Options pattern in ASP.NET Core

By Kirk Larkin and Rick Anderson.

The options pattern uses classes to provide strongly typed access to 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.

This topic provides information on the options pattern in ASP.NET Core. For information on using the options pattern in console apps, see Options pattern in .NET.

View or download sample code (how to download)

Bind hierarchical configuration

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 property 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.

Options interfaces

IOptions<TOptions>:

IOptionsSnapshot<TOptions>:

IOptionsMonitor<TOptions>:

Post-configuration scenarios enable setting or changing 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.

Use IOptionsSnapshot to read updated data

Using IOptionsSnapshot<TOptions>, options are computed once per request when accessed and cached for the lifetime of the request. Changes to the configuration are read after the app starts when using configuration providers that support reading updated configuration values.

The difference between IOptionsMonitor and IOptionsSnapshot is that:

  • IOptionsMonitor is a singleton service that retrieves current option values at any time, which is especially useful in singleton dependencies.
  • IOptionsSnapshot is a scoped service and provides a snapshot of the options at the time the IOptionsSnapshot<T> object is constructed. Options snapshots are designed for use with transient and scoped dependencies.

The following code uses IOptionsSnapshot<TOptions>.

public class TestSnapModel : PageModel
{
    private readonly MyOptions _snapshotOptions;

    public TestSnapModel(IOptionsSnapshot<MyOptions> snapshotOptionsAccessor)
    {
        _snapshotOptions = snapshotOptionsAccessor.Value;
    }

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

The following code registers a configuration instance which MyOptions binds against:

using SampleApp.Models;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

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

var app = builder.Build();

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

IOptionsMonitor

The following code registers a configuration instance which MyOptions binds against.

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 example uses IOptionsMonitor<TOptions>:

public class TestMonitorModel : PageModel
{
    private readonly IOptionsMonitor<MyOptions> _optionsDelegate;

    public TestMonitorModel(IOptionsMonitor<MyOptions> optionsDelegate )
    {
        _optionsDelegate = optionsDelegate;
    }

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

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

Named options support using IConfigureNamedOptions

Named options:

  • Are useful when multiple configuration sections bind to the same properties.
  • Are case sensitive.

Consider the following appsettings.json file:

{
  "TopItem": {
    "Month": {
      "Name": "Green Widget",
      "Model": "GW46"
    },
    "Year": {
      "Name": "Orange Gadget",
      "Model": "OG35"
    }
  }
}

Rather than creating two classes to bind TopItem:Month and TopItem:Year, the following class is used for each section:

public class TopItemSettings
{
    public const string Month = "Month";
    public const string Year = "Year";

    public string Name { get; set; } = string.Empty;
    public string Model { get; set; } = string.Empty;
}

The following code configures the named options:

using SampleApp.Models;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Services.Configure<TopItemSettings>(TopItemSettings.Month,
    builder.Configuration.GetSection("TopItem:Month"));
builder.Services.Configure<TopItemSettings>(TopItemSettings.Year,
    builder.Configuration.GetSection("TopItem:Year"));

var app = builder.Build();

The following code displays the named options:

public class TestNOModel : PageModel
{
    private readonly TopItemSettings _monthTopItem;
    private readonly TopItemSettings _yearTopItem;

    public TestNOModel(IOptionsSnapshot<TopItemSettings> namedOptionsAccessor)
    {
        _monthTopItem = namedOptionsAccessor.Get(TopItemSettings.Month);
        _yearTopItem = namedOptionsAccessor.Get(TopItemSettings.Year);
    }

    public ContentResult OnGet()
    {
        return Content($"Month:Name {_monthTopItem.Name} \n" +
                       $"Month:Model {_monthTopItem.Model} \n\n" +
                       $"Year:Name {_yearTopItem.Name} \n" +
                       $"Year:Model {_yearTopItem.Model} \n"   );
    }
}

All options are named instances. 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.

OptionsBuilder is used in the Options validation section.

See Use AddOptions to configure custom repository for information adding a custom repository.

