ASP.NET Core Feature Flag TagHelper

Releasing stuff is dangerous: you might break things, you might annoy your users or you could screw up in any number of entertaining ways.

Feature flags are a great way to get functionality into production without quite so much risk. You can release your new feature to only a small subset of your users and then roll it out once you’re happy that things aren’t on fire.

The way you define flags will depend on the requirements for roll out – sometimes a configuration setting is sufficient; sometimes you’ll need per-user settings or something more complex. That’s outside the scope of this article though – I’ll leave that part up to you.

Once you have your flags defined you want to start modifying content based on those flags. You coulds do that with a bunch of if statements but when we’re talking about Razor it can get messy fast.

Instead, wouldn’t it be nice to wrap your new stuff in a special tag?

<feature flag="MyCoolNewThing">
  <!-- cool content here -->
</feature>

Or if you want to display something only for users without the new feature?

<feature flag="MyCoolNewThing" disabled>
  Click here to enable my new cool thing!
  <button>Enable Now!</button>
</feature>

Tag Helpers make this easy!

The TagHelper Class

Tag helpers allow you to write server-side code that manipulates the DOM during the render of a Razor view. They can accept dependency-injected services (in the scope of the current request) and can access attributes and child content of their Razor element.

They are implemented by extending the TagHelper class and are used in Razor views by converting the class name to a kebab-cased equivalent. e.g. MyGreatNewTagHelper would be available via the my-great-new tag.

The class then modifies the generated DOM by overriding either the Process or ProcessAsync methods. These methods are passed context objects to allow both interrogation of the current content and modification of the output.

public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
{
  //...
}

Hiding Child Content

In our case, the “modification” we want is really simple: we want to optionally hide all child content. This is directly supported via the TagHelperOutput.SuppressContent() method so we can hide content with a 1 line method:

public override async Task ProcessAsync(
  TagHelperContext context, 
  TagHelperOutput output)
{
  output.SuppressContent();
}

note that we don’t need to be using the ProcessAsync variant as we have no async code yet, but we’ll be adding some shortly

Optionally Hiding Child Content

We only want to hide the child content if a feature flag is disabled, so we need to know the state of the flag. There are a lot of ways that feature flag settings could be implemented (configuration settings, per-user flags, etc.) so we are going to abstract all of that away behind an interface:

public interface IFeatureFlagProvider {
  Task<bool> IsEnabled(FeatureFlag featureFlag);
}

public enunm FeatureFlag {
  Unknown,
  MyCoolNewThing,
  AnotherAwesomeFeature,
  Etc
}

The IFeatureFlagProvider accepts an enum value identifying a feature and asynchronously returns a boolean indicating whether or not the feature is enabled. Any complexity around how you determine the availability of the feature can happily hide behind this facade.

Note: I’m using an enum to define my features but strings are a valid alternative. I prefer enums because you can find all references easily and if you’re adding new features then you’re going to be changing code anyway!

As I said above, tag helpers can accept injected dependencies so as soon as we have registered an implementation of IFeatureFlagProvider we can use it in our helper. We’re also going to add a Flag property which will be set on each instance of the tag.

public class FeatureTagHelper : TagHelper
{
  private readonly IFeatureFlagProvider _featureFlagProvider;

  public FeatureTagHelper(IFeatureFlagProvider featureFlagProvider)
  {
    _featureFlagProvider = featureFlagProvider;
  }

  public FeatureFlag Flag { get; set; }

  public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
  {
    var isFeatureEnabled = await _featureFlagProvider.IsEnabled(this.Flag);
    if (!isFeatureEnabled)
      output.SuppressContent();
  }
}

Support Disabled State

We may want to show content only when a feature is not enabled (e.g. an “enable now” message) so we want to support the inverse. One option would be to create a second tag helper but I felt like a disabled flag made more sense when in Razor:

