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. You can group these name-value pairs 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 (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:

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 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"]}, ");

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. For more information, see the Options topic.

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

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 code above, the environment variables are read last. Any configuration values set through the environment replace those set in the two previous providers.

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

Configuration considerations:

  • IOptionsSnapshot can reload configuration data when it changes. See IOptionsSnapshot for more information.
  • Configuration keys are case insensitive.
  • Specify environment variables last so that the local environment can override settings 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 of the project so that they can't be accidentally committed to your 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 your 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:

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.

GetValue

The following sample demonstrates the GetValue<T> 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.

Bind to an object graph

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

public class AppSettings
{
    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 AppSettings 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 AppSettings();
        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 convenient than using Bind. The following code shows how to use Get<T> with the sample above:

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

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 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:

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

Setup and use the CommandLine configuration 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