Options pattern in ASP.NET Core

By Luke Latham

For the 1.1 version of this topic, download Options pattern in ASP.NET Core (version 1.1, PDF).

The options pattern uses classes to represent groups of related settings. When configuration settings are isolated by scenario into separate classes, the app adheres to two important software engineering principles:

Options also provide a mechanism to validate configuration data. For more information, see the Options validation section.

View or download sample code (how to download)

Prerequisites

Reference the Microsoft.AspNetCore.App metapackage or add a package reference to the Microsoft.Extensions.Options.ConfigurationExtensions package.

Options interfaces

IOptionsMonitor<TOptions> is used to retrieve options and manage options notifications for TOptions instances. IOptionsMonitor<TOptions> supports the following scenarios:

Post-configuration scenarios allow you to set or change options after all IConfigureOptions<TOptions> configuration occurs.

IOptionsFactory<TOptions> is responsible for creating new options instances. It has a single Create method. The default implementation takes all registered IConfigureOptions<TOptions> and IPostConfigureOptions<TOptions> and runs all the configurations first, followed by the post-configuration. It distinguishes between IConfigureNamedOptions<TOptions> and IConfigureOptions<TOptions> and only calls the appropriate interface.

IOptionsMonitorCache<TOptions> is used by IOptionsMonitor<TOptions> to cache TOptions instances. The IOptionsMonitorCache<TOptions> invalidates options instances in the monitor so that the value is recomputed (TryRemove). Values can be manually introduced with TryAdd. The Clear method is used when all named instances should be recreated on demand.

IOptionsSnapshot<TOptions> is useful in scenarios where options should be recomputed on every request. For more information, see the Reload configuration data with IOptionsSnapshot section.

IOptions<TOptions> can be used to support options. However, IOptions<TOptions> doesn't support the preceding scenarios of IOptionsMonitor<TOptions>. You may continue to use IOptions<TOptions> in existing frameworks and libraries that already use the IOptions<TOptions> interface and don't require the scenarios provided by IOptionsMonitor<TOptions>.

General options configuration

General options configuration is demonstrated as Example #1 in the sample app.

An options class must be non-abstract with a public parameterless constructor. The following class, MyOptions, has two properties, Option1 and Option2. Setting default values is optional, but the class constructor in the following example sets the default value of Option1. Option2 has a default value set by initializing the property directly (Models/MyOptions.cs):

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

The MyOptions class is added to the service container with Configure and bound to configuration:

// Example #1: General configuration
// Register the Configuration instance which MyOptions binds against.
services.Configure<MyOptions>(Configuration);

The following page model uses constructor dependency injection with IOptionsMonitor<TOptions> to access the settings (Pages/Index.cshtml.cs):

private readonly MyOptions _options;
public IndexModel(
    IOptionsMonitor<MyOptions> optionsAccessor, 
    IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig, 
    IOptionsMonitor<MySubOptions> subOptionsAccessor, 
    IOptionsSnapshot<MyOptions> snapshotOptionsAccessor, 
    IOptionsSnapshot<MyOptions> namedOptionsAccessor)
{
    _options = optionsAccessor.CurrentValue;
    _optionsWithDelegateConfig = optionsAccessorWithDelegateConfig.CurrentValue;
    _subOptions = subOptionsAccessor.CurrentValue;
    _snapshotOptions = snapshotOptionsAccessor.Value;
    _named_options_1 = namedOptionsAccessor.Get("named_options_1");
    _named_options_2 = namedOptionsAccessor.Get("named_options_2");
}
// Example #1: Simple options
var option1 = _options.Option1;
var option2 = _options.Option2;
SimpleOptions = $"option1 = {option1}, option2 = {option2}";

The sample's appsettings.json file specifies values for option1 and option2:

{
  "option1": "value1_from_json",
  "option2": -1,
  "subsection": {
    "suboption1": "subvalue1_from_json",
    "suboption2": 200
  },
  "Logging": {
    "LogLevel": {
      "Default": "Warning"
    }
  },
  "AllowedHosts": "*"
}

When the app is run, the page model's OnGet method returns a string showing the option class values:

option1 = value1_from_json, option2 = -1

Note

When using a custom ConfigurationBuilder to load options configuration from a settings file, confirm that the base path is set correctly:

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

services.Configure<MyOptions>(config);

Explicitly setting the base path isn't required when loading options configuration from the settings file via CreateDefaultBuilder.