<feature flag="FeatureName" disabled>...</feature>

To achieve this we can use the supplied TagHelperContext to look for a disabled attribute and then combine this with our isFeatureEnabled condition from earlier:

public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
{
    var isFeatureEnabled = await _featureFlagProvider.IsEnabled(this.Flag);
    var isTagDisabled = context.AllAttributes.Where(a => a.Name?.ToLowerInvariant() == "disabled").Any();

    var showContent = isFeatureEnabled != isTagDisabled;
    if (!showContent)
      output.SuppressOutput();
  }
}

Using the TagHelper

Now that we’ve created the tag helper we can start using it in our Razor views.

Before it becomes available we will need to add it to the project in the Views/_ViewImports.cshtml file that is created as part of the ASP.NET Core templates.

You can either add all tag helpers in your project or add each one individually.

<!-- add all in assembly MyProject -->
@addTagHelper *, MyProject
<!-- or add individually -->
@addTagHelper MyProject.TagHelpers.FeatureTagHelper, MyProject

Once that’s done you can use the new tags in any of your Razor pages or views as below:

<feature flag="MyCoolNewThing">
  <!--
    content will only be displayed if FeatureFlag.MyCoolNewThing is enabled
    for the current user
  -->
  <h1>Cool New Thing</h1>
</feature>

<feature flag="MyCoolNewThing" disabled>
  <!--
    content will only be displayed if FeatureFlag.MyCoolNewThing is disabled
    for the current user
  -->
  Click here to enable my new cool thing!
  <button>Enable Now!</button>
</feature>
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Composite Pattern in .NET Core with Dependency Injection

Following on from my previous post where I implemented a decorator pattern using .NET Core dependency injection I realised that I could use the same method to create a composite pattern in a developer-friendly way.

Composite Pattern

Similar to the decorator pattern, the Composite Pattern let’s you wrap existing implementations of an interface to augment the functionality.

The difference between the two is that the decorator wraps a single instance of the interface; a composite wraps many.

interface IService {
  void DoSomething(string value);
}

class Decorator : IService {
  public Decorator(IService wrappedService) {
    //...
  }
}

class Composite : IService {
  public Composite(IEnumerable<IService> wrappedServices) {
    //...
  }
}

This is useful where you have a number of implementations of your service and you don’t want dependent classes to know whether they should call one, some or all of them.

For example, if you have a report generator that wants to send results to multiple sources you might implement several instances of IReporter:

interface IReporter {
  void Send(IReport report);
}

class ConsoleReporter : IReporter {
  public void Send(IReport report) {
    //write details to console
  }
}

class TelemetryReporter : IReporter {
  public void Send(IReport report) {
    //write stats to a telemetry service
  }
}

class EmailReporter : IReporter {
  public void Send(IReport report) {
    //send a report email to stakeholders
  }
}

Your composite reporter would construct on all other implementations of IReporter and call them in order:

class CompositeReporter : IReporter {
  private IEnumerable<IReporter> _reporters;

  public CompositeReporter(IEnumerable<IReporter> reporters) {
    _reporters = reporters;
  }

  public void Send(IReport report) {
    foreach (var reporter in _reporters)
      reporter.Send(report);
  }
}

This means that anything that needs to send a report can request a single IReporter and let the CompositeReporter worry about routing the report through the correct concrete implementations.

Default DI Behaviour

As discussed in the previous post, the default behaviour of the .NET Core Dependency Injection framework is to provide the last-registered copy of an interface, or all registered copies for an IEnumerable.

services.AddScoped<IService, ConcreteService1>();
services.AddScoped<IService, ConcreteService2>();
services.AddScoped<IService, ConcreteService3>();

//later
const service = serviceProvider.GetRequiredService<IService>();
// service is instance of ConcreteService3

const allServices = serviceProvider.GetRequiredService<IEnumerable<IService>>();
// allServices contains one instance of all 3 registered implementations

What we need is a way to register a new type to replace the existing registrations and take them in as a constructor dependency.

class Composite : IService {
  public class Composite(IEnumerable<IService> services) {
    //...
  }
}

//BAD - throws StackOverflowException when resolved!
services.AddScoped<IService, Composite>();

Unfortunately the default behaviour of the DI framework is to attempt to fulfil the request for all IService implementations with…another instance of Composite! One StackOverflowException later and we’re back to the drawing board.