Use DI services to configure options

Services can be accessed from dependency injection while configuring options in two ways:

  • Pass a configuration delegate to Configure on OptionsBuilder<TOptions>. OptionsBuilder<TOptions> provides overloads of Configure that allow use of up to five services to configure options:

    builder.Services.AddOptions<MyOptions>("optionalName")
        .Configure<Service1, Service2, Service3, Service4, Service5>(
            (o, s, s2, s3, s4, s5) => 
                o.Property = DoSomethingWith(s, s2, s3, s4, s5));
    
  • Create a type that implements IConfigureOptions<TOptions> or IConfigureNamedOptions<TOptions> and register the type as a service.

We recommend passing a configuration delegate to Configure, since creating a service is more complex. Creating a type is equivalent to what the framework does when calling Configure. Calling Configure registers a transient generic IConfigureNamedOptions<TOptions>, which has a constructor that accepts the generic service types specified.

Options validation

Options validation enables option values to be validated.

Consider the following appsettings.json file:

{
  "MyConfig": {
    "Key1": "My Key One",
    "Key2": 10,
    "Key3": 32
  }
}

The following class is used to bind to the "MyConfig" configuration section and applies a couple of DataAnnotations rules:

public class MyConfigOptions
{
    public const string MyConfig = "MyConfig";

    [RegularExpression(@"^[a-zA-Z''-'\s]{1,40}$")]
    public string Key1 { get; set; }
    [Range(0, 1000,
        ErrorMessage = "Value for {0} must be between {1} and {2}.")]
    public int Key2 { get; set; }
    public int Key3 { get; set; }
}

The following code:

using OptionsValidationSample.Configuration;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllersWithViews();

builder.Services.AddOptions<MyConfigOptions>()
            .Bind(builder.Configuration.GetSection(MyConfigOptions.MyConfig))
            .ValidateDataAnnotations();

var app = builder.Build();

The ValidateDataAnnotations extension method is defined in the Microsoft.Extensions.Options.DataAnnotations NuGet package. For web apps that use the Microsoft.NET.Sdk.Web SDK, this package is referenced implicitly from the shared framework.

The following code displays the configuration values or the validation errors:

public class HomeController : Controller
{
    private readonly ILogger<HomeController> _logger;
    private readonly IOptions<MyConfigOptions> _config;

    public HomeController(IOptions<MyConfigOptions> config,
                          ILogger<HomeController> logger)
    {
        _config = config;
        _logger = logger;

        try
        {
            var configValue = _config.Value;

        }
        catch (OptionsValidationException ex)
        {
            foreach (var failure in ex.Failures)
            {
                _logger.LogError(failure);
            }
        }
    }

    public ContentResult Index()
    {
        string msg;
        try
        {
            msg = $"Key1: {_config.Value.Key1} \n" +
                  $"Key2: {_config.Value.Key2} \n" +
                  $"Key3: {_config.Value.Key3}";
        }
        catch (OptionsValidationException optValEx)
        {
            return Content(optValEx.Message);
        }
        return Content(msg);
    }

The following code applies a more complex validation rule using a delegate:

using OptionsValidationSample.Configuration;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllersWithViews();

builder.Services.AddOptions<MyConfigOptions>()
            .Bind(builder.Configuration.GetSection(MyConfigOptions.MyConfig))
            .ValidateDataAnnotations()
        .Validate(config =>
        {
            if (config.Key2 != 0)
            {
                return config.Key3 > config.Key2;
            }

            return true;
        }, "Key3 must be > than Key2.");   // Failure message.

var app = builder.Build();