Configure simple options with a delegate

Configuring simple options with a delegate is demonstrated as Example #2 in the sample app.

Use a delegate to set options values. The sample app uses the MyOptionsWithDelegateConfig class (Models/MyOptionsWithDelegateConfig.cs):

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

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

// Example #2: Options bound and configured by a delegate
services.Configure<MyOptionsWithDelegateConfig>(myOptions =>
{
    myOptions.Option1 = "value1_configured_by_delegate";
    myOptions.Option2 = 500;
});

Index.cshtml.cs:

private readonly MyOptionsWithDelegateConfig _optionsWithDelegateConfig;
public IndexModel(
    IOptionsMonitor<MyOptions> optionsAccessor, 
    IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig, 
    IOptionsMonitor<MySubOptions> subOptionsAccessor, 
    IOptionsSnapshot<MyOptions> snapshotOptionsAccessor, 
    IOptionsSnapshot<MyOptions> namedOptionsAccessor)
{
    _options = optionsAccessor.CurrentValue;
    _optionsWithDelegateConfig = optionsAccessorWithDelegateConfig.CurrentValue;
    _subOptions = subOptionsAccessor.CurrentValue;
    _snapshotOptions = snapshotOptionsAccessor.Value;
    _named_options_1 = namedOptionsAccessor.Get("named_options_1");
    _named_options_2 = namedOptionsAccessor.Get("named_options_2");
}
// Example #2: Options configured by delegate
var delegate_config_option1 = _optionsWithDelegateConfig.Option1;
var delegate_config_option2 = _optionsWithDelegateConfig.Option2;
SimpleOptionsWithDelegateConfig = 
    $"delegate_option1 = {delegate_config_option1}, " +
    $"delegate_option2 = {delegate_config_option2}";

You can add multiple configuration providers. Configuration providers are available from NuGet packages and are applied in the order that they're registered. For more information, see Configuration in ASP.NET Core.

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

When more than one configuration service is enabled, the last configuration source specified wins and sets the configuration value. When the app is run, the page model's OnGet method returns a string showing the option class values:

delegate_option1 = value1_configured_by_delgate, delegate_option2 = 500

Suboptions configuration

Suboptions configuration is demonstrated as Example #3 in the sample app.

Apps should create options classes that pertain to specific scenario groups (classes) in the app. Parts of the app that require configuration values should only have access to the configuration values that they use.

When binding options to configuration, each property in the options type is bound to a configuration key of the form property[:sub-property:]. For example, the MyOptions.Option1 property is bound to the key Option1, which is read from the option1 property in appsettings.json.

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

// Example #3: Suboptions
// Bind options using a sub-section of the appsettings.json file.
services.Configure<MySubOptions>(Configuration.GetSection("subsection"));

The GetSection extension method requires the Microsoft.Extensions.Options.ConfigurationExtensions NuGet package. If the app uses the Microsoft.AspNetCore.App metapackage (ASP.NET Core 2.1 or later), the package is automatically included.

The sample's appsettings.json file defines a subsection member with keys for suboption1 and suboption2:

{
  "option1": "value1_from_json",
  "option2": -1,
  "subsection": {
    "suboption1": "subvalue1_from_json",
    "suboption2": 200
  },
  "Logging": {
    "LogLevel": {
      "Default": "Warning"
    }
  },
  "AllowedHosts": "*"
}

The MySubOptions class defines properties, SubOption1 and SubOption2, to hold the options values (Models/MySubOptions.cs):

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

The page model's OnGet method returns a string with the options values (Pages/Index.cshtml.cs):

private readonly MySubOptions _subOptions;
public IndexModel(
    IOptionsMonitor<MyOptions> optionsAccessor, 
    IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig, 
    IOptionsMonitor<MySubOptions> subOptionsAccessor, 
    IOptionsSnapshot<MyOptions> snapshotOptionsAccessor, 
    IOptionsSnapshot<MyOptions> namedOptionsAccessor)
{
    _options = optionsAccessor.CurrentValue;
    _optionsWithDelegateConfig = optionsAccessorWithDelegateConfig.CurrentValue;
    _subOptions = subOptionsAccessor.CurrentValue;
    _snapshotOptions = snapshotOptionsAccessor.Value;
    _named_options_1 = namedOptionsAccessor.Get("named_options_1");
    _named_options_2 = namedOptionsAccessor.Get("named_options_2");
}
// Example #3: Suboptions
var subOption1 = _subOptions.SubOption1;
var subOption2 = _subOptions.SubOption2;
SubOptions = $"subOption1 = {subOption1}, subOption2 = {subOption2}";

When the app is run, the OnGet method returns a string showing the suboption class values:

subOption1 = subvalue1_from_json, subOption2 = 200

Options provided by a view model or with direct view injection

Options provided by a view model or with direct view injection is demonstrated as Example #4 in the sample app.