How can we make this play nicely with DI?

Borrowing from the Last Post

After some digging through the ASP.NET Core source code in the last post we came up with a couple of useful helpers that we can re-use here: CreateFactory and CreateInstance.

ActivatorUtilities.CreateFactory generates a factory function to create an instance of ConcreteType from the service provider with some services provided explicitly.

var objectFactory = ActivatorUtilities.CreateFactory(
  typeof(ConcreteType), 
  new[] { typeof(IService) });

CreateInstance creates an instance of a service from a ServiceDescriptor.
We can get instances of ServiceDescriptor from the service collection and use these to create previously-registered types.

public static object CreateInstance(this IServiceProvider services, ServiceDescriptor descriptor)
{
  if (descriptor.ImplementationInstance != null)
    return descriptor.ImplementationInstance;

  if (descriptor.ImplementationFactory != null)
    return descriptor.ImplementationFactory(services);

  return ActivatorUtilities.GetServiceOrCreateInstance(services, descriptor.ImplementationType);
}

With these tools we can define our desired composite behaviour.

Extract Existing Registrations

When we register a new composite we want to

  1. Remove all existing registered services for the same interface
  2. Insert the composite implementation
  3. Pass instances of all removed implementations into the constructor of the composite

Remove Existing Registrations

IServiceCollection extends IEnumerable so we can filter it down to get the services that match the interface of the composite class.

public static void AddComposite<TInterface, TConcrete>(this IServiceCollection services)
  where TInterface : class
  where TConcrete : class, TInterface
{
  //get a list of existing registrations matching the target interface
  var wrappedDescriptors = services
    .Where(s => s.ServiceType == typeof(TInterface))
    .ToList();

  //remove each from the service collection
  foreach (var descriptor in wrappedDescriptors)
    services.Remove(descriptor);

  //...
}

We call ToList to we get a persistent list of the items and then remove them from the original collection.

Add Composite Implementation

Next up we want to insert the definition of our composite class, and we’re going to use the ActivatorUtilites helper mentioned above.

var objectFactory = ActivatorUtilities.CreateFactory(
  typeof(TConcrete),
  new[] {
    typeof(IEnumerable<TInterface>)
  });

Here we create a factory function that can be used with a service provider to resolve an instance of TConcrete (i.e. our composite class) with any parameters of type IEnumerable{TInterface} manually specified by us.

The objectFactory forms the basis of a new ServiceDescriptor to add to the collection.

var compositeDescriptor = ServiceDescriptor.Describe(
  typeof(TInterface),
  serviceProvider => (TInterface)objectFactory(serviceProvider, new [] {
    /* todo: inject original services here */
  },
  ServiceLifetime.Scoped);
);

services.Add(compositeDescriptor);

Note: in this example I have hard-coded a lifetime of Scoped for the service. We can improve on this below but it will do for now.

Inject Original Services

We still need to inject instances of the original services that we removed. We recorded their service descriptors in wrappedDescriptors and we can now combine those with the CreateInstance extension method above to populate our constructor parameter.

var compositeDescriptor = ServiceDescriptor.Describe(
  typeof(TInterface),
  serviceProvider => (TInterface)objectFactory(serviceProvider, new [] {
    wrappedDescriptors
      .Select(d => serviceProvider.CreateInstance(d))
      .Cast<TInterface>()
  },
  ServiceLifetime.Scoped);
);

Now all of the wrapped services will be created through the service provider and passed to our composite.

