The following code shows the key values for the replaced keys:

public class Test3Model : PageModel
{
    private readonly IConfiguration Config;

    public Test3Model(IConfiguration configuration)
    {
        Config = configuration;
    }

    public ContentResult OnGet()
    {
        return Content(
                $"Key1: '{Config["Key1"]}'\n" +
                $"Key2: '{Config["Key2"]}'\n" +
                $"Key3: '{Config["Key3"]}'\n" +
                $"Key4: '{Config["Key4"]}'\n" +
                $"Key5: '{Config["Key5"]}'\n" +
                $"Key6: '{Config["Key6"]}'");
    }
}

For apps that use switch mappings, the call to CreateDefaultBuilder shouldn't pass arguments. The CreateDefaultBuilder method's AddCommandLine call doesn't include mapped switches, and there's no way to pass the switch-mapping dictionary to CreateDefaultBuilder. The solution isn't to pass the arguments to CreateDefaultBuilder but instead to allow the ConfigurationBuilder method's AddCommandLine method to process both the arguments and the switch-mapping dictionary.

Set environment and command-line arguments with Visual Studio

Environment and command-line arguments can be set in Visual Studio from the launch profiles dialog:

• In Solution Explorer, right click the project and select Properties.
• Select the Debug > General tab and select Open debug launch profiles UI.

Hierarchical configuration data

The Configuration API reads hierarchical configuration data by flattening the hierarchical data with the use of a delimiter in the configuration keys.

The sample download contains the following appsettings.json file:

{
  "Position": {
    "Title": "Editor",
    "Name": "Joe Smith"
  },
  "MyKey": "My appsettings.json Value",
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
}

The following code from the sample download displays several of the configurations settings:

public class TestModel : PageModel
{
    // requires using Microsoft.Extensions.Configuration;
    private readonly IConfiguration Configuration;

    public TestModel(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {
        var myKeyValue = Configuration["MyKey"];
        var title = Configuration["Position:Title"];
        var name = Configuration["Position:Name"];
        var defaultLogLevel = Configuration["Logging:LogLevel:Default"];


        return Content($"MyKey value: {myKeyValue} \n" +
                       $"Title: {title} \n" +
                       $"Name: {name} \n" +
                       $"Default Log Level: {defaultLogLevel}");
    }
}

The preferred way to read hierarchical configuration data is using the options pattern. For more information, see Bind hierarchical configuration data in this document.

GetSection and GetChildren methods are available to isolate sections and children of a section in the configuration data. These methods are described later in GetSection, GetChildren, and Exists.

Configuration keys and values

Warning

This article shows the use of connection strings. With a local database the user doesn't have to be authenticated, but in production, connection strings sometimes include a password to authenticate. A resource owner password credential (ROPC) is a security risk that should be avoided in production databases. Production apps should use the most secure authentication flow available. For more information on authentication for apps deployed to test or production environments, see Secure authentication flows.

Configuration keys:

• Are case-insensitive. For example, ConnectionString and connectionstring are treated as equivalent keys.
• If a key and value is set in more than one configuration provider, the value from the last provider added is used. For more information, see Default configuration.
• Hierarchical keys
  • Within the Configuration API, a colon separator (:) works on all platforms.
  • In environment variables, a colon separator may not work on all platforms. A double underscore, __, is supported by all platforms and is automatically converted into a colon :.
  • In Azure Key Vault, hierarchical keys use -- as a separator. The Azure Key Vault configuration provider automatically replaces -- with a : when the secrets are loaded into the app's configuration.
• The ConfigurationBinder supports binding arrays to objects using array indices in configuration keys. Array binding is described in the Bind an array to a class section.

Configuration values:

• Are strings.
• Null values can't be stored in configuration or bound to objects.

Configuration providers

The following table shows the configuration providers available to ASP.NET Core apps.