Options can be supplied in a view model or by injecting IOptionsMonitor<TOptions> directly into a view (Pages/Index.cshtml.cs):

private readonly MyOptions _options;
public IndexModel(
    IOptionsMonitor<MyOptions> optionsAccessor, 
    IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig, 
    IOptionsMonitor<MySubOptions> subOptionsAccessor, 
    IOptionsSnapshot<MyOptions> snapshotOptionsAccessor, 
    IOptionsSnapshot<MyOptions> namedOptionsAccessor)
{
    _options = optionsAccessor.CurrentValue;
    _optionsWithDelegateConfig = optionsAccessorWithDelegateConfig.CurrentValue;
    _subOptions = subOptionsAccessor.CurrentValue;
    _snapshotOptions = snapshotOptionsAccessor.Value;
    _named_options_1 = namedOptionsAccessor.Get("named_options_1");
    _named_options_2 = namedOptionsAccessor.Get("named_options_2");
}
// Example #4: Bind options directly to the page
MyOptions = _options;

The sample app shows how to inject IOptionsMonitor<MyOptions> with an @inject directive:

@page
@model IndexModel
@using Microsoft.Extensions.Options
@inject IOptionsMonitor<MyOptions> OptionsAccessor
@{
    ViewData["Title"] = "Options Sample";
}

<h1>@ViewData["Title"]</h1>

When the app is run, the options values are shown in the rendered page:

Options values Option1: value1_from_json and Option2: -1 are loaded from the model and by injection into the view.

Reload configuration data with IOptionsSnapshot

Reloading configuration data with IOptionsSnapshot<TOptions> is demonstrated in Example #5 in the sample app.

IOptionsSnapshot<TOptions> supports reloading options with minimal processing overhead.

Options are computed once per request when accessed and cached for the lifetime of the request.

The following example demonstrates how a new IOptionsSnapshot<TOptions> is created after appsettings.json changes (Pages/Index.cshtml.cs). Multiple requests to the server return constant values provided by the appsettings.json file until the file is changed and configuration reloads.

private readonly MyOptions _snapshotOptions;
public IndexModel(
    IOptionsMonitor<MyOptions> optionsAccessor, 
    IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig, 
    IOptionsMonitor<MySubOptions> subOptionsAccessor, 
    IOptionsSnapshot<MyOptions> snapshotOptionsAccessor, 
    IOptionsSnapshot<MyOptions> namedOptionsAccessor)
{
    _options = optionsAccessor.CurrentValue;
    _optionsWithDelegateConfig = optionsAccessorWithDelegateConfig.CurrentValue;
    _subOptions = subOptionsAccessor.CurrentValue;
    _snapshotOptions = snapshotOptionsAccessor.Value;
    _named_options_1 = namedOptionsAccessor.Get("named_options_1");
    _named_options_2 = namedOptionsAccessor.Get("named_options_2");
}
// Example #5: Snapshot options
var snapshotOption1 = _snapshotOptions.Option1;
var snapshotOption2 = _snapshotOptions.Option2;
SnapshotOptions = 
    $"snapshot option1 = {snapshotOption1}, " +
    $"snapshot option2 = {snapshotOption2}";

The following image shows the initial option1 and option2 values loaded from the appsettings.json file:

snapshot option1 = value1_from_json, snapshot option2 = -1

Change the values in the appsettings.json file to value1_from_json UPDATED and 200. Save the appsettings.json file. Refresh the browser to see that the options values are updated:

snapshot option1 = value1_from_json UPDATED, snapshot option2 = 200

Named options support with IConfigureNamedOptions

Named options support with IConfigureNamedOptions<TOptions> is demonstrated as Example #6 in the sample app.