This approach may seem long winded but it has the advantage that any other dependencies of either the wrapped services or our composite will also be injected from the service provider with no further input from us!

Calculate Lifetime Scope

The hard-coded lifetime scope isn’t ideal, and whilst we could push the onus onto the caller to specify a scope we can do slightly better and infer it from the existing registrations.

If the composite depends on a Scoped instance then it can be either Scoped or Transient without a problem, but cannot be Singleton as it would not have access to scoped dependencies.

We can infer the maximum scope of the composite by taking the most specific scope of it’s dependencies. The ServiceLifetime enum is defined with the least specific scope (Singleton) as 0 so we can select the maximum value to get the most specific.

Pull it All Together

Combining all of the above we get the following extension method:

public static void AddComposite<TInterface, TConcrete>(this IServiceCollection services)
  where TInterface : class
  where TConcrete : class, TInterface
{
  var wrappedDescriptors = services.Where(s => s.ServiceType == typeof(TInterface)).ToList();
  foreach (var descriptor in wrappedDescriptors)
    services.Remove(descriptor);

  var objectFactory = ActivatorUtilities.CreateFactory(
    typeof(TConcrete), 
    new[] { typeof(IEnumerable<TInterface>) });

  services.Add(ServiceDescriptor.Describe(
    typeof(TInterface),
    s => (TInterface)objectFactory(s, new[] { wrappedDescriptors.Select(d => s.CreateInstance(d)).Cast<TInterface>() }),
    wrappedDescriptors.Select(d => d.Lifetime).Max())
  );
}

Now we can wrap up as many services as we want in a composite with one line

var services = new ServiceCollection();
services.AddSingleton<IReporter, ConsoleReporter>();
services.AddScoped<IReporter, TelemetryReporter>();
services.AddTransient<IReporter, EmailReporter>();

services.AddComposite<IReporter, CompositeReporter>();

Decorators in .NET Core with Dependency Injection

What is a Decorator?

Feel free to skip this section if you’re already familiar with the Decorator pattern

The Decorator Pattern allows you to add functionality to an implementation of an interface by wrapping it in another implementation. e.g.

interface IService {
  string GetValue();
}

class DbService {
  public string GetValue() => "value from DB";
}

class LoggingService {
  private readonly IService _concreteService;
  private readonly ILogger _logger;

  public LoggingService(IService concreteService, ILogger<LoggingService> logger) {
    _concreteService = concreteService;
    _logger = logger;
  }

  public string GetValue() {
    _logger.LogInformation("Getting value");
    var value = _concreteService.GetValue();
    _logger.LogInformation("Retrieved {0}", value);
    return value;
  }
}

We have a concrete implementation of an IService interface (DbService) that returns a string value. LoggingService then decorates that implementation by wrapping invocation of the concrete instance and logging entry and exit.

This pattern is very useful when you want to augment either your own or framework types with some new behaviour such as logging or exception handling.

Decorators and Dependency Injection

The dependency injection framework in .NET Core is pretty good but out-of-the-box it doesn’t support this pattern very well.

If I register 2 copies of IService then any class attempting to consume IService will receive the one that was registered last. If they try to consume an IEnumerable then they will receive all of them. This is perfectly sensible behaviour but in this case it makes life harder.

Worse, if an implementation of IService depends on an instance of IService (and is the last one registered) then we will get a StackOverflowException when we resolve it!

Ideally we would like to be able to use this pattern with dependency injection without having to put too many constraints on either the decorator or the concrete implementation.

Specifically:

  • the wrapped implementation should be able to be registered in the DI container against the same interface as the decorator. This avoids adding any requirements to the code we are decorating
  • the decorator should depend on an instance of the interface it implements. This allows decorators to be applied to other decorators or implementations and makes the decorator easily testable
  • any dependencies of either the concrete implementation or the decorator should be injected automatically

So how can we achieve all this?