IValidateOptions for complex validation

The following class implements IValidateOptions<TOptions>:

public class MyConfigValidation : IValidateOptions<MyConfigOptions>
{
    public MyConfigOptions _config { get; private set; }

    public  MyConfigValidation(IConfiguration config)
    {
        _config = config.GetSection(MyConfigOptions.MyConfig)
            .Get<MyConfigOptions>();
    }

    public ValidateOptionsResult Validate(string name, MyConfigOptions options)
    {
        string? vor = null;
        var rx = new Regex(@"^[a-zA-Z''-'\s]{1,40}$");
        var match = rx.Match(options.Key1!);

        if (string.IsNullOrEmpty(match.Value))
        {
            vor = $"{options.Key1} doesn't match RegEx \n";
        }

        if ( options.Key2 < 0 || options.Key2 > 1000)
        {
            vor = $"{options.Key2} doesn't match Range 0 - 1000 \n";
        }

        if (_config.Key2 != default)
        {
            if(_config.Key3 <= _config.Key2)
            {
                vor +=  "Key3 must be > than Key2.";
            }
        }

        if (vor != null)
        {
            return ValidateOptionsResult.Fail(vor);
        }

        return ValidateOptionsResult.Success;
    }
}

IValidateOptions enables moving the validation code out of StartUp and into a class.

Using the preceding code, validation is enabled in Program.cs with the following code:

using Microsoft.Extensions.Options;
using OptionsValidationSample.Configuration;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllersWithViews();

builder.Services.Configure<MyConfigOptions>(builder.Configuration.GetSection(
                                        MyConfigOptions.MyConfig));

builder.Services.AddSingleton<IValidateOptions
                              <MyConfigOptions>, MyConfigValidation>();

var app = builder.Build();

Options post-configuration

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

using OptionsValidationSample.Configuration;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllersWithViews();

builder.Services.AddOptions<MyConfigOptions>()
                .Bind(builder.Configuration.GetSection(MyConfigOptions.MyConfig));

builder.Services.PostConfigure<MyConfigOptions>(myOptions =>
{
    myOptions.Key1 = "post_configured_key1_value";
});

PostConfigure is available to post-configure named options:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorPages();

builder.Services.Configure<TopItemSettings>(TopItemSettings.Month,
    builder.Configuration.GetSection("TopItem:Month"));
builder.Services.Configure<TopItemSettings>(TopItemSettings.Year,
    builder.Configuration.GetSection("TopItem:Year"));

builder.Services.PostConfigure<TopItemSettings>("Month", myOptions =>
{
    myOptions.Name = "post_configured_name_value";
    myOptions.Model = "post_configured_model_value";
});

var app = builder.Build();

Use PostConfigureAll to post-configure all configuration instances:

using OptionsValidationSample.Configuration;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllersWithViews();

builder.Services.AddOptions<MyConfigOptions>()
                .Bind(builder.Configuration.GetSection(MyConfigOptions.MyConfig));

builder.Services.PostConfigureAll<MyConfigOptions>(myOptions =>
{
    myOptions.Key1 = "post_configured_key1_value";
});

By Kirk Larkin and Rick Anderson.

The options pattern uses classes to provide strongly typed access to 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.

This topic provides information on the options pattern in ASP.NET Core. For information on using the options pattern in console apps, see Options pattern in .NET.

View or download sample code (how to download)

Bind hierarchical configuration

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; }
    public string Name { get; set; }
}

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 property 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:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<PositionOptions>(Configuration.GetSection(
                                        PositionOptions.Position));
    services.AddRazorPages();
}

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.

Options interfaces

IOptions<TOptions>:

IOptionsSnapshot<TOptions>:

IOptionsMonitor<TOptions>:

Post-configuration scenarios enable setting or changing 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.

Use IOptionsSnapshot to read updated data

Using IOptionsSnapshot<TOptions>, options are computed once per request when accessed and cached for the lifetime of the request. Changes to the configuration are read after the app starts when using configuration providers that support reading updated configuration values.

The difference between IOptionsMonitor and IOptionsSnapshot is that:

  • IOptionsMonitor is a singleton service that retrieves current option values at any time, which is especially useful in singleton dependencies.
  • IOptionsSnapshot is a scoped service and provides a snapshot of the options at the time the IOptionsSnapshot<T> object is constructed. Options snapshots are designed for use with transient and scoped dependencies.