Provider	Provides configuration from
Azure Key Vault configuration provider	Azure Key Vault
Azure App configuration provider	Azure App Configuration
Command-line configuration provider	Command-line parameters
Custom configuration provider	Custom source
Environment Variables configuration provider	Environment variables
File configuration provider	INI, JSON, and XML files
Key-per-file configuration provider	Directory files
Memory configuration provider	In-memory collections
User secrets	File in the user profile directory

Configuration sources are read in the order that their configuration providers are specified. Order configuration providers in code to suit the priorities for the underlying configuration sources that the app requires.

A typical sequence of configuration providers is:

1. appsettings.json
2. appsettings.{Environment}.json
3. User secrets
4. Environment variables using the Environment Variables configuration provider.
5. Command-line arguments using the Command-line configuration provider.

A common practice is to add the Command-line configuration provider last in a series of providers to allow command-line arguments to override configuration set by the other providers.

The preceding sequence of providers is used in the default configuration.

Connection string prefixes

Warning

This article shows the use of connection strings. With a local database the user doesn't have to be authenticated, but in production, connection strings sometimes include a password to authenticate. A resource owner password credential (ROPC) is a security risk that should be avoided in production databases. Production apps should use the most secure authentication flow available. For more information on authentication for apps deployed to test or production environments, see Secure authentication flows.

The Configuration API has special processing rules for four connection string environment variables. These connection strings are involved in configuring Azure connection strings for the app environment. Environment variables with the prefixes shown in the table are loaded into the app with the default configuration or when no prefix is supplied to AddEnvironmentVariables.

Connection string prefix	Provider
CUSTOMCONNSTR_	Custom provider
MYSQLCONNSTR_	MySQL
SQLAZURECONNSTR_	Azure SQL Database
SQLCONNSTR_	SQL Server

When an environment variable is discovered and loaded into configuration with any of the four prefixes shown in the table:

• The configuration key is created by removing the environment variable prefix and adding a configuration key section (ConnectionStrings).
• A new configuration key-value pair is created that represents the database connection provider (except for CUSTOMCONNSTR_, which has no stated provider).

Environment variable key	Converted configuration key	Provider configuration entry
CUSTOMCONNSTR_{KEY}	ConnectionStrings:{KEY}	Configuration entry not created.
MYSQLCONNSTR_{KEY}	ConnectionStrings:{KEY}	Key: ConnectionStrings:{KEY}_ProviderName: Value: MySql.Data.MySqlClient
SQLAZURECONNSTR_{KEY}	ConnectionStrings:{KEY}	Key: ConnectionStrings:{KEY}_ProviderName: Value: System.Data.SqlClient
SQLCONNSTR_{KEY}	ConnectionStrings:{KEY}	Key: ConnectionStrings:{KEY}_ProviderName: Value: System.Data.SqlClient

File configuration provider

FileConfigurationProvider is the base class for loading configuration from the file system. The following configuration providers derive from FileConfigurationProvider:

• INI configuration provider
• JSON configuration provider
• XML configuration provider

INI configuration provider

The IniConfigurationProvider loads configuration from INI file key-value pairs at runtime.

The following code adds several configuration providers:

var builder = WebApplication.CreateBuilder(args);

builder.Configuration
    .AddIniFile("MyIniConfig.ini", optional: true, reloadOnChange: true)
    .AddIniFile($"MyIniConfig.{builder.Environment.EnvironmentName}.ini",
                optional: true, reloadOnChange: true);

builder.Configuration.AddEnvironmentVariables();
builder.Configuration.AddCommandLine(args);

builder.Services.AddRazorPages();

var app = builder.Build();

In the preceding code, settings in the MyIniConfig.ini and MyIniConfig.{Environment}.ini files are overridden by settings in the:

• Environment variables configuration provider
• Command-line configuration provider.