Approach #1: Reference a Concrete Type

Have the decorator explicitly depend on a concrete implementation of the wrapped class.

class Decorator : IService {
  public Decorator(ConcreteService service) {}
}

public static void ConfigureServices(IServiceCollection services) {
  services.Replace(ServiceDescriptor.Scoped<IService, Decorator>());
}
  • ✅ Easy
  • ❌ Can only decorate one implementation
  • ❌ Cannot inject a mock for unit tests
  • ❌ Requires the decorated implementation to be registered against it’s concrete type – not the interface
  • ❌ Cannot take service lifetime (scoped, transient or singleton) from wrapped service

Overall, it’s not great.

Approach #2: Use a Factory Registration

Use a factory method to pass the explicit concrete type to the decorator constructor.

class Decorator : IService {
  public Decorator(IService concreteService) {}
}

public static void ConfigureServices(IServiceCollection services) {
  services.Replace(ServiceDescriptor.Scoped<IService>(s =>
    new Decorator(s.GetRequiredService<ConcreteService>())
  ));
}
  • ✅ Decorator can construct on an interface
  • ❌ Still requires decorated implementation to be registered against it’s concrete type
  • ❌ Forces you to manually maintain other dependencies for Decorator
  • ❌ Cannot take service lifetime from wrapped service

This solution is relatively neat until your Decorator class starts to gain other dependencies.

class Decorator : IService {
  public Decorator(
    IService concreteService,
    IOtherService1 anotherDependency,
    IOtherService2 yetAnotherDependency) {}
}

public static void ConfigureServices(IServiceCollection services) {
  services.Replace(ServiceDescriptor.Scoped<IService>(s =>
    new Decorator(
      s.GetRequiredService<ConcreteService>(),
      s.GetRequiresService<IOtherService1>(),
      s.GetRequiredService<IOtherService2>())
  ));
}

It’s not the end of the world but it is a bit of a headache having to manually update constructor calls when you change a signature.

This approach also fails our requirement that the wrapped implementation is not explicitly specified.

Approach #3: Reusing the Existing Service Descriptor

IServiceCollection is largely a wrapper around a list of ServiceDescriptor objects so we can go grab the existing registration and use that in our factory.

class Decorator : IService {
  public Decorator(
    IService concreteService,
    IOtherService1 anotherDependency) {}
}

public static void ConfigureServices(IServiceCollection services) {
  var wrappedServiceDescriptor = services.First(s => s.ServiceType == typeof(IService));

  services.Replace(ServiceDescriptor.Describe(
    typeof(IService),
    s => new Decorator(
      s.CreateInstance(wrappedServiceDescriptor),
      s.GetRequiresService<IOtherService1>()),
    wrappedServiceDescriptor.Lifetime
  ));
}

public static object CreateInstance(this IServiceProvider services, ServiceDescriptor descriptor)
{
  if (descriptor.ImplementationInstance != null)
    return descriptor.ImplementationInstance;

  if (descriptor.ImplementationFactory != null)
    return descriptor.ImplementationFactory(services);

  return ActivatorUtilities.GetServiceOrCreateInstance(services, descriptor.ImplementationType);
}

Here we use the existing service descriptor to resolve an instance that we can pass into our constructor, as well as copying the defined service lifetime. We’re using the CreateInstance helper method to create an instance of the wrapped type from the service descriptor.

  • ✅ Decorator can construct on an interface
  • ✅ No longer requires the decorated implementation to be registered agaist it’s concrete type
  • ✅ Copies service lifetime from wrapped service
  • ❌ Still forces you to manually maintain other dependencies for Decorator

Closer, but we still have the pain point of managing dependencies when Decorator changes in the future.

Approach #4: Use an Intermediate Scope

Use the dependency injection framework to instantiate the decorator from the services registered so far.