The following code uses IOptionsSnapshot<TOptions>.

public class TestSnapModel : PageModel
{
    private readonly MyOptions _snapshotOptions;

    public TestSnapModel(IOptionsSnapshot<MyOptions> snapshotOptionsAccessor)
    {
        _snapshotOptions = snapshotOptionsAccessor.Value;
    }

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

The following code registers a configuration instance which MyOptions binds against:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<MyOptions>(Configuration.GetSection("MyOptions"));

    services.AddRazorPages();
}

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

IOptionsMonitor

The following code registers a configuration instance which MyOptions binds against.

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<MyOptions>(Configuration.GetSection("MyOptions"));

    services.AddRazorPages();
}

The following example uses IOptionsMonitor<TOptions>:

public class TestMonitorModel : PageModel
{
    private readonly IOptionsMonitor<MyOptions> _optionsDelegate;

    public TestMonitorModel(IOptionsMonitor<MyOptions> optionsDelegate )
    {
        _optionsDelegate = optionsDelegate;
    }

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

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

Named options support using IConfigureNamedOptions

Named options:

  • Are useful when multiple configuration sections bind to the same properties.
  • Are case sensitive.

Consider the following appsettings.json file:

{
  "TopItem": {
    "Month": {
      "Name": "Green Widget",
      "Model": "GW46"
    },
    "Year": {
      "Name": "Orange Gadget",
      "Model": "OG35"
    }
  }
}

Rather than creating two classes to bind TopItem:Month and TopItem:Year, the following class is used for each section:

public class TopItemSettings
{
    public const string Month = "Month";
    public const string Year = "Year";

    public string Name { get; set; }
    public string Model { get; set; }
}

The following code configures the named options:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<TopItemSettings>(TopItemSettings.Month,
                                       Configuration.GetSection("TopItem:Month"));
    services.Configure<TopItemSettings>(TopItemSettings.Year,
                                        Configuration.GetSection("TopItem:Year"));

    services.AddRazorPages();
}

The following code displays the named options:

public class TestNOModel : PageModel
{
    private readonly TopItemSettings _monthTopItem;
    private readonly TopItemSettings _yearTopItem;

    public TestNOModel(IOptionsSnapshot<TopItemSettings> namedOptionsAccessor)
    {
        _monthTopItem = namedOptionsAccessor.Get(TopItemSettings.Month);
        _yearTopItem = namedOptionsAccessor.Get(TopItemSettings.Year);
    }

    public ContentResult OnGet()
    {
        return Content($"Month:Name {_monthTopItem.Name} \n" +
                       $"Month:Model {_monthTopItem.Model} \n\n" +
                       $"Year:Name {_yearTopItem.Name} \n" +
                       $"Year:Model {_yearTopItem.Model} \n"   );
    }
}

All options are named instances. 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.

OptionsBuilder is used in the Options validation section.

See Use AddOptions to configure custom repository for information adding a custom repository.

Use DI services to configure options

Services can be accessed from dependency injection while configuring options in two ways:

  • Pass a configuration delegate to Configure on OptionsBuilder<TOptions>. OptionsBuilder<TOptions> provides overloads of Configure that allow use of 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));
    
  • Create a type that implements IConfigureOptions<TOptions> or IConfigureNamedOptions<TOptions> and register the type as a service.

We recommend passing a configuration delegate to Configure, since creating a service is more complex. Creating a type is equivalent to what the framework does when calling Configure. Calling Configure registers a transient generic IConfigureNamedOptions<TOptions>, which has a constructor that accepts the generic service types specified.

Options validation

Options validation enables option values to be validated.