The sample download contains the following MyIniConfig.ini file:

MyKey="MyIniConfig.ini Value"

[Position]
Title="My INI Config title"
Name="My INI Config name"

[Logging:LogLevel]
Default=Information
Microsoft=Warning

The following code from the sample download displays several of the preceding configurations settings:

public class TestModel : PageModel
{
    // requires using Microsoft.Extensions.Configuration;
    private readonly IConfiguration Configuration;

    public TestModel(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {
        var myKeyValue = Configuration["MyKey"];
        var title = Configuration["Position:Title"];
        var name = Configuration["Position:Name"];
        var defaultLogLevel = Configuration["Logging:LogLevel:Default"];


        return Content($"MyKey value: {myKeyValue} \n" +
                       $"Title: {title} \n" +
                       $"Name: {name} \n" +
                       $"Default Log Level: {defaultLogLevel}");
    }
}

JSON configuration provider

The JsonConfigurationProvider loads configuration from JSON file key-value pairs.

Overloads can specify:

• Whether the file is optional.
• Whether the configuration is reloaded if the file changes.

Consider the following code:

using Microsoft.Extensions.DependencyInjection.ConfigSample.Options;

var builder = WebApplication.CreateBuilder(args);

builder.Configuration.AddJsonFile("MyConfig.json",
        optional: true,
        reloadOnChange: true);

builder.Services.AddRazorPages();

var app = builder.Build();

The preceding code:

• Configures the JSON configuration provider to load the MyConfig.json file with the following options:
  • optional: true: The file is optional.
  • reloadOnChange: true : The file is reloaded when changes are saved.
• Reads the default configuration providers before the MyConfig.json file. Settings in the MyConfig.json file override setting in the default configuration providers, including the Environment variables configuration provider and the Command-line configuration provider.

You typically don't want a custom JSON file overriding values set in the Environment variables configuration provider and the Command-line configuration provider.

XML configuration provider

The XmlConfigurationProvider loads configuration from XML file key-value pairs at runtime.

The following code adds several configuration providers:

var builder = WebApplication.CreateBuilder(args);

builder.Configuration
    .AddXmlFile("MyXMLFile.xml", optional: true, reloadOnChange: true)
    .AddXmlFile($"MyXMLFile.{builder.Environment.EnvironmentName}.xml",
                optional: true, reloadOnChange: true);

builder.Configuration.AddEnvironmentVariables();
builder.Configuration.AddCommandLine(args);

builder.Services.AddRazorPages();

var app = builder.Build();

In the preceding code, settings in the MyXMLFile.xml and MyXMLFile.{Environment}.xml files are overridden by settings in the:

• Environment variables configuration provider
• Command-line configuration provider.

The sample download contains the following MyXMLFile.xml file:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <MyKey>MyXMLFile Value</MyKey>
  <Position>
    <Title>Title from  MyXMLFile</Title>
    <Name>Name from MyXMLFile</Name>
  </Position>
  <Logging>
    <LogLevel>
      <Default>Information</Default>
      <Microsoft>Warning</Microsoft>
    </LogLevel>
  </Logging>
</configuration>

The following code from the sample download displays several of the preceding configurations settings:

public class TestModel : PageModel
{
    // requires using Microsoft.Extensions.Configuration;
    private readonly IConfiguration Configuration;

    public TestModel(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {
        var myKeyValue = Configuration["MyKey"];
        var title = Configuration["Position:Title"];
        var name = Configuration["Position:Name"];
        var defaultLogLevel = Configuration["Logging:LogLevel:Default"];


        return Content($"MyKey value: {myKeyValue} \n" +
                       $"Title: {title} \n" +
                       $"Name: {name} \n" +
                       $"Default Log Level: {defaultLogLevel}");
    }
}

Repeating elements that use the same element name work if the name attribute is used to distinguish the elements:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <section name="section0">
    <key name="key0">value 00</key>
    <key name="key1">value 01</key>
  </section>
  <section name="section1">
    <key name="key0">value 10</key>
    <key name="key1">value 11</key>
  </section>
</configuration>