//Decorator class as above

public static void ConfigureServices(IServiceCollection services) {
  var wrappedServiceDescriptor = services.First(s => s.ServiceType == typeof(IService));

  // add the decorator as a concrete type (with the same lifetime as the wrapped service)
  services.Add(ServiceDescriptor.Describe(typeof(Decorator), typeof(Decorator), wrappedServiceDescriptor.Lifetime);

  // build a new service provider that contains the decorator
  // as a concrete type and the wrapped service as the interface
  // (as well as anything that has already been registered)
  var intermediateProvider = services.BuildServiceProvider();

  // replace the interface registration by resolving the
  // decorator type from the intermediate provider
  services.Replace(ServiceDescriptor.Describe(
    typeof(IService),
    s => intermediateProvider.GetRequiredService<Decorator>(),
    wrappedServiceDescriptor.Lifetime
  ));
}
  • ✅ Decorator can construct on an interface
  • ✅ Can register decorated implementation against interface
  • ✅ Automatically handles dependency changes
  • ❌ Can cause scope issues with other dependencies

This version is pretty good but it does have a subtle problem: the intermediate scope will only contain services that have been registered at the time this code is called. This means that if either Decorator or the wrapped service have other dependencies that are registered later then they won’t be available.

This is possibly not the worst problem in the world but it could lead to a nasty category of bug that would be very difficult to track down. Can we do better?

Going to the Source

By this point we can define our ideal solution as something like:

var existingService = services.First(s => s.ServiceType == typeof(IService));
services.Replace(ServiceDescriptor.Describe(
  typeof(IService),
  s => s.GetRequiredService<Decorator>(/* somehow pass in existingService here */),
  existingService.Lifetime)
);

i.e. we want to use the existing service registration but only while resolving the decorator. Anything else resolving IService from the container should get the decorator.

While researching this problem, the closest analogue that came to mind was the way in which HttpClientFactory injects specific HttpClient instances into resolved classes. After a few minutes digging through the source (open source FTW!) I found their implementation including a few useful utilities that can help us out here.

ActivatorUtilities & ObjectFactory

The ActivatorUtilities.CreateFactory helper method creates a factory function for a concrete type:

var objectFactory = ActivatorUtilities.CreateFactory(
  typeof(Decorator), 
  new[] { typeof(IService) });

The second parameter is an array of the types that we can explicitly provide to the created objectFactory function. In this case, we will provide an explicitly-specified instance of IService but every other constructor parameter should be sourced from a service provider.

We invoke the objectFactory by passing in a service provider as well as an array of concrete instances of the types specified above.

ServiceDescriptor wrappedDescriptor;
IServiceProvider services;
var instanceOfDecorator = objectFactory(
  services, 
  new [] { services.CreateInstance(wrappedDescriptor) });

These 2 methods together give us exactly what we need:

  • ✅ Decorator can construct on an interface
  • ✅ Wrapped implementation can be registered against the interface
  • ✅ Decorator scope is taken from wrapped service
  • ✅ Dependencies in wrapped implementation are injected automatically
  • ✅ Dependencies in decorator are injected automatically

Here’s our final implementation, converted into a generic extension method on the IServiceCollection:

public static class ServiceCollectionExtensions {

  public static void Decorate<TInterface, TDecorator>(this IServiceCollection services)
    where TInterface : class
    where TDecorator : class, TInterface
  {
    // grab the existing registration
    var wrappedDescriptor = services.FirstOrDefault(
      s => s.ServiceType == typeof(TInterface));

    // check it's valid
    if (wrappedDescriptor == null) 
      throw new InvalidOperationException($"{typeof(TInterface).Name} is not registered");

    // create the object factory for our decorator type,
    // specifying that we will supply TInterface explicitly
    var objectFactory = ActivatorUtilities.CreateFactory(
      typeof(TDecorator), 
      new[] { typeof(TInterface) });