Named options support allows the app to distinguish between named options configurations. In the sample app, named options are declared with Configure. Configure calls the extension method Configure method:

// Example #6: Named options (named_options_1)
// Register the ConfigurationBuilder instance which MyOptions binds against.
// Specify that the options loaded from configuration are named
// "named_options_1".
services.Configure<MyOptions>("named_options_1", Configuration);

// Example #6: Named options (named_options_2)
// Specify that the options loaded from the MyOptions class are named
// "named_options_2".
// Use a delegate to configure option values.
services.Configure<MyOptions>("named_options_2", myOptions =>
{
    myOptions.Option1 = "named_options_2_value1_from_action";
});

The sample app accesses the named options with Get (Pages/Index.cshtml.cs):

private readonly MyOptions _named_options_1;
private readonly MyOptions _named_options_2;
public IndexModel(
    IOptionsMonitor<MyOptions> optionsAccessor, 
    IOptionsMonitor<MyOptionsWithDelegateConfig> optionsAccessorWithDelegateConfig, 
    IOptionsMonitor<MySubOptions> subOptionsAccessor, 
    IOptionsSnapshot<MyOptions> snapshotOptionsAccessor, 
    IOptionsSnapshot<MyOptions> namedOptionsAccessor)
{
    _options = optionsAccessor.CurrentValue;
    _optionsWithDelegateConfig = optionsAccessorWithDelegateConfig.CurrentValue;
    _subOptions = subOptionsAccessor.CurrentValue;
    _snapshotOptions = snapshotOptionsAccessor.Value;
    _named_options_1 = namedOptionsAccessor.Get("named_options_1");
    _named_options_2 = namedOptionsAccessor.Get("named_options_2");
}
// Example #6: Named options
var named_options_1 = 
    $"named_options_1: option1 = {_named_options_1.Option1}, " +
    $"option2 = {_named_options_1.Option2}";
var named_options_2 = 
    $"named_options_2: option1 = {_named_options_2.Option1}, " +
    $"option2 = {_named_options_2.Option2}";
NamedOptions = $"{named_options_1} {named_options_2}";

Running the sample app, the named options are returned:

named_options_1: option1 = value1_from_json, option2 = -1
named_options_2: option1 = named_options_2_value1_from_action, option2 = 5

named_options_1 values are provided from configuration, which are loaded from the appsettings.json file. named_options_2 values are provided by:

  • The named_options_2 delegate in ConfigureServices for Option1.
  • The default value for Option2 provided by the MyOptions class.

Configure all options with the ConfigureAll method

Configure all options instances with the ConfigureAll method. The following code configures Option1 for all configuration instances with a common value. Add the following code manually to the Startup.ConfigureServices method:

services.ConfigureAll<MyOptions>(myOptions => 
{
    myOptions.Option1 = "ConfigureAll replacement value";
});

Running the sample app after adding the code produces the following result:

named_options_1: option1 = ConfigureAll replacement value, option2 = -1
named_options_2: option1 = ConfigureAll replacement value, option2 = 5

Note

All options are named instances. Existing IConfigureOptions<TOptions> instances are treated as targeting the Options.DefaultName instance, which is string.Empty. IConfigureNamedOptions<TOptions> also implements IConfigureOptions<TOptions>. The default implementation of the IOptionsFactory<TOptions> has logic to use each appropriately. The null named option is used to target all of the named instances instead of a specific named instance (ConfigureAll and PostConfigureAll use this convention).

OptionsBuilder API

OptionsBuilder<TOptions> is used to configure TOptions instances. OptionsBuilder streamlines creating named options as it's only a single parameter to the initial AddOptions<TOptions>(string optionsName) call instead of appearing in all of the subsequent calls. Options validation and the ConfigureOptions overloads that accept service dependencies are only available via OptionsBuilder.

// Options.DefaultName = "" is used.
services.AddOptions<MyOptions>().Configure(o => o.Property = "default");

services.AddOptions<MyOptions>("optionalName")
    .Configure(o => o.Property = "named");

Configure<TOptions, TDep1, ... TDep4> method

Using services from DI to configure options by implementing IConfigure[Named]Options in a boilerplate manner is verbose. Overloads for ConfigureOptions on OptionsBuilder<TOptions> allow you to use up to five services to configure options:

services.AddOptions<MyOptions>("optionalName")
    .Configure<Service1, Service2, Service3, Service4, Service5>(
        (o, s, s2, s3, s4, s5) => 
            o.Property = DoSomethingWith(s, s2, s3, s4, s5));

The overload registers a transient generic IConfigureNamedOptions<TOptions>, which has a constructor that accepts the generic service types specified.