The following code reads the previous configuration file and displays the keys and values:

public class IndexModel : PageModel
{
    private readonly IConfiguration Configuration;

    public IndexModel(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {
        var key00 = "section:section0:key:key0";
        var key01 = "section:section0:key:key1";
        var key10 = "section:section1:key:key0";
        var key11 = "section:section1:key:key1";

        var val00 = Configuration[key00];
        var val01 = Configuration[key01];
        var val10 = Configuration[key10];
        var val11 = Configuration[key11];

        return Content($"{key00} value: {val00} \n" +
                       $"{key01} value: {val01} \n" +
                       $"{key10} value: {val10} \n" +
                       $"{key10} value: {val11} \n"
                       );
    }
}

Attributes can be used to supply values:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <key attribute="value" />
  <section>
    <key attribute="value" />
  </section>
</configuration>

The previous configuration file loads the following keys with value:

• key:attribute
• section:key:attribute

Key-per-file configuration provider

The KeyPerFileConfigurationProvider uses a directory's files as configuration key-value pairs. The key is the file name. The value contains the file's contents. The Key-per-file configuration provider is used in Docker hosting scenarios.

To activate key-per-file configuration, call the AddKeyPerFile extension method on an instance of ConfigurationBuilder. The directoryPath to the files must be an absolute path.

Overloads permit specifying:

• An Action<KeyPerFileConfigurationSource> delegate that configures the source.
• Whether the directory is optional and the path to the directory.

The double-underscore (__) is used as a configuration key delimiter in file names. For example, the file name Logging__LogLevel__System produces the configuration key Logging:LogLevel:System.

Call ConfigureAppConfiguration when building the host to specify the app's configuration:

.ConfigureAppConfiguration((hostingContext, config) =>
{
    var path = Path.Combine(
        Directory.GetCurrentDirectory(), "path/to/files");
    config.AddKeyPerFile(directoryPath: path, optional: true);
})

Memory configuration provider

The MemoryConfigurationProvider uses an in-memory collection as configuration key-value pairs.

The following code adds a memory collection to the configuration system:

var builder = WebApplication.CreateBuilder(args);

var Dict = new Dictionary<string, string>
        {
           {"MyKey", "Dictionary MyKey Value"},
           {"Position:Title", "Dictionary_Title"},
           {"Position:Name", "Dictionary_Name" },
           {"Logging:LogLevel:Default", "Warning"}
        };

builder.Configuration.AddInMemoryCollection(Dict);
builder.Configuration.AddEnvironmentVariables();
builder.Configuration.AddCommandLine(args);

builder.Services.AddRazorPages();

var app = builder.Build();

The following code from the sample download displays the preceding configurations settings:

public class TestModel : PageModel
{
    // requires using Microsoft.Extensions.Configuration;
    private readonly IConfiguration Configuration;

    public TestModel(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {
        var myKeyValue = Configuration["MyKey"];
        var title = Configuration["Position:Title"];
        var name = Configuration["Position:Name"];
        var defaultLogLevel = Configuration["Logging:LogLevel:Default"];


        return Content($"MyKey value: {myKeyValue} \n" +
                       $"Title: {title} \n" +
                       $"Name: {name} \n" +
                       $"Default Log Level: {defaultLogLevel}");
    }
}

In the preceding code, config.AddInMemoryCollection(Dict) is added after the default configuration providers. For an example of ordering the configuration providers, see JSON configuration provider.

See Bind an array for another example using MemoryConfigurationProvider.

Kestrel endpoint configuration

Kestrel specific endpoint configuration overrides all cross-server endpoint configurations. Cross-server endpoint configurations include:

• UseUrls
• --urls on the command line
• The environment variable ASPNETCORE_URLS

Consider the following appsettings.json file used in an ASP.NET Core web app:

{
  "Kestrel": {
    "Endpoints": {
      "Https": {
        "Url": "https://localhost:9999"
      }
    }
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
} 

When the preceding highlighted markup is used in an ASP.NET Core web app and the app is launched on the command line with the following cross-server endpoint configuration:

dotnet run --urls="https://localhost:7777"

Kestrel binds to the endpoint configured specifically for Kestrel in the appsettings.json file (https://localhost:9999) and not https://localhost:7777.

