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conf is a Dart package for defining configuration schemas and loading configuration values from from multiple sources, such as:

  • command line arguments
  • environment variables
  • JSON string from command line arguments or environment variables
  • JSON files
  • YAML files

Sources override each other in a configurable order.


Add conf as a dependency in your pubspec.yaml file:

dart pub add conf


See the example package for a complete example.


Before you can load configuration values, you need to define a schema. I recommend breaking your schema up into multiple classes, each of which holds related configuration values. For example, if you have a server that needs to connect to a database, you might define a DatabaseConfiguration class that contains the database URL, username and password:

class DatabaseConfiguration {
    required this.url,
    required this.username,
    required this.password,

  factory DatabaseConfiguration._factory(Map<String, Object?> map) =>
        url: map['url']! as Uri,
        username: map['username']! as String,
        password: map['password']! as String,

  static final schema = ConfObject(
    propertiesMap: {
      'url': ConfUri(),
      'username': ConfString(),
      'password': ConfString(),
    factory: DatabaseConfiguration._factory,

  final Uri url;
  final String username;
  final String password;

Note that the DatabaseConfiguration class has a schema field that defines the configuration schema for the class. The schema field is a ConfObject that defines the configuration properties of the class and how to create an instance of DatabaseConfiguration from a map of property values.

Now lets define a ServerConfiguration class that contains the DatabaseConfiguration as well as the port and address to listen on:

class ServerConfiguration {
    required this.port,
    required this.address,
    required this.database,

  factory ServerConfiguration._factory(Map<String, Object?> map) =>
        port: map['port']! as int,
        address: map['address']! as InternetAddress,
        database: map['database']! as DatabaseConfiguration,

  static final schema = ConfObject(
    propertiesMap: {
      'port': ConfDefault(ConfInteger(), defaultValue: 8080),
      'address': ConfDefault(
        defaultValue: InternetAddress.loopbackIPv4,
      'database': DatabaseConfiguration.schema,
    factory: ServerConfiguration._factory,

  final int port;
  final InternetAddress address;
  final DatabaseConfiguration database;

Here we use the ConfDefault class to define a default value for the port and address properties. We also use the DatabaseConfiguration.schema field to define the database property.

Scalar values

conf provides a number of builtin schema classes to load scalar values:

  • ConfBool
  • ConfNumber
  • ConfInteger
  • ConfDouble
  • ConfString
  • ConfDateTime
  • ConfUri
  • ConfInternetAddress
  • ConfEnum

You can also define your own scalar value schema classes by extending the ConfScalar class, or one of its subclasses. For example, here is the implementation of the ConfInternetAddress class:

class ConfInternetAddress extends ParseConfScalar<InternetAddress> {
  ConfInternetAddress() : super('InternetAddress');

  InternetAddress parse(String value) {
    final address = InternetAddress.tryParse(value);
    if (address == null) {
      throw FormatException(
        'Expected an IPv4 or IPv6 address but got "$value".',
    return address;

Loading configuration

To load the ServerConfiguration we need a ConfigurationSource that provides the configuration values. For simplicity, we'll provide the configuration values directly in code:

final source = CombiningSource([
    'PORT': '4567',
    'DATABASE_URL': 'postgres://localhost:5432/db',
    'DATABASE_PASSWORD': 'password',

try {
  final configuration = await ServerConfiguration.schema.load(source);
  // Do something with the configuration.
} on ConfigurationException catch (error) {
  exitCode = 1;

If loading the configuration fails, load throws a ConfigurationException. conf does not stop after encountering the first error. Instead, it collects all errors and makes them available in ConfigurationException.errors. This allows you to display all errors at once, instead of fixing one error at a time.

The example above demonstrates one of the core features of conf: The ability to load configuration values from multiples sources, which have different formats.

conf inferred a environment variable name from the schema for each configuration value. For example, the port property is loaded from the PORT environment variable and the database.url property is loaded from the DATABASE_URL.

Formatting of configuration values from the command line is slightly different. For example, the database.username property is loaded from the --database.username command line argument.

Because we specified the CommandLineSource first, the database.username property is loaded from the command line argument instead of the environment variable.

Configuration sources

A ConfigurationSource is a lower-level representation of configuration values that makes it easy to load configuration values from different sources, such as:

  • CommandLineSource: Loads configuration values from command line arguments.
  • EnvironmentSource: Loads configuration values from environment variables.
  • DataSource: Loads configuration values from a JSON-style data structure. Typically used to load configuration values from a JSON and YAML files.
  • CombiningSource: Combines multiple sources into a single source.


AppSources provide an easy way to load configuration sources in an opinionated way that is suitable for Dart applications, such as servers.

AppSources assumes that the application defines a set of profiles. A profile is a named set of configuration values. For example, a server might have a dev profile for development and a prod profile for production. Multiple profiles can be active at the same time and are represented by the Profiles class.

enum Profile {

  /// The currently active profiles.
  static Profiles<Profile> get active => as Profiles<Profile>;

You specifying the active profiles as a comma separated list in the --profiles command line argument or the PROFILES environment. For example:

$ dart run server.dart --profiles="dev,test"

Continuing with the example from the previous section, we can use AppSources.load to load a CombiningSource that combines the command line and environment sources as well as configuration file sources from a well-known location.

class ServerConfiguration {

  // ...

  static Future<ServerConfiguration> load(
    List<String> arguments, {
    Set<Profile>? additionalProfiles,
  }) async {
    final sources = await AppSources.load(
      arguments: arguments,
      allProfiles: Profile.values,
      defaultProfiles: {},
      additionalProfiles: additionalProfiles,
    return schema.load(sources);

  // ...


After AppSources.load returns, contains the active profiles.

Configuration files are loaded from the following locations, in order or precedence:

  1. For each profile in alphabetical order:
    1. config/application.$profile.json
    2. config/application.$profile.yaml
    3. config/application.$profile.yml
  2. The base configuration:
    1. config/application.json
    2. config/application.yaml
    3. config/application.yml

The paths are relative to the current working directory.

When multiple profiles are active, the precedence between profiles is determined by sorting the profiles in alphabetical order. This is usually not something that should be relied on. Instead profiles that are going to be activated simultaneously should not have overlapping configuration values.

Profile configuration files always have a higher precedence than the base configuration files.