Options validation

Options validation allows you to validate options when options are configured. Call Validate with a validation method that returns true if options are valid and false if they aren't valid:

// Registration
services.AddOptions<MyOptions>("optionalOptionsName")
    .Configure(o => { }) // Configure the options
    .Validate(o => YourValidationShouldReturnTrueIfValid(o), 
        "custom error");

// Consumption
var monitor = services.BuildServiceProvider()
    .GetService<IOptionsMonitor<MyOptions>>();

try
{
    var options = monitor.Get("optionalOptionsName");
}
catch (OptionsValidationException e) 
{
   // e.OptionsName returns "optionalOptionsName"
   // e.OptionsType returns typeof(MyOptions)
   // e.Failures returns a list of errors, which would contain 
   //     "custom error"
}

The preceding example sets the named options instance to optionalOptionsName. The default options instance is Options.DefaultName.

Validation runs when the options instance is created. Your options instance is guaranteed to pass validation the first time it's accessed.

Important

Options validation doesn't guard against options modifications after the options are initially configured and validated.

The Validate method accepts a Func<TOptions, bool>. To fully customize validation, implement IValidateOptions<TOptions>, which allows:

  • Validation of multiple options types: class ValidateTwo : IValidateOptions<Option1>, IValidationOptions<Option2>
  • Validation that depends on another option type: public DependsOnAnotherOptionValidator(IOptionsMonitor<AnotherOption> options)

IValidateOptions validates:

  • A specific named options instance.
  • All options when name is null.

Return a ValidateOptionsResult from your implementation of the interface:

public interface IValidateOptions<TOptions> where TOptions : class
{
    ValidateOptionsResult Validate(string name, TOptions options);
}

Data Annotation-based validation is available from the Microsoft.Extensions.Options.DataAnnotations package by calling the ValidateDataAnnotations method on OptionsBuilder<TOptions>. Microsoft.Extensions.Options.DataAnnotations is included in the Microsoft.AspNetCore.App metapackage (ASP.NET Core 2.1 or later).

private class AnnotatedOptions
{
    [Required]
    public string Required { get; set; }

    [StringLength(5, ErrorMessage = "Too long.")]
    public string StringLength { get; set; }

    [Range(-5, 5, ErrorMessage = "Out of range.")]
    public int IntRange { get; set; }
}

[Fact]
public void CanValidateDataAnnotations()
{
    var services = new ServiceCollection();
    services.AddOptions<AnnotatedOptions>()
        .Configure(o =>
        {
            o.StringLength = "111111";
            o.IntRange = 10;
            o.Custom = "nowhere";
        })
        .ValidateDataAnnotations();

    var sp = services.BuildServiceProvider();

    var error = Assert.Throws<OptionsValidationException>(() => 
        sp.GetRequiredService<IOptionsMonitor<AnnotatedOptions>>().Value);
    ValidateFailure<AnnotatedOptions>(error, Options.DefaultName, 1,
        "DataAnnotation validation failed for members Required " +
            "with the error 'The Required field is required.'.",
        "DataAnnotation validation failed for members StringLength " +
            "with the error 'Too long.'.",
        "DataAnnotation validation failed for members IntRange " +
            "with the error 'Out of range.'.");
}

Eager validation (fail fast at startup) is under consideration for a future release.

Options post-configuration

Set post-configuration with IPostConfigureOptions<TOptions>. Post-configuration runs after all IConfigureOptions<TOptions> configuration occurs:

services.PostConfigure<MyOptions>(myOptions =>
{
    myOptions.Option1 = "post_configured_option1_value";
});

PostConfigure is available to post-configure named options:

services.PostConfigure<MyOptions>("named_options_1", myOptions =>
{
    myOptions.Option1 = "post_configured_option1_value";
});

Use PostConfigureAll to post-configure all configuration instances:

services.PostConfigureAll<MyOptions>(myOptions =>
{
    myOptions.Option1 = "post_configured_option1_value";
});

Accessing options during startup

IOptions<TOptions> and IOptionsMonitor<TOptions> can be used in Startup.Configure, since services are built before the Configure method executes.

public void Configure(IApplicationBuilder app, IOptionsMonitor<MyOptions> optionsAccessor)
{
    var option1 = optionsAccessor.CurrentValue.Option1;
}

Don't use IOptions<TOptions> or IOptionsMonitor<TOptions> in Startup.ConfigureServices. An inconsistent options state may exist due to the ordering of service registrations.

Additional resources