Consider the Kestrel specific endpoint configured as an environment variable:

set Kestrel__Endpoints__Https__Url=https://localhost:8888

In the preceding environment variable, Https is the name of the Kestrel specific endpoint. The preceding appsettings.json file also defines a Kestrel specific endpoint named Https. By default, environment variables using the Environment Variables configuration provider are read after appsettings.{Environment}.json, therefore, the preceding environment variable is used for the Https endpoint.

GetValue

ConfigurationBinder.GetValue extracts a single value from configuration with a specified key and converts it to the specified type:

public class TestNumModel : PageModel
{
    private readonly IConfiguration Configuration;

    public TestNumModel(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public ContentResult OnGet()
    {
        var number = Configuration.GetValue<int>("NumberKey", 99);
        return Content($"{number}");
    }
}

In the preceding code, if NumberKey isn't found in the configuration, the default value of 99 is used.

GetSection, GetChildren, and Exists

For the examples that follow, consider the following MySubsection.json file:

{
  "section0": {
    "key0": "value00",
    "key1": "value01"
  },
  "section1": {
    "key0": "value10",
    "key1": "value11"
  },
  "section2": {
    "subsection0": {
      "key0": "value200",
      "key1": "value201"
    },
    "subsection1": {
      "key0": "value210",
      "key1": "value211"
    }
  }
}

The following code adds MySubsection.json to the configuration providers:

var builder = WebApplication.CreateBuilder(args);

builder.Configuration
    .AddJsonFile("MySubsection.json",
                 optional: true,
                 reloadOnChange: true);

builder.Services.AddRazorPages();

var app = builder.Build();

GetSection

IConfiguration.GetSection returns a configuration subsection with the specified subsection key.

The following code returns values for section1:

public class TestSectionModel : PageModel
{
    private readonly IConfiguration Config;

    public TestSectionModel(IConfiguration configuration)
    {
        Config = configuration.GetSection("section1");
    }

    public ContentResult OnGet()
    {
        return Content(
                $"section1:key0: '{Config["key0"]}'\n" +
                $"section1:key1: '{Config["key1"]}'");
    }
}

The following code returns values for section2:subsection0:

public class TestSection2Model : PageModel
{
    private readonly IConfiguration Config;

    public TestSection2Model(IConfiguration configuration)
    {
        Config = configuration.GetSection("section2:subsection0");
    }

    public ContentResult OnGet()
    {
        return Content(
                $"section2:subsection0:key0 '{Config["key0"]}'\n" +
                $"section2:subsection0:key1:'{Config["key1"]}'");
    }
}

GetSection never returns null. If a matching section isn't found, an empty IConfigurationSection is returned.

When GetSection returns a matching section, Value isn't populated. A Key and Path are returned when the section exists.

GetChildren and Exists

The following code calls IConfiguration.GetChildren and returns values for section2:subsection0:

public class TestSection4Model : PageModel
{
    private readonly IConfiguration Config;

    public TestSection4Model(IConfiguration configuration)
    {
        Config = configuration;
    }

    public ContentResult OnGet()
    {
        string s = "";
        var selection = Config.GetSection("section2");
        if (!selection.Exists())
        {
            throw new Exception("section2 does not exist.");
        }
        var children = selection.GetChildren();

        foreach (var subSection in children)
        {
            int i = 0;
            var key1 = subSection.Key + ":key" + i++.ToString();
            var key2 = subSection.Key + ":key" + i.ToString();
            s += key1 + " value: " + selection[key1] + "\n";
            s += key2 + " value: " + selection[key2] + "\n";
        }
        return Content(s);
    }
}

The preceding code calls ConfigurationExtensions.Exists to verify the section exists:

Bind an array

The ConfigurationBinder.Bind supports binding arrays to objects using array indices in configuration keys. Any array format that exposes a numeric key segment is capable of array binding to a POCO class array.