    // replace the existing registration with one
    // that passes an instance of the existing registration
    // to the object factory for the decorator
    services.Replace(ServiceDescriptor.Describe(
      typeof(TInterface),
      s => (TInterface)objectFactory(s, new[] { s.CreateInstance(wrappedDescriptor) }),
      wrappedDescriptor.Lifetime)
    );
  }

  private static object CreateInstance(this IServiceProvider services, ServiceDescriptor descriptor)
  {
    if (descriptor.ImplementationInstance != null)
      return descriptor.ImplementationInstance;

    if (descriptor.ImplementationFactory != null)
      return descriptor.ImplementationFactory(services);

    return ActivatorUtilities.GetServiceOrCreateInstance(services, descriptor.ImplementationType);
  }
}

Our sample usage is now nice and easy. We can even stack more than one decorator with each one wrapping the previously-registered type!

public static void ConfigureServices(IServiceCollection services) {
  services.AddScoped<IService, DbService>();
  services.Decorate<IService, LoggingService>();
  services.Decorate<IService, ExceptionHandlingService>(); //etc.

  // IService resolves to:
  //  an ExceptionHandlingService
  //    wrapping a LoggingService
  //      wrapping a DbService
}

I’m not sure why this was such a complicated thing to achieve with the dependency injection framework – it is certainly easier in, for example, Autofac – but it goes to show the great advantage of open source: you can almost always find a way!

Post-Logout Redirect with ASP.NET Core and ADFS 2016

Redirect after logout

When a user logs out from your app you have the option to log them out of the provider as well by redirecting the browser to the logout endpoint. By default this means that the user will end up sat on your providers “You have signed out” page – not brilliant.

You can, however, tell your provider to redirect back to your app once they’re done with logout by specifying a post_logout_redirect_uri.

For ASP.NET Core Identity you can specify this redirection as a parameter on the SignOutResult.

[Route("auth")]
public class AuthController
{
  [HttpPost("logout")]
  public IActionResult Logout() => new SignOutResult(
    OpenIdConnectDefaults.AuthenticationScheme,
    new AuthenticationProperties { RedirectUri = Url.Action(nameof(LogoutSuccess))});

  [HttpGet("logoutSuccess")]
  public IActionResult LogoutSuccess() => View();
}

Useless ADFS error messages

For ADFS 2016 you need to do a little bit more than just set the redirect URL. On first inspection you can see that the above will set the parameter in the ADFS URL but ADFS will silently ignore it and your user will sit forever on the ADFS sign-out page.

Digging into the event logs you will find the following error message:

The specified redirect URL did not match any of the OAuth client’s redirect URIs. The logout was successful but the client will not be redirected

If you’re unlucky, this wonderfully-misleading error message can take you down a rabbit hole of further configuration. It’s a shame, then, that no-one thought to expose something a little more accurate:

That redirect looks good but you need to specify id_token_hint or we’ll ignore you

Thanks ADFS!

Sending ID Token Hint

To be fair to ADFS, sending an id_token_hint is recommended by the spec. This parameter needs to be set to the id_token that was sent to your app when the user first logged in; provide this value and ADFS will happily redirect back to your app.

The only problem here is that you probably don’t still have that id_token. ASP.NET Core Identity uses the identity information to create an auth cookie and then (by default) discards it.

Happily, the OpenIdConnectOptions exposes a SaveTokens property to persist the received token to the auth cookie. Even better: the OIDC logout mechanism will automatically pick this up once enabled so you should be good to go as soon as you set the flag:

public class Startup {
  public void ConfigureServices(IServiceCollection services)
  {
    services.AddAuthentication()
      .AddOpenIdConnect(options => {
        options.SaveTokens = true;
        //...set other options
      });

    //...other service config
  }
}

This does have one important downside though: you’re now storing much more information in your auth cookie and that adds extra data to every client request, maybe even doubling the cookie size.

Whether or not this is a problem for your app is another decision – at least your logout redirect works now!

