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title	author	description	manager	ms.author	ms.custom	ms.date	ms.prod	ms.technology	ms.topic	uid
Configuration in ASP.NET Core	rick-anderson	Use the Configuration API to configure an ASP.NET Core app by multiple methods.	wpickett	riande	mvc	01/11/2018	asp.net-core	aspnet	article	fundamentals/configuration/index

Configure an ASP.NET Core App

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

The Configuration API provides a way to configure an ASP.NET Core web app based on a list of name-value pairs. Configuration is read at runtime from multiple sources. 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.
The unencrypted Secret Manager storage.
An encrypted user store, such as Azure Key Vault.
Custom providers (installed or created).

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).

The options pattern uses options classes to represent groups of related settings. For more information on using the options pattern, see the Options topic.

View or download sample code (how to download)

JSON configuration

The following console app uses the JSON configuration provider:

[!code-csharp]

The app reads and displays the following configuration settings:

[!code-json]

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:

[!code-csharp]

To work with arrays in JSON-formatted configuration sources, use an 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"]}");
// Output: Gandalf

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

The preceding sample uses the configuration indexer to read values. To access configuration outside of Startup, use the options pattern. For more information, see the Options topic.

XML configuration

To work with arrays in XML-formatted configuration sources, provide a name index to each element. Use the index to access the values:

<wizards>
  <wizard name="Gandalf">
    <age>1000</age>
  </wizard>
  <wizard name="Harry">
    <age>17</age>
  </wizard>
</wizards>

Console.Write($"{Configuration["wizard:Harry:age"]}");
// Output: 17

Configuration by environment

It's typical to have different configuration settings for different environments, for example, development, testing, 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

ASP.NET Core 1.x apps need to call AddJsonFile and AddEnvironmentVariables.

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

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

Consider the following appsettings.Staging.json file:

[!code-json]

When the environment is set to Staging, the following Configure method reads the value of MyConfig:

[!code-csharp]

The environment is typically set to Development, Staging, or Production. For more information, see Working with multiple environments.

Configuration considerations:

IOptionsSnapshot can reload configuration data when it changes. For more information, see IOptionsSnapshot.,
Configuration keys are not case-sensitive.
Never store passwords or other sensitive data in configuration provider code or in plain text configuration files. 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. Learn more about working with multiple environments and managing safe storage of app secrets during development.
If a colon (:) can't be used in environment variables on a system, replace the colon (:) with a double-underscore (__).

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:

[!code-csharp]

Configuration values are returned as strings, but binding enables the construction of objects. Binding allows the retrieval of POCO objects or even entire object graphs.

GetValue

The following sample demonstrates the GetValue<T> extension method:

[!code-csharp]

The ConfigurationBinder's GetValue<T> method allows the specification of a default value (80 in the sample). GetValue<T> is for simple scenarios and doesn't bind to entire sections. GetValue<T> obtains scalar values from GetSection(key).Value converted to a specific type.

Bind to an object graph

Each object in a class can be recursively bound. Consider the following AppSettings class:

[!code-csharp]

The following sample binds to the AppSettings class:

[!code-csharp]

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

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

Using the following appsettings.json file:

[!code-json]

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 settings = new AppSettings();
    config.GetSection("App").Bind(settings);

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

Create an 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:

[!code-csharp]

Add a ConfigurationContext to store and access the configured values:

[!code-csharp]

Create a class that implements IConfigurationSource:

[!code-csharp]

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

[!code-csharp]

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

An EFConfigSource extension method for adding the configuration source can be used:

[!code-csharp]

The following code shows how to use the custom EFConfigProvider:

[!code-csharp]

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:

[!code-json]

The following output 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

Setup and use the CommandLine configuration provider

Basic Configuration

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

[!code-csharp]

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:

[!code-csharp]

ASP.NET Core 2.x

Typical ASP.NET Core 2.x apps use the static convenience method CreateDefaultBuilder to build the host:

[!code-csharp]

CreateDefaultBuilder loads optional configuration from appsettings.json, appsettings.{Environment}.json, user secrets (in the Development environment), environment variables, and command-line arguments. The CommandLine configuration provider is called last. Calling the provider last allows the command-line arguments passed at runtime to override configuration set by the other configuration providers called earlier.

For appsettings files where:

reloadOnChange is enabled.
Contain the same setting in the command-line arguments and an appsettings file.
The appsettings file containing the matching command-line argument is changed after the app starts.

If all the preceding conditions are true, the command-line arguments are overridden.

ASP.NET Core 2.x app can use WebHostBuilder instead of CreateDefaultBuilder. When using WebHostBuilder, manually set configuration with ConfigurationBuilder. See the ASP.NET Core 1.x tab for more information.

ASP.NET Core 1.x

Create a ConfigurationBuilder and call the AddCommandLine method to use the CommandLine configuration provider. Calling the provider last allows the command-line arguments passed at runtime to override configuration set by the other configuration providers called earlier. Apply the configuration to WebHostBuilder with the UseConfiguration method:

[!code-csharp]

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, a switch mappings dictionary can be added to the AddCommandLine method. Switch mappings allow 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 command-line arguments to use a single dash (-) key prefix and avoid leading subkey prefixes.

[!code-csharp]

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

web.config file

A web.config file is required when hosting the app in IIS or IIS Express. Settings in web.config enable the ASP.NET Core Module to launch the app and configure other IIS settings and modules. If the web.config file isn't present and the project file includes <Project Sdk="Microsoft.NET.Sdk.Web">, publishing the project creates a web.config file in the published output (the publish folder). For more information, see Host ASP.NET Core on Windows with IIS.

Access configuration during startup

To access configuration within ConfigureServices or Configure during startup, see the examples in the Application startup topic.

Access configuration in a Razor Page or MVC view

To access configuration settings in a Razor Pages page or an MVC view, add a using directive (C# reference: using directive) for the Microsoft.Extensions.Configuration namespace and inject IConfiguration into the page or view.

In a Razor Pages page:

@page
@model IndexModel

@using Microsoft.Extensions.Configuration
@inject IConfiguration Configuration

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Index Page</title>
</head>
<body>
    <h1>Access configuration in a Razor Pages page</h1>
    <p>Configuration[&quot;key&quot;]: @Configuration["key"]</p>
</body>
</html>

In an MVC view:

@using Microsoft.Extensions.Configuration
@inject IConfiguration Configuration

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Index View</title>
</head>
<body>
    <h1>Access configuration in an MVC view</h1>
    <p>Configuration[&quot;key&quot;]: @Configuration["key"]</p>
</body>
</html>

Additional notes

Dependency Injection (DI) isn't set up until after ConfigureServices is invoked.
The configuration system isn't 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.
- Use IConfigurationRoot when reloading configuration or for access to each provider. Neither of these situations are common.

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Configure an ASP.NET Core App

JSON configuration

XML configuration

Configuration by environment

In-memory provider and binding to a POCO class

GetValue

Bind to an object graph

Create an Entity Framework custom provider

CommandLine configuration provider

Setup and use the CommandLine configuration provider

Basic Configuration

ASP.NET Core 2.x

ASP.NET Core 1.x

Arguments

Duplicate keys

Switch mappings

web.config file

Access configuration during startup

Access configuration in a Razor Page or MVC view

Additional notes

Additional resources

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