Consider MyArray.json from the sample download:

{
  "array": {
    "entries": {
      "0": "value00",
      "1": "value10",
      "2": "value20",
      "4": "value40",
      "5": "value50"
    }
  }
}

The following code adds MyArray.json to the configuration providers:

var builder = WebApplication.CreateBuilder(args);

builder.Configuration
    .AddJsonFile("MyArray.json",
                 optional: true,
                 reloadOnChange: true);

builder.Services.AddRazorPages();

var app = builder.Build();

The following code reads the configuration and displays the values:

public class ArrayModel : PageModel
{
    private readonly IConfiguration Config;
    public ArrayExample? _array { get; private set; }

    public ArrayModel(IConfiguration config)
    {
        Config = config;
    }

    public ContentResult OnGet()
    {
       _array = Config.GetSection("array").Get<ArrayExample>();
        if (_array == null)
        {
            throw new ArgumentNullException(nameof(_array));
        }
        string s = String.Empty;

        for (int j = 0; j < _array.Entries.Length; j++)
        {
            s += $"Index: {j}  Value:  {_array.Entries[j]} \n";
        }

        return Content(s);
    }
}

public class ArrayExample
{
    public string[]? Entries { get; set; } 
}

The preceding code returns the following output:

Index: 0  Value: value00
Index: 1  Value: value10
Index: 2  Value: value20
Index: 3  Value: value40
Index: 4  Value: value50

In the preceding output, Index 3 has value value40, corresponding to "4": "value40", in MyArray.json. The bound array indices are continuous and not bound to the configuration key index. The configuration binder isn't capable of binding null values or creating null entries in bound objects.

Custom configuration provider

The sample app demonstrates how to create a basic configuration provider that reads configuration key-value pairs from a database using Entity Framework (EF).

The provider has the following characteristics:

• The EF in-memory database is used for demonstration purposes. To use a database that requires a connection string, implement a secondary ConfigurationBuilder to supply the connection string from another configuration provider.
• The provider reads a database table into configuration at startup. The provider doesn't query the database on a per-key basis.
• Reload-on-change isn't implemented, so updating the database after the app starts has no effect on the app's configuration.

Define an EFConfigurationValue entity for storing configuration values in the database.

Models/EFConfigurationValue.cs:

public class EFConfigurationValue
{
    public string Id { get; set; } = String.Empty;
    public string Value { get; set; } = String.Empty;
}

Add an EFConfigurationContext to store and access the configured values.

EFConfigurationProvider/EFConfigurationContext.cs:

public class EFConfigurationContext : DbContext
{
    public EFConfigurationContext(DbContextOptions<EFConfigurationContext> options) : base(options)
    {
    }

    public DbSet<EFConfigurationValue> Values => Set<EFConfigurationValue>();
}

Create a class that implements IConfigurationSource.

EFConfigurationProvider/EFConfigurationSource.cs:

public class EFConfigurationSource : IConfigurationSource
{
    private readonly Action<DbContextOptionsBuilder> _optionsAction;

    public EFConfigurationSource(Action<DbContextOptionsBuilder> optionsAction) => _optionsAction = optionsAction;

    public IConfigurationProvider Build(IConfigurationBuilder builder) => new EFConfigurationProvider(_optionsAction);
}

Create the custom configuration provider by inheriting from ConfigurationProvider. The configuration provider initializes the database when it's empty. Since configuration keys are case-insensitive, the dictionary used to initialize the database is created with the case-insensitive comparer (StringComparer.OrdinalIgnoreCase).