Consider the following appsettings.json file:

{
  "MyConfig": {
    "Key1": "My Key One",
    "Key2": 10,
    "Key3": 32
  }
}

The following class binds to the "MyConfig" configuration section and applies a couple of DataAnnotations rules:

public class MyConfigOptions
{
    public const string MyConfig = "MyConfig";

    [RegularExpression(@"^[a-zA-Z''-'\s]{1,40}$")]
    public string Key1 { get; set; }
    [Range(0, 1000,
        ErrorMessage = "Value for {0} must be between {1} and {2}.")]
    public int Key2 { get; set; }
    public int Key3 { get; set; }
}

The following code:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddOptions<MyConfigOptions>()
            .Bind(Configuration.GetSection(MyConfigOptions.MyConfig))
            .ValidateDataAnnotations();

        services.AddControllersWithViews();
    }

The ValidateDataAnnotations extension method is defined in the Microsoft.Extensions.Options.DataAnnotations NuGet package. For web apps that use the Microsoft.NET.Sdk.Web SDK, this package is referenced implicitly from the shared framework.

The following code displays the configuration values or the validation errors:

public class HomeController : Controller
{
    private readonly ILogger<HomeController> _logger;
    private readonly IOptions<MyConfigOptions> _config;

    public HomeController(IOptions<MyConfigOptions> config,
                          ILogger<HomeController> logger)
    {
        _config = config;
        _logger = logger;

        try
        {
            var configValue = _config.Value;
           
        }
        catch (OptionsValidationException ex)
        {
            foreach (var failure in ex.Failures)
            {
                _logger.LogError(failure);
            }
        }
    }

    public ContentResult Index()
    {
        string msg;
        try
        {
             msg = $"Key1: {_config.Value.Key1} \n" +
                   $"Key2: {_config.Value.Key2} \n" +
                   $"Key3: {_config.Value.Key3}";
        }
        catch (OptionsValidationException optValEx)
        {
            return Content(optValEx.Message);
        }
        return Content(msg);
    }

The following code applies a more complex validation rule using a delegate:

public void ConfigureServices(IServiceCollection services)
{
    services.AddOptions<MyConfigOptions>()
        .Bind(Configuration.GetSection(MyConfigOptions.MyConfig))
        .ValidateDataAnnotations()
        .Validate(config =>
        {
            if (config.Key2 != 0)
            {
                return config.Key3 > config.Key2;
            }

            return true;
        }, "Key3 must be > than Key2.");   // Failure message.

    services.AddControllersWithViews();
}

IValidateOptions for complex validation

The following class implements IValidateOptions<TOptions>:

public class MyConfigValidation : IValidateOptions<MyConfigOptions>
{
    public MyConfigOptions _config { get; private set; }

    public  MyConfigValidation(IConfiguration config)
    {
        _config = config.GetSection(MyConfigOptions.MyConfig)
            .Get<MyConfigOptions>();
    }

    public ValidateOptionsResult Validate(string name, MyConfigOptions options)
    {
        string vor=null;
        var rx = new Regex(@"^[a-zA-Z''-'\s]{1,40}$");
        var match = rx.Match(options.Key1);

        if (string.IsNullOrEmpty(match.Value))
        {
            vor = $"{options.Key1} doesn't match RegEx \n";
        }

        if ( options.Key2 < 0 || options.Key2 > 1000)
        {
            vor = $"{options.Key2} doesn't match Range 0 - 1000 \n";
        }

        if (_config.Key2 != default)
        {
            if(_config.Key3 <= _config.Key2)
            {
                vor +=  "Key3 must be > than Key2.";
            }
        }

        if (vor != null)
        {
            return ValidateOptionsResult.Fail(vor);
        }

        return ValidateOptionsResult.Success;
    }
}

IValidateOptions enables moving the validation code out of StartUp and into a class.

Using the preceding code, validation is enabled in Startup.ConfigureServices with the following code:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<MyConfigOptions>(Configuration.GetSection(
                                        MyConfigOptions.MyConfig));
    services.TryAddEnumerable(ServiceDescriptor.Singleton<IValidateOptions
                              <MyConfigOptions>, MyConfigValidation>());
    services.AddControllersWithViews();
}

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.

Options.ConfigurationExtensions NuGet package

The Microsoft.Extensions.Options.ConfigurationExtensions package is implicitly referenced in ASP.NET Core apps.