Find .NET Core packages in all projects

In a quick aide-memoire for next time I need to use it: here’s a Powershell snippet that will return a list of all unique package names in all .NET Core projects under the current folder:

get-childitem -recurse -filter *.csproj |
  foreach { get-content $_.FullName } |
  select-string -pattern "PackageReference Include=""(.+?)""" |
  % { $_.matches[0].Groups[1].Value } |
  sort-object -unique

Line by line…

  • Get all *.csproj in the current and child folders
  • Read the content of each one
  • Extract every line that matches the specific regex and extract the name of the package
  • Grab the package name group
  • Sort & filter to unique values

This outputs something like

CommandLineParser
CsvHelper
Microsoft.Extensions.DependencyInjection
Microsoft.Extensions.Logging
Microsoft.Extensions.Logging.Console

Generic behaviour in ASP.NET Core with Action Filters

Everyone hates copy/pasting code, and Action Filters in ASP.NET Core MVC offer a great way to avoid filling your controllers with boilerplate.

Filters offer you entry points into the execution pipeline for an action where you can examine the incoming parameters or generated results and modify them to suit your needs.

Here are a couple of examples of how this can help.

Treat a null result as a 404 Not Found

By default an ASP.NET Core controller will return a 204 No Content response if you return null from an action:

[Route("api/example")]
public class ExampleApiController : Controller
{
  [HttpGet("")]
  public string GetExample()
  {
    return null;
  }
}

In some cases, however, you might not want to treat null as No Content. If your API is looking up a resource by ID, for example, then a 404 Not Found response would be more useful:

[HttpGet("{id}")]
public MyDtoObject GetById(int id)
{
  if (!_store.ContainsId(id))
    return null; //should return 404

  //...
}

We can use an action filter to automatically replace the null result with a NotFoundResult:

//filter
public class NullAsNotFoundAttribute : ActionFilterAttribute
{
  public override void OnActionExecuted(ActionExecutedContext context)
  {
    var objectResult = context.Result as ObjectResult;

    if (objectResult?.Value == null)
      context.Result = new NotFoundResult();
  }
}

//controller
[HttpGet("{id}")]
[NullAsNotFound]
public MyDtoObject GetById(int id)
{
  //...
}

Here we override OnActionExecuted to invoke our code after the action method has generated a result but before that result is processed.

If the generated result is an ObjectResult with a null value then we replace it with an empty NotFoundResult and our controller will now return a 404 response.

Treat invalid models as a 400 Bad Request

It is very common to see the following pattern in MVC controllers:

[HttpPost("")]
public IActionResult Create(MyModel model)
{
  if (!ModelState.IsValid)
    return new BadRequestObjectResult(ModelState); //or a View, or other validation behaviour

  //...eventually return created model
  return Ok(model);
}

This has 2 downsides:
* Boilerplate code in every action that needs to validate
* Return type must be IActionResult to accomodate 2 result types

The second point is fairly minor but worth noting. By exposing IActionResult instead of the concrete type we lose metadata about the action.
That metadata is useful for things like generating swagger docs, and losing it can mean you need to decorate the method with response types (though this is improved in ASP.NET Core 2.1 with IActionResult).

In any case, we can make this behaviour generic by moving the validation check into another action filter:

public class InvalidModelAsBadRequestAttribute : ActionFilterAttribute
{
  public override void OnActionExecuting(ActionExecutingContext context)
  {
    if (!context.ModelState.IsValid)
      context.Result = new BadRequestObjectResult(context.ModelState);
  }
}

This time we are overriding OnActionExecuting instead of OnActionExecuted so our code gets run before the controller action. We can tell if the model is invalid before hitting our controller so we can skip the action entirely if we know it should be replaced with a 400.

Other Possibilities

Wherever you find yourself writing duplicate code in many actions it is worth considering whether it can be pulled out into a filter (or middleware) to keep your controllers clean and focussed on their intent.