EFConfigurationProvider/EFConfigurationProvider.cs:

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

    Action<DbContextOptionsBuilder> OptionsAction { get; }

    public override void Load()
    {
        var builder = new DbContextOptionsBuilder<EFConfigurationContext>();

        OptionsAction(builder);

        using (var dbContext = new EFConfigurationContext(builder.Options))
        {
            if (dbContext == null || dbContext.Values == null)
            {
                throw new Exception("Null DB context");
            }
            dbContext.Database.EnsureCreated();

            Data = !dbContext.Values.Any()
                ? CreateAndSaveDefaultValues(dbContext)
                : dbContext.Values.ToDictionary(c => c.Id, c => c.Value);
        }
    }

    private static IDictionary<string, string> CreateAndSaveDefaultValues(
        EFConfigurationContext dbContext)
    {
        // Quotes (c)2005 Universal Pictures: Serenity
        // https://www.uphe.com/movies/serenity-2005
        var configValues =
            new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
            {
                    { "quote1", "I aim to misbehave." },
                    { "quote2", "I swallowed a bug." },
                    { "quote3", "You can't stop the signal, Mal." }
            };

        if (dbContext == null || dbContext.Values == null)
        {
            throw new Exception("Null DB context");
        }

        dbContext.Values.AddRange(configValues
            .Select(kvp => new EFConfigurationValue
            {
                Id = kvp.Key,
                Value = kvp.Value
            })
            .ToArray());

        dbContext.SaveChanges();

        return configValues;
    }
}

An AddEFConfiguration extension method permits adding the configuration source to a ConfigurationBuilder.

Extensions/EntityFrameworkExtensions.cs:

public static class EntityFrameworkExtensions
{
    public static IConfigurationBuilder AddEFConfiguration(
               this IConfigurationBuilder builder,
               Action<DbContextOptionsBuilder> optionsAction)
    {
        return builder.Add(new EFConfigurationSource(optionsAction));
    }
}

The following code shows how to use the custom EFConfigurationProvider in Program.cs:

//using Microsoft.EntityFrameworkCore;

var builder = WebApplication.CreateBuilder(args);

builder.Configuration.AddEFConfiguration(
    opt => opt.UseInMemoryDatabase("InMemoryDb"));

var app = builder.Build();

app.Run();

Access configuration with Dependency Injection (DI)

Configuration can be injected into services using Dependency Injection (DI) by resolving the IConfiguration service:

public class Service
{
    private readonly IConfiguration _config;

    public Service(IConfiguration config) =>
        _config = config;

    public void DoSomething()
    {
        var configSettingValue = _config["ConfigSetting"];

        // ...
    }
}

For information on how to access values using IConfiguration, see GetValue and GetSection, GetChildren, and Exists in this article.

Access configuration in Razor Pages

The following code displays configuration data in a Razor Page:

@page
@model Test5Model
@using Microsoft.Extensions.Configuration
@inject IConfiguration Configuration

Configuration value for 'MyKey': @Configuration["MyKey"]

In the following code, MyOptions is added to the service container with Configure and bound to configuration:

using SampleApp.Models;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Services.Configure<MyOptions>(
    builder.Configuration.GetSection("MyOptions"));

var app = builder.Build();

The following markup uses the @inject Razor directive to resolve and display the options values:

@page
@model SampleApp.Pages.Test3Model
@using Microsoft.Extensions.Options
@using SampleApp.Models
@inject IOptions<MyOptions> optionsAccessor


<p><b>Option1:</b> @optionsAccessor.Value.Option1</p>
<p><b>Option2:</b> @optionsAccessor.Value.Option2</p>

Access configuration in a MVC view file

The following code displays configuration data in a MVC view:

@using Microsoft.Extensions.Configuration
@inject IConfiguration Configuration

Configuration value for 'MyKey': @Configuration["MyKey"]

Access configuration in Program.cs

The following code accesses configuration in the Program.cs file.

var builder = WebApplication.CreateBuilder(args);

var key1 = builder.Configuration.GetValue<string>("KeyOne");

var app = builder.Build();

app.MapGet("/", () => "Hello World!");

var key2 = app.Configuration.GetValue<int>("KeyTwo");
var key3 = app.Configuration.GetValue<bool>("KeyThree");

app.Logger.LogInformation("KeyOne: {KeyOne}", key1);
app.Logger.LogInformation("KeyTwo: {KeyTwo}", key2);
app.Logger.LogInformation("KeyThree: {KeyThree}", key3);

app.Run();

In appsettings.json for the preceding example:

{
  ...
  "KeyOne": "Key One Value",
  "KeyTwo": 1999,
  "KeyThree": true
}

Configure options with a delegate

Options configured in a delegate override values set in the configuration providers.

In the following code, an IConfigureOptions<TOptions> service is added to the service container. It uses a delegate to configure values for MyOptions:

using SampleApp.Models;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Services.Configure<MyOptions>(myOptions =>
{
    myOptions.Option1 = "Value configured in delegate";
    myOptions.Option2 = 500;
});

var app = builder.Build();

The following code displays the options values:

public class Test2Model : PageModel
{
    private readonly IOptions<MyOptions> _optionsDelegate;

    public Test2Model(IOptions<MyOptions> optionsDelegate )
    {
        _optionsDelegate = optionsDelegate;
    }

    public ContentResult OnGet()
    {
        return Content($"Option1: {_optionsDelegate.Value.Option1} \n" +
                       $"Option2: {_optionsDelegate.Value.Option2}");
    }
}

In the preceding example, the values of Option1 and Option2 are specified in appsettings.json and then overridden by the configured delegate.

Host versus app configuration

Before the app is configured and started, a host is configured and launched. The host is responsible for app startup and lifetime management. Both the app and the host are configured using the configuration providers described in this topic. Host configuration key-value pairs are also included in the app's configuration. For more information on how the configuration providers are used when the host is built and how configuration sources affect host configuration, see ASP.NET Core fundamentals overview.

Default host configuration

For details on the default configuration when using the Web Host, see the ASP.NET Core 2.2 version of this topic.

• Host configuration is provided from:
  • Environment variables prefixed with DOTNET_ (for example, DOTNET_ENVIRONMENT) using the Environment Variables configuration provider. The prefix (DOTNET_) is stripped when the configuration key-value pairs are loaded.
  • Command-line arguments using the Command-line configuration provider.
• Web Host default configuration is established (ConfigureWebHostDefaults):
  • Kestrel is used as the web server and configured using the app's configuration providers.
  • Add Host Filtering Middleware.
  • Add Forwarded Headers Middleware if the ASPNETCORE_FORWARDEDHEADERS_ENABLED environment variable is set to true.
  • Enable IIS integration.

Other configuration

This topic only pertains to app configuration. Other aspects of running and hosting ASP.NET Core apps are configured using configuration files not covered in this topic:

• launch.json/launchSettings.json are tooling configuration files for the Development environment, described:
  • In Use multiple environments in ASP.NET Core.
  • Across the documentation set where the files are used to configure ASP.NET Core apps for Development scenarios.
• web.config is a server configuration file, described in the following topics:
  • Host ASP.NET Core on Windows with IIS
  • ASP.NET Core Module (ANCM) for IIS

Environment variables set in launchSettings.json override those set in the system environment.

For more information on migrating app configuration from earlier versions of ASP.NET, see Update from ASP.NET to ASP.NET Core.

Add configuration from an external assembly

An IHostingStartup implementation allows adding enhancements to an app at startup from an external assembly outside of the app's Startup class. For more information, see Use hosting startup assemblies in ASP.NET Core.

Configuration-binding source generator

The Configuration-binding source generator provides AOT and trim-friendly configuration. For more information, see Configuration-binding source generator.

Additional resources

• Configuration source code
• WebApplicationBuilder source code
• View or download sample code (how to download)
• Options pattern in ASP.NET Core
• ASP.NET Core Blazor configuration